Roof Drain Sizing Calculator — Leader Size, Horizontal Storm Drain and the Secondary System

Roof Drain Leader Sizing

A leader, also called a vertical conductor or a downspout where it runs outside, is the vertical pipe that takes water from one roof drain down through the building. It is sized on the area that one drain takes, and that is the first place a sizing goes wrong.

Where a path has one drain, the area it takes is the whole design area. Where a path has several, each drain takes a share, and the honest question is which share. Dividing the area by the number of drains produces an average, and roof geometry rarely divides equally. Three drains on 3,000 square feet average 1,000 square feet each; if the real shares are 1,400, 900 and 700, the leader sized on the average is undersized for the drain that actually takes the most, by 40 percent of area.

In the 2009 through 2015 editions the leader comes out of Table 1106.2(1), which is indexed by maximum projected roof area at a stated rainfall rate. The published column for 1 inch per hour is the one to reason from, because every other column is that column divided by the rate. At 1 inch per hour a 2 inch leader carries 2,880 square feet, a 3 inch carries 8,800, a 4 inch carries 18,400, a 5 inch carries 34,600, a 6 inch carries 54,000 and an 8 inch carries 116,000. At 4 inches per hour, divide each by four.

In the 2018 through 2024 editions the leader comes out of Table 1106.3, which is indexed by flow: 30 gallons per minute at 2 inches, 54 at 2.5, 92 at 3, 192 at 4, 360 at 5, 563 at 6 and 1,208 at 8. The 2.5 inch row exists only in the flow table, which is a small illustration of why the edition has to be named before a size is quoted.

One thing the tables never do is interpolate downward. The smallest tabulated size that carries the requirement is the answer, and where nothing in the table carries it, the correct output is that nothing fits rather than the largest size. Splitting the area between more drains, increasing the slope on the horizontal, or an engineered alternative accepted by the authority are the routes out.

The 100-Year Hourly Rainfall Rate

The rate that indexes the tables is the 100-year, 1-hour rainfall rate for the location. Code requires vertical conductors and leaders, building storm drains, building storm sewers and their horizontal branches to be based on that figure as indicated in the code rainfall figures, or on other rainfall rates determined from approved local weather data. The figures trace back to National Weather Service and NOAA data.

Read the name of that quantity carefully, because each word is doing work. It is hourly, so it is an intensity rather than a volume. It is a 1-hour duration, which is the duration a roof drainage system is sized against because roofs have almost no storage. And it is a 100-year event, meaning a storm severe enough that its expected return period is a century, not a bad afternoon.

Across the United States the design figure runs from roughly 1 to 8 inches per hour. Published guidance notes Dallas requiring 6 inches per hour as one example of a high figure, which is an example rather than a default. There is no national number and there is no safe default, which is why this calculator refuses to supply one.

The failure that matters is entering an annual total into the hourly field. Annual rainfall across much of the country runs 20 to 60 inches, so a figure of 42 typed into a field expecting 4 still computes cleanly and returns a pipe five to ten times too large. On a 2,500 square foot roof, 4 inches per hour calls for a 4 inch leader and 42 inches per hour calls for an 8 inch one. Note the direction: the annual mistake produces oversizing, so the building is not in danger, but the cost is real and the drawing is wrong.

A plausibility band catches that mistake when the figure lands outside the hourly range and misses it entirely when it lands inside. A monthly total of 6 inches sits squarely in the plausible hourly band. That is why this page asks for the basis of the rate as a separate question rather than inferring it from the number, and why an unnamed source is flagged even when the number is right: the size is only as traceable as the rate behind it.

The Vertical Wall Addition

Where a vertical wall sheds rain onto a roof, code adds one half of the wall face area to the projected roof area for the purpose of sizing vertical conductors, leaders and horizontal storm drainage piping.

The walls that count are the ones that divert water onto this roof: a taller adjacent building face, a stair or lift overrun, a screen wall around rooftop plant, the side of an upper storey where a lower roof abuts it. A parapet on the roof itself is not one of them. A parapet retains water already on the roof rather than adding more, and confusing the two adds area that is not there.

The rule takes half rather than all because a vertical surface intercepts rain arriving at an angle rather than falling straight onto it. It is a simplification of a wind-driven rain problem, and like most code simplifications it is chosen to be conservative and easy to apply rather than exact.

The effect is larger than it looks. A 4,000 square foot roof at 4 inches per hour sheds 166.7 gallons per minute and calls for a 4 inch leader. Add a 2,000 square foot wall, which contributes 1,000 square feet, and the design area is 5,000, the flow is 208.3 gallons per minute, and the leader moves to 5 inches. One question on a site visit changed a pipe size.

This is also why the wall condition on this page has three answers rather than two. A blank field cannot tell a roof with no adjacent wall apart from a roof where nobody has looked yet, and those two states deserve different results. Not checked is reported as not evaluated, never as none.

Horizontal Storm Drain Sizing and Slope

The horizontal storm drain is a different problem from the leader, sized from a different table, and the size it returns is usually larger: one or two nominal pipe sizes larger, depending on where the area falls between table rows.

It is larger for a structural reason: a vertical pipe runs full under gravity at the speed the fall gives it, and a sloped pipe runs part full at the speed the slope gives it. The leader is sized on the area one drain takes. The horizontal segment is sized on everything that segment carries, which is the same area at the top of a run and an accumulated area further down, so it also grows as you follow the pipe. At 4 inches per hour under the area tables a 4,000 square foot roof gives a 4 inch leader and a 6 inch horizontal drain at one eighth in 12, two nominal steps apart, and a smaller area can land one step apart instead. How wide the gap is depends on where each area sits between table rows rather than being a fixed relationship.

Slope is the lever. In the 2009 through 2015 editions Table 1106.3 tabulates maximum projected roof area at three slopes: one eighth unit vertical in 12 units horizontal, which is a 1 percent grade, one quarter in 12, which is 2 percent, and one half in 12, which is 4 percent. At 1 inch per hour a 6 inch drain carries 21,400 square feet at one eighth, 30,200 at one quarter and 42,800 at one half.

Doubling the slope multiplies capacity by about the square root of two, which is what Manning's equation predicts for the same pipe at the same depth of flow. It is worth knowing that relationship and worth being clear that it never produces a size on this page: the sizes come from table lookups, and the square root relationship is an explanation of why the table reads the way it does.

On a marginal case the slope is a whole pipe size. A 4,000 square foot design area at 4 inches per hour needs a 6 inch horizontal drain at one eighth in 12 and a 5 inch drain at one quarter, because the 5 inch row carries 3,340 square feet at the shallow slope and 4,720 at the steeper one. Whether you can use the steeper slope is a question about ceiling space and invert levels rather than about hydraulics, which is why the calculator asks rather than assumes.

One caution about the published tables: two cells in the horizontal table disagree with the rest of the table's own internal arithmetic across the jurisdictions that reproduce it. This calculator uses the smaller of the two figures in each case, which returns the same size or a larger one and never a smaller one.

Area Tables Versus Flow Tables by IPC Edition

The method changed between editions, and the two methods do not always return the same size.

In IPC 2009, 2012 and 2015 the tables are indexed by projected roof area at a rainfall rate. Table 1106.2(1) sizes circular vertical conductors and leaders, Table 1106.3 sizes building storm drains and sewers at a stated slope, and Equation 11-1 determines the equivalent circular diameter of rectangular piping.

In IPC 2018, 2021 and 2024 the tables are indexed by flow in gallons per minute. Table 1106.2 gives the maximum flow rate in storm drain piping by slope, Table 1106.3 gives the maximum flow rate through vertical leaders, and Equation 11-1 is now the conversion from a rainfall rate on a roof surface to a flow. The same equation number is a different equation. The area based method survives as an alternative in some adoptions, at Section 1106.2.2.

The conversion between the two is the code equation itself: flow in gallons per minute is the design area in square feet multiplied by the rainfall rate in inches per hour and divided by 96. One square foot at one inch per hour is one twelfth of a cubic foot per hour, 0.6234 gallons per hour, or 0.010390 gallons per minute, and 96 is the reciprocal rounded. The published worked example of 2,500 square feet at 4 inches per hour returning 104.2 gallons per minute is reproduced exactly by that divisor.

Here is where the two methods diverge. Take that same case, 2,500 square feet at 4 inches per hour, 104.2 gallons per minute, and size the horizontal drain at one eighth in 12. The flow table gives 4 inches, because a 4 inch drain at that slope carries 115 gallons per minute. The area table gives 5 inches, because a 4 inch drain at that slope carries 7,520 square feet at 1 inch per hour, which is 1,880 at 4 inches per hour, well short of 2,500. Translated into flow, the older table is crediting that 4 inch pipe with 78 gallons per minute against the newer table's 115.

That is not a transcription error in either direction; it is two generations of the same table built on different assumptions about depth of flow. It is also the practical reason the edition has to be named before anyone quotes a size, and the reason a set of drawings should never mix table values from one edition with section numbers from another.

Secondary and Overflow Roof Drainage

Where the roof perimeter construction extends above the roof in such a manner that water will be entrapped if the primary drains allow buildup for any reason, code requires secondary emergency overflow roof drains or scuppers.

Read the trigger rather than the roof type. It is entrapment, not the word parapet. An equipment curb, a raised edge detail, or a wall abutting a roof that drains freely everywhere else can each entrap water, and one such detail puts the roof back inside the requirement. Conversely a roof genuinely draining off its edge on every side is outside it.

The sizing rule is the part that surprises people. The secondary system is sized on the rainfall rate the primary was sized for, using the same tables, with no credit for the primary at all. For scuppers the code is explicit that the flow through the primary system shall not be considered when locating and sizing secondary scuppers. So the secondary is a complete second drainage system for the whole design storm, not a partial backup carrying some remainder.

That is not conservatism for its own sake. It follows from what the secondary is for. The primary is assumed blocked, because that is the only scenario in which the secondary ever runs, and a blocked drain contributes nothing. Primary drains block with leaves, with debris, with ice, and they block silently.

Which is why the discharge requirement matters as much as the size. Code requires the secondary system to have its end point of discharge separate from the primary system, above grade, and in a location that would normally be observed by the building occupants or maintenance personnel. That is a requirement about human attention. If the secondary discharge joins the primary pipe, or falls somewhere nobody looks, the evidence of a blocked primary is lost and the building runs on its backup indefinitely. A scupper pouring water onto a walkway is doing exactly its job the first time anyone notices it, and the correct response is to go and clear the primary drains.

Scupper Sizing

A scupper is an opening through a parapet that lets water off a roof, and it is one of the two accepted routes for secondary drainage.

Code requires scupper openings of not less than 4 inches in height, with a width equal to or greater than the circumference of a roof drain sized for the same roof area. That width rule is worth reading twice: it ties the opening back to the drain the scupper is replacing rather than to a rule of thumb. Where the roof area calls for a 4 inch roof drain, the circumference is pi times 4, or 12.57 inches, and that is the minimum scupper width.

The 4 inch minimum height is a floor, not a design value. The controlling requirement is elsewhere: where scuppers serve as primary drainage, secondary drainage or both, the quantity, size, location and inlet elevation are chosen to prevent the depth of ponding water on the roof from exceeding the maximum depth the roof structure can carry. Inlet elevation is a design decision made against a structural number.

Parapet wall scupper and overflow scupper location also have to comply with the building code, not only the plumbing code, so the dimension is necessary but not sufficient.

The choice between scuppers and overflow roof drains is mostly about the building. Scuppers suit a parapet with an accessible external wall face and give a highly visible discharge, which is the point. Overflow roof drains suit a roof where a second internal piping route is available, and they need a raised inlet elevation to stop them running in ordinary rain.

Ponding, Head and Full Bore Flow

The capacities in the code tables are full bore flow rates, and a pipe does not run full bore because it is raining.

Manufacturer engineering guidance is explicit about the mechanism: to achieve full flow a certain head of water must cover the roof drain, and it is this head in relation to the ability of the drain to discharge a given flow that creates ponding. So a drain does not deliver its tabulated capacity on a wet roof with water running toward it. It delivers it once water has built up over the inlet, and that build-up is the ponding. The same guidance notes the head and the resulting flow vary with rainfall intensity, ambient temperature, wind and roof slope, so the depth is not a single number.

That statement is manufacturer engineering guidance rather than a code statement, and it deserves to be attributed that way. The 2018 and later editions do carry the related requirement as code: Section 1105.2 states that the published roof drain flow rate, based on the head of water above the roof drain, shall be used to size the storm drainage system.

The secondary inlet height is the other half of the picture. A secondary inlet set 2 inches above the roof surface will not run until 2 inches of water have accumulated, which is exactly what stops it draining in ordinary rain, and 2 inches of water is 10.4 pounds per square foot of dead load that was not on the structural drawing as rain.

Water weighs 62.4 pounds per cubic foot, so one inch over one square foot is 5.2 pounds. That figure is the whole of the ponding arithmetic. What it cannot tell you is whether the roof carries it. The allowable ponding depth comes from the structural engineer, and this calculator reports the load at the depth you enter and asserts nothing about the structure's capacity to take it. Where the secondary inlet sits above the allowable depth, the roof reaches its structural limit before the backup starts working, and that is a coordination failure rather than a plumbing one.

The Drain Body Against the Pipe Table

The code tables size pipe. They say nothing about the roof drain body, and the two are separate selections that have to agree.

A roof drain body with its strainer, sump and outlet has its own hydraulic characteristic, published by the manufacturer as a flow rate at a stated head of water above the drain. A correctly sized leader below an under-capacity drain body is still a restricted system, and the restriction is at the inlet where nothing in the pipe table looks.

In the 2018 and later editions this stopped being a refinement and became a step. Section 1106.2 requires the calculated flow rate to be checked against the roof drain manufacturer's published flow rate for the specific roof drain model and size.

One detail decides whether a published flow means anything: the head it is quoted at. A drain published at 120 gallons per minute at 3 inches of head and a drain published at 120 gallons per minute at 1 inch of head are not the same product, because the second reaches that flow with a third of the ponding. A flow rate without its head is not a capacity, and a specification that quotes one without the other has not been checked.

What Is a Roof Drain Leader

A leader is the vertical pipe that carries water from a roof drain down through or alongside the building to the horizontal storm drain. Inside the building it is more often called a vertical conductor; outside, running down a wall from a gutter, it is a downspout. The code tables use conductor and leader interchangeably and size all of them the same way.

A roof drainage system is four parts in series. The roof drain body sits in the roof deck with a strainer over it. The leader takes the flow down. The horizontal building storm drain collects several leaders and runs to the building line. The building storm sewer takes it from there to the point of disposal.

Each part is sized on a different quantity, which is the single most useful thing to hold onto. The drain body is selected on flow at a head from manufacturer data. The leader is sized on the area one drain takes. The horizontal drain is sized segment by segment on accumulated area at a slope. The sewer beyond the building is a site drainage problem with its own rules.

This page covers the middle two, which are the ones the plumbing code tables answer directly. Gutters are sized from a separate table on the flow from the roof surface. Siphonic and controlled flow roof drainage run full bore by design and are sized by entirely different methods, and none of these tables applies to them.