How to Waterproof a Flat Roof Skylight with EPDM

How to Waterproof a Flat Roof Skylight with EPDM

A skylight is the biggest hole you will ever deliberately cut in a flat roof — and the detail around it decides whether you get a bright, dry room or a stained ceiling. The good news is that the correct detail is simple, mechanical and completely reliable: a raised timber kerb, membrane dressed up and over it in one continuous path, and the rooflight seated on top. This guide covers flat glass rooflights, roof lanterns and dome lights on EPDM rubber roofs, from building the kerb to the final bead of sealant.

The principle: the water never reaches the hole

Every skylight detail that works follows the same rule as every other penetration in an EPDM roof: the membrane stays continuous, and the opening is lifted above the water. The skylight does not sit in the roof — it sits on a raised kerb, and the membrane climbs the kerb and crosses its top, so rain landing anywhere near the opening is standing on unbroken rubber. The glazing unit then overhangs and seals onto that finished, waterproofed kerb.

Sealant is the garnish, not the meal. If a skylight detail relies on a perimeter bead of sealant to keep water out of the room below, it has already failed — it just has not rained hard enough yet.

Flat roofed brick house extension finished in black EPDM rubber with a flush square rooflight (illustration)
A flat glass rooflight seated on a membrane-wrapped kerb — the standard detail on extension roofs (illustration).

Build the kerb right and the rest follows

The kerb is a timber upstand fixed around the opening, and it has one non-negotiable dimension: its top must finish at least 150mm above the finished roof surface. That height is what keeps wind-driven rain and standing snow below the level of the glazing seal. On a roof with very shallow falls, or where drifting snow is likely, go taller rather than shorter.

Practical kerb rules:

  • Height: 150mm minimum above the finished membrane; 225mm is a sensible upper limit for a timber kerb before it starts to look like a wall and needs its own engineering.
  • Material: treated softwood or built-up timber sections, faced with 18mm exterior-grade ply or OSB3 where the sides span more than a joist bay. The membrane needs a smooth, continuous face to bond to.
  • Fixing: the kerb fixes to the roof structure — the trimmed joists around the opening — not just to the deck boards. The rooflight's wind loads travel through it.
  • Corners: square, tight and flush. Every step in the kerb face telegraphs through the rubber.
  • Match the rooflight: order the rooflight first and build the kerb to the manufacturer's stated kerb dimensions. The frame must overhang the finished, membrane-wrapped kerb — allow for the membrane thickness on each face.

If your rooflight comes with its own uPVC or composite upstand instead of seating on a site-built timber kerb, the detail changes — email sales@rubberroofuk.uk and we will specify the right approach for that unit.

Section through a skylight kerb Cross-section through a flat roof skylight kerb showing the timber kerb rising at least 150mm above the finished roof, the EPDM membrane dressed continuously up the kerb face and across its top, and the rooflight frame overhanging the wrapped kerb and sealed to the membrane. The skylight kerb detail Section: the membrane climbs the kerb, crosses the top, and the frame seals onto it. min 150mm rooflight frame overhangs the wrapped kerb frame sealed to the membrane on the kerb top membrane crosses the kerb top one continuous piece up the kerb face field membrane fully bonded to the deck No face fixings Nothing is screwed through the membrane on the kerb face — the frame fixes down into the kerb top. EPDM membrane timber deck and kerb rooflight frame glazing RUBBER ROOF UK
The kerb lifts the opening above the water; the membrane makes the kerb part of the roof.

Waterproofing the kerb, step by step

Step 1 — Build the kerb before the membrane goes down

Trim the opening, fix the kerb to the structure and face it smooth. Round or arris the top outside edge of the kerb with a plane or sander — a sharp 90° edge is the single most common place a dressed membrane wears through. Sweep the whole roof clean.

Step 2 — Cut the membrane void smaller than the opening

With the sheet positioned dry on the roof, mark the kerb's outside footprint on the membrane, then cut the opening well inside that line — the material between your cut and the kerb line is what dresses up the kerb faces. For a 150mm kerb, cut roughly 100–120mm inside the kerb line on all four sides, then make a 45° cut into each corner, stopping just short of the kerb corner itself. Cut less rather than more; you can always trim excess at the top, but you cannot put material back.

Step 3 — Bond the field membrane

Bond the sheet to the deck exactly as on any roof — water-based deck adhesive across the field on timber, worked in stages, rolled as you go — stopping around 150mm short of the kerb on all sides. Everything within 150mm of the kerb, and the kerb faces themselves, are contact-adhesive territory.

Step 4 — Dress the flaps up and over

Coat the kerb faces, the kerb top and the back of each membrane flap with contact adhesive, let it flash off until touch-dry, then dress each flap up the face and over the top of the kerb, working from the centre of each side out towards the corners. Press the rubber into the base angle first — where kerb meets deck — then smooth upwards. Roll firmly with a seam roller, especially along the base angle and the top edge.

Step 5 — Form the corners

At each corner the two flaps meet and leave a small triangle of kerb exposed. Fold and bond the overlapping material around the corner, then cover every corner with a moulded external corner patch (or a site-formed patch of uncured flashing tape), dressed around both faces and rolled hard. The corner patch is not optional — the four kerb corners are where skylight details leak.

Step 6 — Trim the top

Trim the membrane neatly on the kerb top so it finishes flat where the rooflight seal will land. If the manufacturer's instructions show the membrane terminating on the inside edge of the kerb, take it over and down a few millimetres and trim there — never terminate the rubber on the outer face where the water is.

Step 7 — Seat and fix the rooflight

Bed the rooflight frame on the kerb top following the manufacturer's instructions — most seat on a foam gasket or a continuous bead of low-modulus sealant on the membrane. Fix through the frame down into the kerb top, never through the membrane on the kerb face. Torque fixings evenly so the gasket compresses uniformly.

Step 8 — Seal and inspect

Run lap sealant along any exposed membrane termination and check the finished detail: no bridging or hollow rubber in the base angle, corners patched and rolled tight, frame overhanging the kerb on all sides, and no fixing anywhere through the weathered faces. Then test it with a hose before the ceiling goes in — five minutes with running water is cheaper than redecorating.

Cutting the membrane at a skylight opening Plan view of an EPDM sheet at a skylight kerb showing the cut-out made around 100 to 120 millimetres inside the kerb line, 45 degree cuts into each corner, the four flaps dressing up the kerb faces, and an external corner patch at each of the four corners. Cutting the void — plan view Cut inside the kerb line and the offcut becomes the upstand. Cut to the line and you have nothing left to dress. kerb outside line (dashed white) cut-out 100–120mm inside the kerb line 45° cut into each corner, stop short flaps dress up the four kerb faces external corner patch at every corner Cut less, trim later Excess rubber on the kerb top can be trimmed in seconds. A void cut too big means patching the whole detail. EPDM sheet (plan) cut-out and corner cuts RUBBER ROOF UK
The material inside the kerb line is not waste — it is the upstand.

Roof lanterns, dome lights and sun tunnels

Roof lanterns

A lantern is weathered identically to a flat rooflight — it is simply a taller, heavier unit on the same membrane-wrapped kerb. Two differences matter: the kerb must be built to the lantern manufacturer's exact dimensions because the cill sections are not adjustable, and the extra weight and sail area mean the kerb fixings into the structure need real attention.

Dome lights

Older dome units often come with their own integral upstand that the membrane dresses against rather than under. Where the dome has a flange designed to be trapped, the membrane runs under the flange across the kerb top and the flange is bedded in sealant and fixed down through it into the kerb — a trapped, mechanically clamped termination that works well.

Sun tunnels

A sun tunnel's tube is a round penetration, not a kerb — treat it like a large pipe and weather it with the flashing supplied by the tunnel manufacturer or a moulded detail, as covered in the pipes and penetrations guide.

Condensation: the leak that is not a leak

A surprising share of "leaking skylight" complaints are condensation. A cold kerb bridges warm room air straight to the outside, and moisture condenses on the inner faces and drips. On a warm-roof build-up, carry the insulation up the inside of the kerb so the kerb is inside the thermal envelope, and make sure the vapour control layer is continuous up to the frame. Specify double or triple glazed units with a thermally broken frame — on an insulated roof, a cold single-skin dome will stream with condensation every winter morning. The house extension guide covers warm-roof build-ups in detail.

Pricing a roof with skylights

Our EPDM roof kit calculator handles skylights natively: add a Skylight Void to the plan, drag it into position, and enter the void size and upstand height. The calculator subtracts the opening, adds the membrane needed to dress the upstand, and prices the corner patches and adhesive the detail needs — for as many skylights as the roof has. The shed and outbuilding calculator does the same for garden buildings.

Mistakes that cause skylight leaks

A kerb that is too low

Anything under 150mm puts the glazing seal within reach of standing water and drifted snow. This is the most common defect on failed rooflight details, and the only fix is rebuilding the kerb.

Cutting the void to the full opening size

Cut the membrane to the kerb line and there is nothing left to dress the upstand — the kerb faces end up patched from offcuts with four times the seams. Cut well inside the line and trim later.

Skipping the corner patches

The four kerb corners carry folded, stressed rubber. Uncovered, they open with thermal movement. Every corner gets a patch, rolled hard.

Fixing through the kerb face

A screw through the dressed membrane face is a hole below the seal line. All fixings go down through the frame into the kerb top.

Bridging the base angle

If the membrane spans the deck-to-kerb angle instead of being pressed into it, foot traffic and snow load will eventually puncture the unsupported rubber. Press into the angle first, then dress upwards.

Relying on the perimeter sealant

The frame gasket and the membrane do the waterproofing. If a bead of sealant is the only thing keeping water out, the detail is wrong and will fail when the bead ages.

Frequently asked questions

How high does a skylight upstand need to be on a flat roof?

At least 150mm from the finished roof surface to the underside of the glazing seal. Go higher on very shallow falls or where snow drifts against the kerb.

Can I fit a skylight to an existing EPDM roof?

Yes. Cut the opening, build and fix the kerb, then dress the kerb with a bonded EPDM cover piece lapped onto the existing field membrane with seam tape and finished with corner patches. It is a bigger job than weathering a kerb on a new roof, but entirely standard.

Does the membrane go under the rooflight frame?

Yes — the membrane crosses the kerb top so the kerb is fully waterproofed before the rooflight arrives, and the frame seats and seals on top of the rubber.

Do I really need corner patches on a skylight kerb?

Yes. The corners are folded, stressed rubber and are the most likely failure point of the whole detail. A moulded external corner patch on each corner, rolled firmly, is standard practice.

Why does my skylight drip in cold weather but not when it rains?

That is condensation, not a leak — warm room air condensing on a cold kerb or frame. Insulate the kerb inside the thermal envelope, keep the vapour control layer continuous, and specify a thermally broken, double glazed unit.

Can one EPDM sheet cover a roof with a skylight in it?

Yes — one of EPDM's biggest advantages. The sheet is cut and dressed around the kerb, so the roof still has no seams in the field. Our calculator prices exactly this: add a skylight void to the plan and the membrane allowance is worked out automatically.

What about rooflights with their own uPVC upstand?

Units with integral uPVC or composite upstands terminate differently and vary by manufacturer. Email sales@rubberroofuk.uk with the unit details and we will specify the termination.

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