How to Install Drain Outlets on an EPDM Rubber Roof
How to Install Drain Outlets on an EPDM Rubber Roof
A rubber roof does not leak because water lands on it — it leaks because water stays on it. Every flat roof is designed to move rain to one or more collection points and get it off the roof, and the outlet is where that job either works or fails. This guide covers the two outlet families used on EPDM roofs — internal outlets that drop water down through the deck, and horizontal outlets that push it out through the edge — including how many you need, how to fit each type, and the overflow rule that saves parapet roofs.
Two ways off the roof
An internal drain outlet sits in the field of the roof at its lowest point, connected to a pipe that runs down through the building. It is the standard detail on parapet roofs and larger flat roofs where the water cannot simply be tipped over an edge. A leaf grate over the bowl keeps debris out of the pipework.
A horizontal drain outlet passes through a raised edge, parapet or fascia and discharges into an external gutter or hopper. It keeps all the pipework outside the building — no ceiling penetrations, no internal downpipe — which makes it the first choice for extensions, garden rooms and any roof with an accessible edge.
Many roofs use neither, and simply shed water over a drip trim into a gutter along one whole edge. If that is your roof, the flashing and detailing guide covers the drip edge; this guide is for roofs where the water has to leave through a point outlet.
How many outlets, and where
The rules that matter:
- Outlets go at the low point. The roof's falls are designed first, the outlet position comes from them — never the other way round. An outlet at anything other than the lowest point leaves a permanent pond.
- One outlet is never enough on an enclosed roof. A parapet roof with a single outlet is one blocked grate away from becoming a swimming pool. Fit at least two outlets, or one outlet plus an overflow.
- Coverage: as a working rule, allow one 68mm internal outlet per 35–40 m² of roof with proper falls. Bigger roofs, shallow falls and high local rainfall all push you towards more outlets, not bigger ones.
- Keep outlets clear of details. Like every penetration, an outlet flange wants 150mm of flat, unstressed membrane around it — not jammed against an upstand or corner.
Installing an internal drain outlet, step by step
Step 1 — Form the falls to the outlet position
Confirm the deck falls actually deliver water to the outlet position — check with a level, or on a re-roof, look at where the old roof ponded. Correct the falls with firrings before any rubber goes down; no outlet rescues a roof that drains the wrong way.
Step 2 — Cut the deck and connect the pipework
Cut the deck opening for the outlet spigot and connect it to the downpipe below. Support the pipework independently — the outlet body should never carry the weight of the pipe run.
Step 3 — Fix the outlet body
Seat the outlet body so its flange sits flush with, or fractionally below, the deck surface — recess the flange into the deck if needed. A flange standing proud of the deck creates a dam around the very point the water is supposed to reach. Fix it to the deck through the flange.
Step 4 — Bond the membrane and cut the opening
Bond the field membrane across the outlet position as normal, then cut the opening in the rubber slightly smaller than the bowl so the membrane can be dressed down into it. Do not cut flush with the flange — the rubber must turn down into the bowl, not stop at its rim.
Step 5 — Bond the membrane into the outlet
Contact-adhesive the flange and the underside of the membrane, dress the rubber down into the bowl, and roll it firmly so the membrane follows the flange with no bridging and no trapped air. Where the outlet is supplied with a clamping ring, fit it now to mechanically trap the membrane; otherwise finish the bonded edge with lap sealant.
Step 6 — Fit the leaf grate and test
Clip or screw the leaf grate over the bowl and test the detail with a bucket of water or a hose: the water should pull straight down the outlet with no hesitation and no ring of standing water around the flange.
Installing a horizontal drain outlet, step by step
Step 1 — Mark the opening at the true low point
Find the lowest point of the raised edge or parapet the falls run to and mark the chute opening there, sized to the outlet you are fitting. Set the opening so the chute's base sits level with the finished roof surface — any step up creates a pond in front of it.
Step 2 — Cut through the upstand
Cut the opening through the timber upstand or parapet cleanly. Check the chute will fall slightly towards the outside so water cannot sit in it.
Step 3 — Dress the membrane through the opening
Bond the field membrane up to and through the opening, dressing it onto the base of the opening so the rubber lines the bottom of the hole before the chute goes in. Water that gets beside the chute must still be landing on rubber.
Step 4 — Fit and bond the chute
Insert the chute so its flanges sit over the dressed membrane inside the roof, bond the flanges with contact adhesive, roll them, and seal the edges with lap sealant. The chute should project far enough outside to discharge into the middle of the gutter or hopper, not dribble down the fascia.
Step 5 — Seal and test
Seal around the outside of the chute where it passes through the upstand, then hose-test: water should run through the chute and drop into the gutter, with nothing tracking back along the underside towards the fascia.
The overflow rule
Any roof where the water cannot escape over an edge — a parapet roof above all — needs a plan for the day the outlet blocks. That is what an overflow is: a second horizontal outlet set 25–50mm higher than the main outlet, through the parapet, ideally somewhere visible. If water ever pours from the overflow you get a warning you can see from the ground, instead of a ceiling collapse. Building regulations expect one; leaks caused by a blocked single outlet are among the most expensive flat-roof failures there are, and the overflow that prevents them costs a few pounds of chute.
Mistakes that cause drainage failures
The outlet is not at the low point
The commonest fault by far. Water finds the real low point, not the intended one, and ponds there while the outlet sits dry. Falls first, outlet position second.
One outlet and no overflow on an enclosed roof
Every grate blocks eventually. On a parapet roof, a single blocked outlet turns the roof into a tank whose water level rises until it finds a way in. Two outlets or an outlet plus overflow, always.
The flange stands proud of the deck
A proud flange dams the last 5mm of water permanently. Recess the flange so the membrane falls into the bowl without a lip.
Membrane bridging the bowl
If the rubber is not dressed down into the outlet it spans the bowl like a drum skin, and the bond takes all the load every time water pulls at it. Dress in, roll hard.
No leaf grate, or a grate never cleaned
The grate is a maintenance item, not an ornament. Clear it every autumn and after storms — it takes seconds and it is the whole reason the outlet keeps working. The maintenance guide has the full annual routine.
Sealant doing the waterproofing
As at every detail on a rubber roof: the bonded membrane does the waterproofing, sealant protects edges. A bead of sealant bridging a gap between membrane and flange is a countdown, not a detail.
Frequently asked questions
How many drain outlets does a flat roof need?
As a working rule, one 68mm internal outlet per 35–40 m² with proper falls — and never fewer than two escape routes on an enclosed roof (two outlets, or one outlet plus an overflow).
Should I choose an internal or horizontal outlet?
If the roof has an accessible edge and an external gutter, a horizontal outlet is simpler and keeps pipework outside. Internal outlets are for parapet roofs and larger roofs where the water must go down through the building.
Can I add a drain outlet to an existing EPDM roof?
Yes. Cut the opening at the true low point, fit the outlet, and weather it with a bonded EPDM collar piece lapped onto the existing membrane with seam tape, dressed into the bowl and rolled. It is the same detail as on a new roof with one extra lap joint.
Do I really need an overflow?
On any roof enclosed by parapets or upstands, yes — set 25–50mm above the main outlet, somewhere visible. It is the difference between a warning you can see and a ceiling you replace.
Why does water pond around my outlet?
Either the outlet is not at the low point, or its flange stands proud of the deck and dams the last few millimetres. The first needs the falls corrected; the second needs the flange recessed.
How often should the leaf grate be cleared?
Check it every autumn once the leaves are down, and after any big storm. A blocked grate is the single most common cause of standing water on an otherwise healthy roof.
Does the calculator include drain outlets?
Yes — both types. In our roof kit calculator, set the number of internal drain outlets and horizontal drain kits in the Trims & Extras step and they are priced into your kit with everything else.
Shop related products
- EPDM roof drain outlet with leaf grate
- EPDM horizontal drain kit
- All drainage and outlets
- Contact adhesive
- EPDM seam tape 75mm
- Price a complete kit with outlets — roof kit calculator