Roof Drain Sizing and Installation Guide

How to size a roof drain and its leader, where the overflow has to sit, what the different drain types are for, and how the assembly goes together. Figures here come from Table 1103.1 of the 2021 Uniform Plumbing Code, from the International Plumbing Code, and from Sioux Chief's published installation literature. Local code always takes precedence.

To buy the parts, see roof drains, nozzles and accessories.

What is a roof drain?

A roof drain is the fitting set into the low point of a flat or low-slope roof that carries rainwater through the deck and into the building's storm piping. It is the only planned path off the roof, which is why its sizing and its backup are treated as structural questions rather than plumbing details.

A complete assembly is more than the body. It is a sump receiver set flush with the deck, the drain body, a combination membrane clamping collar and gravel guard, a dome strainer, usually an underdeck clamp, and a No-Hub coupling down to the pipe. Commercial cast iron drains such as the Sioux Chief 868 series are built to ASME A112.6.4.

What is the drain on a roof called?

The names change with position. On the roof surface the assembly is a roof drain. The vertical pipe below it is the leader, also called a conductor. An opening through the parapet wall instead of through the deck is a scupper. The fitting that finishes the discharge at grade or through a wall face is a downspout nozzle. The perforated cap on top of the drain is the dome strainer. The ring that grips the roofing membrane against the drain flange is the clamping collar, which on most cast iron drains doubles as the gravel guard.

What are the different types of roof drains?

• Main or primary roof drain. The working drain at the roof low point. Fifteen-inch cast iron bodies are the commercial standard, with 3", 4", 6" and 8" No-Hub outlets.
• Combination drain and overflow. Primary and secondary in one body, the overflow inlet raised above the primary. Code requires the two paths to remain independent of each other.
• Overflow or secondary drain. A separate drain at a higher elevation, piped to discharge where somebody will see it running.
• Scupper drain. Takes water out through the parapet rather than through the deck. Common as the secondary path on buildings with a parapet.
• Promenade drain. Flush grate for roof decks, terraces and walking surfaces.
• Drain with intake weir. Used where the drain sits in a channel or trough rather than a sump.
• Downspout nozzle. The discharge fitting at the end of the leader. A bird screen keeps nesting out of an open outlet.
• Siphonic drain. An engineered system with an air baffle that primes the pipe to run full bore, moving the same flow through smaller pipe with little or no slope. It is not a drop-in substitute for a conventional drain and requires a designed layout for the whole roof.

Drains are also separated by connection type. No-Hub outlets couple to cast iron with a shielded band clamp and are the usual commercial choice. Inside caulk outlets seat into a hub below. Material matters for the roof side too: cast iron bodies take traffic and heat, while ABS and PVC bodies suit lighter residential and small-area work.

Sizing a roof drain and its leader

Drain count and leader size come from roof area and local rainfall rate. The pipe already in the building is not the answer. Two methods reach the same result.

Method 1: by roof area

1. Calculate the total roof area, length times width.
2. Determine the maximum hourly rainfall for the location, from the local weather bureau or the code authority.
3. Pick a leader size and read the area one leader of that size will drain at that rainfall rate.
4. Divide total roof area by that figure. Round any fraction up. That is the number of drains.

Method 2: by flow

Total GPM = 0.0104 × rainfall rate in inches per hour × roof area in square feet. Divide that total by the maximum flow one leader of the chosen size will carry, and round up.

Leader capacity table

Maximum flow per vertical leader at 1.75" head, and the maximum horizontal projected roof area in square feet that one leader will drain at each rainfall rate. From Table 1103.1 of the 2021 Uniform Plumbing Code.

Two worked examples

By area. A roof 500 ft by 200 ft is 100,000 square feet. At a 4"/hr rainfall rate, one 4" leader handles 4,600 square feet. 100,000 divided by 4,600 is 21.7, so 22 drains with 4" leaders.

By flow. The same 100,000 square foot roof at 4"/hr gives 0.0104 × 4 × 100,000 = 4,160 GPM. One 6" leader carries 563 GPM. 4,160 divided by 563 is 7.38, so 8 drains with 6" leaders.

The two examples show the trade directly: bigger leaders mean fewer roof penetrations, and each penetration is a place the membrane can fail.

Overflow and secondary drainage

Every primary drain needs a second path. Under IPC 1108.1 the secondary drain inlet sits not less than 2 inches and not more than 4 inches above the low point of the roof surface, and where a primary and a secondary drain are built as a single assembly, the inlet and the outlet of each drain must be independent. Sioux Chief recommends a one-to-one ratio of drains to overflows.

The raised inlet is the entire design intent. The overflow stays dry in ordinary rain and only discharges once the primary is restricted. Pipe it somewhere visible, through a wall face or a scupper rather than into the storm system, so that water running out of it is read as the alarm it is.

The reason the code is written this way is load. Water weighs about 5.2 pounds per square foot per inch of depth. Four inches of standing water over 10,000 square feet is roughly 104 tons on a deck that was never designed to hold it, and ponding is self-reinforcing: the deck deflects, which deepens the pond, which deflects the deck further.

What are the signs of a failing roof drain?

• Ponding that does not clear. Water still standing 48 hours after the rain stops means the drain, the leader or the roof slope is not doing its job.
• A missing dome strainer. The most common missing part on an older roof. Without it, leaves and gravel go straight into the leader and block the horizontal run instead of the drain, which is much harder to reach and clear.
• The overflow running in ordinary rain. The secondary is a warning device. If it discharges in normal weather, the primary is already restricted.
• Staining or drips on the deck below the drain. Usually the clamping collar has loosened or the membrane at the flange has failed, not the drain body.
• Debris ringing the drain after every storm. The gravel guard is working and nobody is clearing it. That is a maintenance schedule problem, and it ends as a blockage.
• Slow drawdown after heavy rain. Points at an undersized or partially blocked leader rather than the drain itself.
• A cracked or rocking drain body. Cast iron takes traffic, but a body that moves under foot has lost its underdeck clamp or its sump receiver fastening.

Roof drain installation detail

1. Confirm the drain location is within waterproofed roofing material. Drains belong nowhere else.
2. Set the sump receiver flush with the roof deck and fasten it with self-tapping screws.
3. Set the drain body into the receiver opening so it rests evenly in the groove.
4. Lay the waterproofing material over the sump receiver and the drain flange.
5. Bolt the clamping collar to the drain body, keeping full-circle contact with the waterproofing, and tighten enough to make a watertight seal between body, membrane and collar.
6. Place the dome strainer in the clamping collar and twist to lock it into the retaining clips.
7. For an underdeck clamp, thread the three studs into the taps on the underside of the drain body from below, fit the clamp, and secure with a washer and nut on each stud.
8. Connect the drain outlet to the leader with a No-Hub coupling.

A sump receiver with an underdeck clamp, or a top-mount sump receiver, is recommended for most installations. A water dam raises the standing level around the drain where the design calls for controlled retention rather than immediate discharge.

Parts people forget to order

The drain body is rarely the item that holds up a job. Order the dome strainer, the underdeck clamp kit, the sump receiver and the No-Hub coupling at the same time, and check whether the specification calls for a water dam or an extension for the roofing build-up. On the discharge end, a downspout nozzle and a bird screen finish the run.

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Sizing figures are reproduced from Table 1103.1 of the 2021 Uniform Plumbing Code and are offered as a guide. Overflow elevation is from IPC 1108.1. Installation steps follow Sioux Chief's published 868 Series installation guide. Always confirm sizing, overflow provision and drain placement against the code in force at the job site.