How to Install a Flat Roof Drain: Sump, Drain Body, Clamping Ring, Membrane and Strainer

Step by Step

1. 1Step 1: Locate the low point and confirm the drain size

   The drain goes at the lowest point of the roof area it serves, not beside it. Check the location against the slope plan and the structure below. A drain must fall between joists or beams and line up with the storm pipe. Drain sizes are set by the plumbing design based on roof area and rainfall, often 3 or 4 inches for smaller roofs. Do not substitute a smaller drain.

   When replacing an existing drain, check that it really is at the low point. A drain that was installed high is a cause of ponding, and a replacement in the same place repeats the error.

2. 2Step 2: Build the sump in the insulation

   The sump is a shallow depression around the drain, often roughly 24 inches square, cut or built from tapered boards so the drain flange is set about 1/2 to 1 inch below the surrounding roof surface, as the drain manufacturer and designer specify. Water that flows toward the drain then always finds a lower exit.

   Tapered insulation around the drain is usually the thinnest on the roof, and follows the taper plan. See how to install tapered insulation and crickets. Some drains use a sump pan or sump receiver to form the depression.

3. 3Step 3: Cut the deck opening and set the drain body

   Cut the opening just large enough for the drain body and its outlet, following the drain maker's template. Check that framing, pipes and wiring below are clear. Set the drain body with its flange flat on the deck or in the sump pan, at a height that puts the flange slightly below the finished surface.

   Fasten the flange to the deck with the screws or bolts supplied. The plumber then connects the outlet to the storm pipe with a watertight coupling, and the connection should be tested before the roof is finished. A leak at the pipe connection shows up as a ceiling stain, not a roof leak, and it is often misdiagnosed.

   Watch out: Never leave the deck opening uncovered at the end of the day. Rain through an open drain hole goes straight into the building.

4. 4Step 4: Install the membrane over the drain flange

   Lay the field membrane over the sump and let it conform to the depression. Do not stretch it tight, because stretched membrane bridges the sump and pulls off the flange. Cut relief where needed and patch. For single-ply, a hand welder and roller join the sheets at the sump, and T-joints get round patches. For torch-on, a base sheet and cap sheet are dressed into the sump with heat and a flashing piece is added.

   Some drain flanges have a flat ring that accepts a membrane patch, and others need a bed of sealant or flashing cement under the membrane. Follow the drain and membrane manufacturers' details. The membrane should run over the flange to the clamping ring bolts.

5. 5Step 5: Clamp the membrane and cut the opening

   Set the clamping ring on top of the membrane and tighten the bolts evenly in a cross pattern, like wheel nuts, so the ring seats flat without pinching the membrane on one side. Stainless bolts resist corrosion at the wettest point on the roof. The ring presses the membrane onto the drain flange, and it is the main seal of the detail.

   After the ring is on, cut out the membrane inside the ring, following the inside edge of the drain opening. Do not cut into the flange or the ring. A sealant bead around the ring edge is sometimes specified as a second barrier. The clamp carries the seal, and the sealant only backs it up.

6. 6Step 6: Flash and seal the drain edge

   Patches or flashing pieces around the drain are welded or torched down with laps of at least 1-1/2 inches for single-ply, and the edges are probed. See how to hot-air weld TPO seams for the method. Every T-joint in the sump must be patched.

   For torch-on roofs, see how to flash drains and scuppers in torch-on, and for EPDM, see the EPDM guides in this library.

7. 7Step 7: Fit the strainer and test the drain

   A strainer, often a cast or plastic dome, sits over the drain and keeps leaves and needles from entering the pipe. The strainer should be large enough that it is not blocked by the first handful of debris, and it should be removable for cleaning.

   Run water from a hose into the sump and watch it flow. Check that the water flows to the drain from every side, that the joint at the pipe does not leak, and that no water lifts at the membrane edge. Then check below for any drips.

Overflow Drains and Secondary Drainage

Maintenance

Common Mistakes

• Drain set above the surrounding surface. The drain becomes a high spot and water ponds around it.
• Membrane stretched over the sump. It bridges the depression and pulls off the flange under load.
• Uneven clamping bolts. The ring tilts, pinches one side and leaves a path for water on the other.
• No strainer or a strainer that is too small. Debris blocks the pipe and the roof fills.
• Untested plumbing connection. A leak at the pipe coupling is mistaken for a roof leak.
• Missing T-joint patches in the sump. Three layers meet in a small space and leak if not covered.