Roofing Calculator — A Roofing Square Is Not Always What the Product Covers

Calculate

This selector governs every field, label, result and export here and takes priority over the site header switch. Switching converts what you entered rather than reinterpreting it, so a 40 ft eave becomes 12.192 m and returns to exactly 40. The roofing square stays 100 square feet, which is 9.290304 square metres, because it is an Imperial trade unit rather than a conversion.

The Roof

The fields below follow from this answer, and the three routes are different calculations rather than three ways of typing the same one. Plane by plane is the route for a roof with a garage, a porch or a lean-to at a different pitch, because each plane then carries its own slope factor.

Waste and Covering

This drives the published waste band and nothing else. It creates no ridge length, no hip length and no valley, because a length this page has not been given is a length it does not know.

No shingle data sheet consulted publishes a waste percentage, so this is a judgment about your roof rather than a figure to look up. None is an answer and is offered as one.

Published bundle counts run three per square for strip and standard laminate and three to four for architectural and dimensional, and one source gives three, four or five. There is no default that is right across product lines.

Product Coverage (optional)

In square feet, from the wrapper or the data sheet. Leave it blank to use the nominal square of 100 square feet, which the result then names as assumed.

A wrapper prints both and they differ by the bundles per square. The published 98.4 per square at three bundles per square is 32.8 per bundle, and on 2,500 square feet the two readings give 77 bundles and 229.

Underlayment (optional)

In square feet, what the roll holds. This is the figure most products are quoted on.

In square feet, what the roll puts on the roof after the laps. One published sheet gives 905.67 against contents of 1,001.

The Linear Items (optional)

In feet, along every peak ridge. Nothing in the square count knows how long it is.

In feet, along every hip. The same product covers ridge and hip, so the two are added.

In linear feet per bundle, from the product. Published figures run 29.53 to 33, which is too wide a spread to guess inside, so without this the cap is reported as a length only.

In feet, along every eave. Starter follows the eave and drip edge follows the eave and the rake.

In feet, along every rake. Drip edge runs here as well as along the eave.

In linear feet per unit, from the product. Without it the eave is reported as a length and no unit count is issued.

In feet, the length of one piece. Without it the drip edge is reported as a length rather than a count.

Weight and Cost (optional)

In pounds per square, from your product. Without it the order weight is reported as the published 230 to 430 pound range with no midpoint taken.

A whole number, for the tear off weight. It comes off the roof squares rather than the order squares, because the roof already on the house carries none of your waste allowance.

Your price for one square of field shingles. This page holds no price data of its own.

Your price for one bundle. It is applied to the rounded up bundle count, because bundles are bought whole.

Your price for one roll of underlayment. It is applied to the coverage count where you entered one, and to the contents count otherwise.

Overview

A roofing square is 100 square feet of roof surface. It is the unit the trade measures in, prices in and orders in, and one manufacturer states it on its own data sheet.

A nominal square of laminate shingle can cover 98.4 square feet. Owens Corning publishes that figure for Oakridge, alongside three bundles per square.

That is 1.6 percent, which is 40 square feet on a 25 square roof, and it shows up at the last course rather than the first.

The same gap runs through the rest of the order and it runs a different way each time. An underlayment roll containing 1,001 square feet covers 905.67 once the laps are taken out. Ridge cap is sold by the linear foot and bought by the bundle. Starter follows the eave and drip edge follows the eave and the rake, and neither has anything to do with the square count.

Before any of it, the surface has to be the right one. A ground measurement gives a footprint and a pitched roof is always larger. A 2,000 square foot footprint under a 6:12 gable is 2,236 square feet of roof, which is 74 bundles at ten percent waste against 66 ordered off the footprint.

And the waste percentage every calculator offers as a setting is in no data sheet at all.

What to Look at First

Read the measurement basis and the slope factor before the bundle count. The bundles are arithmetic on the roof area, and the roof area is decided by two answers most calculators never ask for: whether your figures are horizontal or already on the roof, and whether they include the overhang. A surface measurement multiplied by the slope factor is 11.8 percent over on a 6:12 roof and looks entirely reasonable on the screen.

How to Use This Calculator

  1. Say whether your measurements are horizontal or taken on the roof. A tape run up the slope already carries the pitch, and applying the slope factor to it counts the pitch twice.

  2. Enter the roof plan rather than the building footprint, or say that your footprint excludes the overhang. A one foot overhang all round a 40 by 30 house adds 12 percent before any slope factor.

  3. Give each plane its own slope where they differ. A main roof at 6:12 with a garage at 4:12 is two calculations, and one factor across both is wrong for both.

  4. Set the waste allowance yourself. No shingle data sheet publishes one, so this is a judgment about your roof rather than a figure to look up, and the published bands are guidance.

  5. Enter the coverage from your shingle wrapper if you have it. Where it is not 100 square feet per square, the bundle count comes from the product rather than from the unit.

  6. Measure the ridge, the hips, the eaves and the rakes. Nothing in the square count knows how long any of them are.

The bundle count is arithmetic on a roof area and a coverage figure, and both of them are decisions rather than measurements the page can make for you. The area depends on the basis and the overhang, and the coverage depends on the wrapper of the product you are actually buying.

Inputs & Outputs

Inputs

•Unit System : Options: US / Imperial (ft, sq ft, lb), SI / Metric (m, sq m, kg)
•How The Roof Is Entered : Options: Not selected, A roof plan area and a single slope, Plane by plane, each with its own slope, A sloped area already known
•Measurement Basis : Options: Not selected, Horizontal projection, from a plan or measured across the ground, Already measured on the roof surface, with a tape run up the slope
•Roof Plan Length
•Roof Plan Width
•Overhang Treatment : Options: Not selected, My figures are the roof plan and include the overhang, My figures are the building footprint only
•Eave Overhang
•Rake Overhang
•Slope, As A Rise In Twelve
•Plane 1 Length
•Plane 1 Width
•Plane 1 Slope
•Plane 2 Length
•Plane 2 Width
•Plane 2 Slope
•Plane 3 Length
•Plane 3 Width
•Plane 3 Slope
•Plane 4 Length
•Plane 4 Width
•Plane 4 Slope
•Sloped Roof Area
•Roof Complexity : Options: Not selected, A simple gable with few penetrations, A gable with one or two valleys, A hip roof, Complex, with dormers, multiple pitches or many inside corners
•Waste Allowance : Options: Not selected, The published band for the complexity entered, A stated percentage, None
•Stated Waste Percentage
•Covering : Options: Not selected, Strip shingles, Standard laminate, Architectural or dimensional, A stated bundles per square
•Bundles Per Square
•Product Coverage
•Product Coverage Basis : Options: Not selected, Per square, Per bundle
•Underlayment Roll Contents
•Underlayment Roll Coverage
•Ridge Length
•Hip Length
•Ridge Cap Coverage
•Eave Length
•Rake Length
•Starter Coverage
•Drip Edge Piece Length
•Weight Per Square
•Existing Layers
•Price Per Square
•Price Per Bundle
•Price Per Roll

Outputs

•The areas: the measurement basis, the overhang treatment, the plan area, the slope factor, the sloped area and each plane separately
•The squares: the roof squares before the allowance and the order squares after it, with the allowance on its own line and the published band beside it
•The field shingles: the bundle count, the coverage it came from, and whether that was your product or the nominal square
•The underlayment: rolls on what the roll contains and on what it covers, each rounded up separately
•The linear items: ridge and hip cap, starter and drip edge, each from its own measurement rather than from the square count
•The weight: the order weight from the order squares, and the tear off weight from the roof squares where layers were named
•The system warranty note: what manufacturer system warranties require of the underlayment, cap and starter
•The boundaries: what this page reports and what it deliberately does not

Roofing Formula

Every quantity here comes from one roof area, one waste allowance and one product figure, with each product carried separately to its own count.

The Slope Factor and the Areas

slope_f = (1 + (rise_in / 12) ^ 2) ^ 0.5
plan_sf = plan_len_ft * plan_wid_ft
fp_len_adj_ft = fp_len_ft + 2 * eave_oh_ft
fp_wid_adj_ft = fp_wid_ft + 2 * rake_oh_ft
plan_sf = fp_len_adj_ft * fp_wid_adj_ft
roof_sf = plan_sf * slope_f

The slope factor is the hypotenuse of a twelve inch run and the rise, divided by that run. It is written as a square root because that is arithmetic; the equivalent form using a cosine is a function and stays in prose.

The overhang equations run before the slope factor, so an omitted overhang and an omitted slope factor multiply rather than add.

One Plane, Two Equations

plane_sf = pl_len_ft * pl_wid_ft * slope_f
plane_sf = pl_len_ft * pl_wid_ft
roof_sf = plane_1_sf + plane_2_sf + plane_3_sf

There are two plane equations and the page uses exactly one of them. A dimension from a plan or measured across the ground is a horizontal projection and takes the slope factor. A dimension taken with a tape run up the slope is already on the roof surface and takes none: a plane measured at 40 by 18 feet on a 6:12 roof is 720 square feet, and applying the factor to it returns 805, which is 11.8 percent of a roof that does not exist.

Each plane carries its own slope factor. A main plane at 6:12 with a garage plane at 4:12 is two calculations, and summing the footprints before applying one factor is wrong for both. The summation line above extends to as many planes as you enter.

The Squares and the Allowance

order_sf = roof_sf * (1 + waste_pct / 100)
roof_sq = roof_sf / 100
order_sq = order_sf / 100

The roof squares and the order squares are separate figures and are never one identifier. Roof squares are the roof area divided by a hundred and carry no waste. Order squares carry the allowance. Bundles and the order weight come from the order squares; the tear off weight comes from the roof squares, because the waste allowance is for cutting new shingles and a roof already on the house has none.

The Bundles

bundles = order_sq * bun_per_sq
bundles = order_sf / cov_sq_sf * bun_per_sq
bundles = order_sf / cov_bun_sf

The three bundle equations are alternatives and exactly one runs. Where a coverage was entered the order squares figure is still reported, because it is the unit the supplier prices in, but the bundle count comes from the coverage.

A wrapper prints coverage two ways and the two differ by the bundles per square. Owens Corning gives 98.4 square feet per square for Oakridge with three bundles per square, which is 32.8 square feet per bundle. On 2,500 square feet the per square figure gives 77 bundles and the same number read as a per bundle figure gives 229, so the basis is asked rather than assumed.

Underlayment, Cap, Starter and Drip Edge

rolls_con = roof_sf / roll_con_sf
rolls_cov = roof_sf / roll_cov_sf
cap_bun = (ridge_ft + hip_ft) / cap_ft_bun
start_un = eave_ft / start_ft_un
drip_ft = eave_ft + rake_ft
drip_pc = drip_ft / drip_pc_ft

The underlayment equations divide the roof area rather than the order area. The shingle waste allowance is not applied to underlayment, because the roll coverage figure already accounts for the laps, and the result says so rather than leaving a reader to assume the allowance ran everywhere.

The two underlayment equations are both reported and each rounds up separately. Contents is what the roll holds and coverage is what it puts on the roof after the laps.

The drip edge pieces equation runs only where a piece length was entered. Without one the drip edge is reported as a length.

The Weight

order_lb = order_sq * lb_per_sq
tear_lb = roof_sq * lb_per_sq * layers

On a 2,236 square foot roof at ten percent, two layers at 300 pounds a square is 13,416 pounds from the roof squares and 14,758 from the order squares, and the second figure is a demolition estimate carrying an allowance for material nobody is cutting.

Unit Conversions

sq m = sq ft * 0.09290304
m = ft * 0.3048
kg = lb * 0.453592

A roofing square is 100 square feet, which is 9.290304 square metres.

All bundle, roll and unit counts round up once, at the end. Rounding up is a function rather than an arithmetic expression and is stated here rather than in the boxes above.

Everything else on this page is a condition rather than an equation and belongs in prose.

Footprint Versus Roof Plan Versus Roof Surface

Three areas get called the roof and they are three different numbers.

The building footprint is what the house covers on the ground. It is what a plan shows and what a tape run around the outside of the walls gives.

The roof plan is the footprint plus the overhangs. A one foot overhang all round a 40 by 30 foot house gives a roof plan of 42 by 32, which is 1,344 square feet against 1,200.

The roof surface is the roof plan multiplied by the slope factor, because a pitched plane is longer than its own shadow. At 6:12 that factor is 1.1180.

Those two corrections compound, because the overhang is added before the factor. On that house the footprint gives 1,342 square feet of roof and the true plan gives 1,503, which at ten percent waste is 14.76 order squares against 16.53.

There is a fourth number that is none of the three, and it is where the most expensive mistake lives. A tape run up the slope measures the roof surface directly. That figure already carries the pitch, and multiplying it by the slope factor counts the pitch twice: a plane measured at 40 by 18 feet on a 6:12 roof is 720 square feet, and the factor returns 805.

Nothing about the number says which it is. Forty feet is forty feet whether the tape was on the ground or on the shingles, which is why this page asks the basis before it applies anything.

Roofing Squares and Order Squares

A roofing square is 100 square feet of roof surface. It is the unit the trade measures in, prices in and orders in.

Two different square figures come out of one roof and they are used for different things.

Roof squares are the roof surface divided by 100. They describe the building. A 2,236 square foot roof is 22.36 roof squares.

Order squares are the same figure with the waste allowance in it. At ten percent that roof is 24.60 order squares.

Both belong on a result, and they are separate figures rather than one number reported twice.

Bundles come from the order squares, because bundles are what you buy.

The order weight comes from the order squares, for the same reason: the delivery is the material ordered.

The tear off weight comes from the roof squares. A roof already on the house has no waste allowance in it, because the allowance is for cutting new shingles. On that roof, two existing layers at 300 pounds a square is 13,416 pounds from the roof squares and 14,758 from the order squares, and the second figure is a demolition estimate carrying an allowance for material nobody is cutting.

That is why the two are named separately here. With one squares variable the wrong base is one line of code away.

Shingle Bundles and Product Coverage

Three bundles to a square is the common answer and it is a convention rather than a measurement.

Published figures give three bundles per square for strip shingles and standard laminate, three to four for architectural and dimensional products, and one source gives three, four or five. Published guidance is explicit that the count is not assumed across product lines.

The square itself is a convention too. It is 100 square feet by definition, and one manufacturer states on its own data sheet that one sales square covers 100 square feet. Owens Corning publishes a Coverage Per Square of 98.4 square feet for Oakridge laminate shingles.

On a 25 square roof that 1.6 percent is 40 square feet, which is 0.41 of a square, and it arrives at the last course rather than the first.

So where a reader has the wrapper, the bundle count comes from what the wrapper says the product covers, and where they do not, the nominal square is used and the result says it was assumed.

One trap sits inside that. A wrapper prints coverage two ways, per square and per bundle, and the two differ by the bundles per square. The 98.4 figure at three bundles per square is 32.8 square feet per bundle. Both numbers are on the same package.

On 2,500 square feet the per square figure gives 77 bundles. The same number entered as a per square figure when it was actually per bundle gives 229. That is a factor of three, and the only thing that separates them is which line of the wrapper the reader was looking at.

That is why the page asks which basis a coverage figure is on rather than assuming the field it was typed into.

Roofing Waste Allowance

Every roofing calculator offers a waste factor as a setting, usually with ten percent preselected.

Published guidance states that across six shingle data sheets from three manufacturers, not one prints a waste percentage. One manufacturer's estimating guide works a sample job at a ten percent allowance, and that is the only waste figure in any of those documents.

So the number every calculator treats as the standard input has no specification behind it. It is a judgment about this roof: how many valleys, how many hips, how many dormers, how cut up the planes are, and how well the crew nests cuts.

Trade guidance gives bands, and this page reports them as guidance. Ten percent for a simple gable with few penetrations. Twelve for a gable with one or two valleys. Fifteen for a hip roof. Fifteen to twenty for complex roofs with dormers, multiple pitches or many inside corners.

That last band is two figures rather than one, and nothing on this page asks which end a particular roof is, so no midpoint is taken: the squares, the bundles and the weight are reported at both ends, and a stated percentage is the input for choosing one.

Two more things follow from waste being a judgment rather than a lookup.

A figure above the bands is flagged for review and computed, not blocked. Complex roofs exceed twenty percent in practice, and a deliberate entry is not an error. Only a negative or absurd value stops the calculation.

And the allowance goes on its own line rather than into the square count. Published guidance recommends putting it on a quote as its own number: a customer who can see the allowance can argue about it, and a customer who cannot will re-measure the roof, get a different figure, and have no way to find the difference.

The same principle bounds the complexity input. Choosing a hip roof sets a waste band and creates no ridge length, hip length or valley, because a length this page has not been given is a length it does not know.

Underlayment Contents Versus Coverage

A roll of underlayment holds more than it puts on the roof, and both figures are printed on the same data sheet.

One published sheet gives a synthetic underlayment roll containing 1,001 square feet and covering 905.67. The difference is the laps, and it is 9.5 percent of the roll.

On a small roof the two agree. 1,252 square feet is 1.25 rolls on contents and 1.38 on coverage, and both order as two.

On a larger one they do not. 2,800 square feet is 2.80 rolls on contents and 3.09 on coverage, which orders as three rolls against four.

So this page reports both, each rounded up separately, wherever both figures are entered. A product quoted only on contents is the commoner case, and where that is all a reader has the page reports on it and names the coverage as not entered.

One thing about the underlayment count is worth stating plainly because it looks like an omission. The rolls are computed from the roof area, not from the order area with the shingle waste in it. A coverage figure already accounts for the laps, which is the underlayment equivalent of waste, so applying the shingle allowance on top would take the same allowance twice.

The shingle waste allowance and the underlayment lap loss are two different corrections on two different products, and neither is applied to the other.

Ridge Cap, Starter and Drip Edge

A bundle count is not a material order, and three of the things missing from it are measured in a different unit entirely.

Published guidance lists what a roofing job consumes beyond the field shingles: underlayment, ice and water shield at the eaves, drip edge, starter strip and ridge cap, and states that forgetting any one of them means a separate delivery charge or a contractor sitting idle.

Ridge cap goes by the linear foot of ridge and hip, because the same product covers both. One manufacturer's hip and ridge product covers 29.53 linear feet per bundle, so a 40 foot ridge takes 1.35 bundles and orders as two. Published standard bundles run 30 to 33 linear feet.

Starter goes by the eave length and its own product coverage, which is again a linear figure.

Drip edge goes along the eaves and the rakes together, which is a third measurement. It is reported as a length unless a piece length is entered, in which case it is also reported in pieces.

None of the three can be derived from the square count. A 25 square roof might have 40 feet of ridge or 140, depending on whether it is a simple gable or a hip roof with dormers, and the square count is identical either way.

That is why this page asks for the lengths rather than estimating them, and where a length was entered without its product coverage it reports the linear feet and no count. Published cap coverage runs 29.53 to 33 linear feet per bundle across products, which is too wide a spread to guess inside.

Roof Weight and Tear-Off Weight

Two weights come off one roof and they use different bases.

Published guidance gives asphalt shingles at roughly 230 to 430 pounds per square depending on type, with architectural products nearer the upper end and strip shingles nearer the lower.

That is a wide range and this page reports it as a range. No midpoint is taken, because a midpoint is a figure no source published. Where a reader has a weight from the product data sheet, that figure is used instead and named as theirs.

The order weight is what arrives. It comes from the order squares, because the delivery is the material ordered including the waste allowance. On a 25 square order at 230 to 430 pounds per square that is 5,750 to 10,750 pounds.

The tear off weight is what leaves. It comes from the roof squares, before the waste allowance, because the roof already on the house was not cut from your order and carries none of your allowance. It multiplies by the number of existing layers.

On a 2,236 square foot roof with two existing layers at 300 pounds a square, the tear off is 13,416 pounds. Computed from the order squares it would be 14,758, which is a demolition estimate carrying an allowance for material nobody is cutting.

Both matter before the truck arrives. A ten square roof is a pickup and a twenty five square roof with two layers to remove is a skip and a structural question about where the bundles are stacked while the work is done.

What Is a Roofing Calculation

A roofing calculation turns a house into a delivery, and it passes through three conversions that each lose people.

The first is the surface. A plan or a ground measurement gives a horizontal area, and a pitched roof is larger than its own shadow by the slope factor. That factor is the square root of one plus the square of the rise over twelve, so a 6:12 roof is 11.8 percent larger than its plan and a 12:12 roof is 41.4 percent larger.

The second is the unit. Roofing is counted in squares of 100 square feet, priced by the square, and delivered in bundles. Three bundles to a square is the common answer and it is not the only one, and the coverage a square actually delivers is a product figure rather than a definition.

The third is everything that is not a field shingle. Underlayment goes by the roll and loses area to laps. Ridge cap goes by the linear foot of ridge and hip. Starter goes by the eave, drip edge by the eave and the rake. None of them can be derived from the square count, because a 25 square roof might have 40 feet of ridge or 140.

What sits over all three is a waste allowance, and it is the one number here with no specification behind it at all.

Key Facts

  • A roofing square is 100 square feet of roof surface, and one manufacturer states on its own data sheet that one sales square covers 100 square feet.
  • Owens Corning publishes Coverage Per Square of 98.4 square feet for Oakridge laminate shingles, with three bundles per square.
  • On a 25 square roof a 98.4 square foot square is 40 square feet short of the nominal, which is 0.41 of a square and turns up at the last course.
  • The slope factor is the square root of one plus the square of the rise over twelve. At 4:12 it is 1.0541, at 6:12 it is 1.1180, at 8:12 it is 1.2019 and at 12:12 it is 1.4142.
  • A 2,000 square foot footprint under a 6:12 gable is 2,236 square feet of roof, which is 22.36 roof squares and 24.60 order squares at ten percent waste.
  • Ordered off that footprint instead it is 22.00 order squares and 66 bundles against 74, a shortfall of 8 bundles on one house.
  • The same 1,200 square foot plan is 12.65 squares at 4:12 and 16.97 at 12:12, a factor of 1.34 on one house.
  • Dimensions taken with a tape on the roof surface already carry the pitch. Applying the slope factor to them counts it twice, which is 11.8 percent over on a 6:12 roof.
  • A building footprint is not a roof plan. A one foot overhang all round a 40 by 30 house adds 12 percent to the plan area before any slope factor is applied.
  • Published bundle counts run three per square for strip and standard laminate, three to four for architectural and dimensional, and one source gives three, four or five.
  • Across six shingle data sheets from three manufacturers, not one prints a waste percentage.
  • One manufacturer's estimating guide works a sample job at a ten percent waste allowance, which is the only waste figure in any of those documents.
  • Trade guidance gives waste bands of ten percent for a simple gable, twelve for a gable with one or two valleys, fifteen for a hip roof, and fifteen to twenty for complex roofs with dormers, multiple pitches or many inside corners.
  • Published guidance recommends putting waste on a quote as its own number rather than burying it in a square count a customer can check.
  • One published data sheet gives a synthetic underlayment roll containing 1,001 square feet and covering 905.67, because the laps take 9.5 percent.
  • On 2,800 square feet that roll is 2.80 on contents and 3.09 on coverage, which rounds to three rolls against four.
  • One manufacturer's hip and ridge product covers 29.53 linear feet per bundle, so a 40 foot ridge takes 1.35 bundles and orders as two. Another source gives standard bundles at 30 to 33 linear feet.
  • Asphalt shingles weigh roughly 230 to 430 pounds per square depending on type, and a tear off of multiple layers multiplies accordingly.
  • Manufacturer system warranties often require the underlayment, ridge cap and starter strip to come from the same product line, and mixing brands can void coverage.
  • A shortfall means a second supplier run and makeup bundles that may come from a different colour lot.

Applications

  • A homeowner with a plan drawing finds out that the roof is larger than the drawing by the slope factor, and by how many bundles.
  • Someone who climbed up with a tape finds the input that stops the page applying the pitch to a measurement that already carries it.
  • A reader whose house has a main roof and a lower garage roof gets two slope factors rather than one average.
  • Someone holding a shingle wrapper enters the coverage printed on it and sees the bundle count move off the nominal square.
  • A reader ordering underlayment sees the roll count on what the roll contains and on what it covers, and finds they differ by one on a large roof.
  • Someone pricing a job puts the waste allowance on the quote as its own line, because a square count a customer can re-measure will not match otherwise.
  • A reader who measured the ridge finds the cap bundles from the product's linear coverage rather than from a ratio to the area.
  • Someone planning a delivery sees the weight of the order before the truck arrives.

Example Calculations

Example 1. A tape on the roof, and the pitch counted twice

Given: a plane measured with a tape run up the slope, 40 feet by 18 feet, on a 6:12 roof.

That measurement is already on the roof surface. The plane is 720 square feet, which is 7.20 roof squares.

Applying the slope factor of 1.1180 to it gives 805 square feet, or 8.05.

Result: 720, not 805. The difference is 11.8 percent, about three bundles, and nothing on the screen looks wrong either way.

Example 2. Two planes, two factors

Given: a main roof plane of 40 by 16 feet on a 6:12, and a garage plane of 24 by 14 on a 4:12, both horizontal projections.

The main plane is 40 times 16 times 1.1180, which is 716 square feet. The garage plane is 24 times 14 times 1.0541, which is 354.

Together they are 1,070 square feet. Applying the 6:12 factor to the summed footprints instead gives 1,091.

Result: 21 square feet out on two planes. The error grows with every plane at the lower slope, so a house with a large low pitched porch or lean-to loses considerably more.

Example 3. The coverage figure read off the wrong line

Given: 2,500 square feet to cover, Oakridge shingles, whose wrapper gives 98.4 square feet per square and three bundles per square.

Read as a per square figure: 2,500 divided by 98.4, times three, is 77 bundles.

The same wrapper implies 32.8 square feet per bundle. Entered as a per square figure, 2,500 divided by 32.8 times three is 229 bundles.

Result: 77 or 229, a factor of three, from two numbers printed on the same package. This is why the page asks which basis a coverage figure is on rather than trusting the field it was typed into.

Example 4. Starter, cap and drip edge on one roof

Given: 120 feet of eave, 60 feet of rake, a 40 foot ridge, starter covering 105 linear feet per unit, cap covering 29.53 linear feet per bundle.

Starter: 120 divided by 105 is 1.14 units, which orders as two.

Ridge cap: 40 divided by 29.53 is 1.35 bundles, which orders as two.

Drip edge: 120 plus 60 is 180 linear feet, reported as a length because no piece length was entered.

Result: three separate orders from three separate measurements, and the square count for this roof appears in none of them.

Standards & References

This page reports quantities and issues no pass or fail. There is no minimum quantity of roofing to test against, and whether a covering is permitted at a given slope is a separate question answered by the Roof Pitch Calculator.

The 98.4 square feet per square figure is published by Owens Corning for Oakridge shingles and is a product figure rather than a general one; where no product coverage is entered this page uses the nominal 100 square feet and says that it assumed it. Bundle counts of three, four and five per square come from published trade sources and from product data sheets, and the wrapper on the product being bought overrides any table here.

The waste bands are trade guidance and not a specification: across six shingle data sheets from three manufacturers not one prints a waste percentage, and the only waste figure in those documents is a sample calculation at ten percent in one manufacturer's estimating guide, so the allowance on this page is a judgment the estimator owns.

The underlayment contents and coverage figures come from a published data sheet for one synthetic product and vary by product; both are reported wherever both are entered. Ridge cap coverage of 29.53 linear feet per bundle is one manufacturer's product figure and published standard bundles run 30 to 33 linear feet. The shingle weight range of 230 to 430 pounds per square is trade guidance.

Any cost is arithmetic on a price the reader enters. This page does not size rafters, assess a deck, design flashing or penetrations, determine ice barrier extent, or select a product.

Units

Areas are entered and reported in square feet and square metres at 0.09290304. A 2,236 square foot roof is 207.7 square metres.

A roofing square is 100 square feet, which is 9.290304 square metres. The square is an Imperial trade unit, and metric markets sell roofing by the square metre, which is a different product list rather than a conversion of a square.

Lengths are in feet with metres alongside at 0.3048. A 40 foot ridge is 12.19 metres and a 29.53 linear foot bundle covers 9.00 metres.

Weight is in pounds with kilograms alongside at 0.453592. A 25 square order at 230 to 430 pounds per square is 5,750 to 10,750 pounds, or 2,608 to 4,876 kilograms.

The slope factor, the bundles per square, the waste percentage and the square count are dimensionless and identical in both systems.

Slope is entered as a rise in twelve, which is a ratio rather than a measurement and does not convert. A 6:12 is a 6:12 in both systems, and it is 26.57 degrees and 50 percent if a drawing gives it either of those ways.

The internal unit selector governs the fields, labels, math, on-screen result and any exported result, and it takes priority over any site-wide unit switch. Switching converts the values you entered rather than reinterpreting them.

Limitations

  • This page reports quantities and issues no pass or fail. There is no minimum quantity of roofing to test against.
  • It does not rule on whether a covering is permitted at a slope. That is the Roof Pitch Calculator, and duplicating the question here would give two pages in one category that disagree the first time either table moves.
  • It does not derive geometry from complexity. Choosing a hip roof sets the published waste band and creates no ridge length, hip length or valley, because a length this page has not been given is a length it does not know.
  • The single slope mode is a simplified estimate. Roofs with hips, valleys, dormers, different pitches or an attached garage should be entered plane by plane.
  • It does not compute ice and water shield extent, which needs climate data and an eave detail.
  • It does not compute nails or fasteners by published spacing.
  • It does not size rafters, assess a deck, or design flashing, valleys, vents or chimney details, all of which are part of why a waste allowance exists.
  • It does not check a system warranty. It names the requirement and reports the accessories as separate lines.
  • The product coverage, bundle count, underlayment and ridge cap figures here are from named products and published trade sources, and the wrapper on what you are buying overrides all of them.
  • The waste bands are trade guidance rather than a specification, because no data sheet consulted publishes one. Where a band has two ends and nothing on the page asks which end your roof is, both ends are reported and no midpoint is taken.
  • It takes rectangular planes in the plane by plane mode. A triangular or trapezoidal plane is broken into rectangles or entered as a known sloped area.
  • It does not apply the shingle waste allowance to underlayment, because a roll coverage figure already accounts for the laps.
  • Any cost is arithmetic on a price you entered.

Common Mistakes to Avoid

  • Ordering off the footprint. A pitched roof is always larger than its plan, and on a 2,000 square foot footprint at 6:12 the gap is 8 bundles at the same ten percent allowance on both sides.
  • Applying the slope factor to a measurement taken on the roof. A tape run up the slope already carries the pitch, and multiplying it again is 11.8 percent over at 6:12.
  • Taking a building footprint as the roof plan. A one foot overhang all round a 40 by 30 house is 12 percent more plan area before the slope factor.
  • Using one slope factor on a roof with two pitches. Each plane carries its own, and a low pitched garage or porch under a steeper main roof loses the most.
  • Treating the waste percentage as a specification. No shingle data sheet consulted publishes one, and the only figure in those documents is a sample calculation in an estimating guide.
  • Burying the waste in the square count. A customer who re-measures the roof will get a different number and no way to see where the difference came from.
  • Assuming a square covers 100 square feet of product. It is 100 square feet by definition, and Owens Corning publishes 98.4 for Oakridge.
  • Assuming three bundles per square across product lines. Published figures run three, four and five, and the wrapper is the source.
  • Ordering underlayment on what the roll contains. One published roll contains 1,001 square feet and covers 905.67, and on 2,800 square feet that is three rolls against four.
  • Deriving ridge cap from the square count. A 25 square roof might have 40 feet of ridge or 140, and the square count is identical either way.
  • Forgetting the linear items. Starter, drip edge and cap are separate orders, and missing one is a separate delivery charge.
  • Mixing product lines on the accessories. System warranties often require the underlayment, cap and starter from the same line as the shingles.
  • Planning to top up a shortfall later. Makeup bundles may come from a different colour lot, which is visible on a roof.

Frequently Asked Questions

How many squares is my roof?
Multiply the roof plan area by the slope factor for your pitch, then divide by 100. A 2,000 square foot plan at 6:12 is 2,236 square feet of roof, which is 22.36 squares before waste.
How many bundles of shingles are in a square?
Three for strip shingles and standard laminate. Architectural and dimensional run three to four, and one source gives three, four or five. Check the wrapper, because it varies by product line.
What is the slope factor for my pitch?
The square root of one plus the square of the rise over twelve. At 4:12 it is 1.0541, at 6:12 it is 1.1180, at 8:12 it is 1.2019 and at 12:12 it is 1.4142.
How much waste should I add?
That is a judgment about your roof rather than a figure to look up: no shingle data sheet consulted publishes a waste percentage. Trade guidance gives ten percent for a simple gable, twelve with one or two valleys, fifteen for a hip roof and fifteen to twenty for complex roofs.
Does a roofing square always cover 100 square feet?
It is 100 square feet by definition and that is not always what the product delivers. Owens Corning publishes 98.4 square feet per square for Oakridge, which is 40 square feet short of nominal on a 25 square roof.
How many rolls of underlayment do I need?
It depends whether you count what the roll contains or what it covers after the laps. One published roll contains 1,001 square feet and covers 905.67, and on 2,800 square feet that is three rolls on contents and four on coverage.
How much ridge cap do I need?
Measure every peak ridge and hip in linear feet, then divide by the product's coverage per bundle. One manufacturer's product covers 29.53 linear feet, so a 40 foot ridge is 1.35 bundles and orders as two. Standard bundles run 30 to 33 linear feet.
I measured the roof myself with a tape. Do I still apply the slope factor?
No. A tape run up the slope already measures the roof surface, so the pitch is in the figure. Applying the factor to it counts the pitch twice, which is 11.8 percent over on a 6:12 roof.
What is the difference between roof squares and order squares?
Roof squares are the roof area divided by 100 and carry no waste. Order squares include the waste allowance. Bundles and the order weight come from the order squares, and the tear off weight comes from the roof squares.
Does the waste allowance apply to underlayment?
No. Underlayment is calculated from the roof area using the product's stated coverage, which already accounts for the laps. Applying the shingle allowance on top would take the same kind of allowance twice.
My wrapper gives coverage per bundle rather than per square. Which do I enter?
Either, as long as you say which. They differ by the bundles per square, so a per bundle figure entered as a per square figure gives three times the bundles on a three bundle product.
Does this tell me whether shingles are allowed on my slope?
No. That is the Roof Pitch Calculator. This page calculates quantities only.

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