How to Form Internal and External Corners on an EPDM Rubber Roof
How to Form Internal and External Corners on an EPDM Rubber Roof
Ask anyone who repairs flat roofs for a living where rubber roofs fail, and they will give you the same answer: at the corners. The field of an EPDM roof is a single sheet with nothing to go wrong; the corners are where flat rubber has to become three-dimensional. The good news is that both corner types — internal and external — have a standard detail that works every time, and neither involves cutting the membrane. This guide covers both, step by step.
The golden rule: fold, never cut
A corner asks a flat sheet to cover three planes that meet at a point. There are only two ways to do that: cut the sheet and rejoin it, or fold the excess. Cutting creates edges, and every cut edge at a corner is a future leak sitting exactly where water concentrates. Folding keeps the membrane continuous — the rubber that seems to be "in the way" is folded flat, bonded, and covered with a moulded patch.
So the rule for every corner on the roof: the scissors stay in the bag. The membrane is dressed, folded and patched. If you have already cut a corner, it is not lost — it needs a properly formed patch over the cut, which the patching steps below also cover.
Know which corner you are looking at
An internal corner is one the rubber folds into — where two upstands meet, such as the corner of a parapet, the junction where a kerb meets a wall, or the inside corner of an L-shaped roof. The membrane has too much material at an internal corner, and the excess gathers.
An external corner is one the rubber wraps around — the outside corner of a rooflight kerb, a chimney, or a raised plinth. The membrane has to stretch around an external corner, and the two dressed faces leave a triangle of the corner exposed.
Every corner detail on a rubber roof is one of these two, and each has its own fold and its own moulded patch.
Forming an internal corner, step by step
Step 1 — Bond the field and the faces first
Bond the field membrane and dress both upstand faces with contact adhesive, working towards the corner from both directions and pressing the rubber firmly into the base angle. Stop bonding roughly 100mm short of the corner itself so you have free material to form the fold.
Step 2 — Gather the excess into a "pig's ear"
Push the surplus rubber at the corner into a single neat vertical pleat standing proud of the corner — the fold roofers call a pig's ear. Work it with your thumbs so the rubber sits tight into the corner behind the pleat, with no air trapped and no bridging across the base angle.
Step 3 — Fold the pleat flat to one side
Coat the inside faces of the pleat with contact adhesive, let it flash off, then fold the pleat flat against one upstand face — flatten it towards the face that sheds water, not against it, so the fold's open edge faces downhill. Roll it hard with a seam roller.
Step 4 — Cover with an internal corner patch
Bond a moulded internal corner patch (or a site-formed patch of uncured flashing tape) over the whole corner, centred on the fold, dressed onto the deck and both faces. Roll every edge firmly, working from the centre out. The patch is what makes the detail permanent — the fold takes the stress, the patch seals it.
Step 5 — Check and seal the edges
Look for lifted patch edges, voids behind the fold and bridging in the base angle, then run lap sealant around the exposed patch edge. The finished corner should feel solid when pressed — a hollow sound or give behind the rubber means an air pocket that needs rolling out.
Forming an external corner, step by step
Step 1 — Dress both faces past the corner
Contact-adhesive both faces and dress the membrane around each face in turn, letting the material from each side run past the corner arris. Press each face tight before worrying about the corner itself.
Step 2 — Fold the overlap around the corner
One face's material wraps around the arris and lies over the other, leaving a small diagonal fold at the top or bottom. Adhesive the mating surfaces, wrap, and smooth the fold flat — around the corner, never across the open angle.
Step 3 — Fit the external corner patch
Centre a moulded external corner patch on the arris so it laps equally onto both faces and, where the corner meets the deck or kerb top, onto that surface too. Bond it, then roll it hard from the arris outwards on both faces — the patch edges are the detail's weak point if under-rolled.
Step 4 — Seal and inspect
Run lap sealant around the patch edge, then sight down the corner: the rubber should follow the arris crisply with no gape, no stretched drum-tight panel, and no exposed fold edge facing uphill into the water.
Uncured flashing tape: the site-formed alternative
Moulded patches suit standard 90° corners. For odd angles, tight clusters or repairs, uncured flashing tape does the same job: because it has not been vulcanised, it stretches and moulds around a shape and then cures in place. Cut a patch with generous laps and rounded corners — square patch corners peel — press it into every change of plane with a roller, and treat the edges exactly like a moulded patch. Our flashing and detailing guide covers the material in depth.
Corners you will actually meet
Rooflight and skylight kerbs
Four external corners per kerb, every one needing a patch — the full kerb detail, including the membrane cutting pattern, is in the skylight waterproofing guide.
L-shaped roofs
The inside angle of an L-shaped roof puts an internal corner in the middle of the perimeter, and it governs how the whole sheet is handled — covered start to finish in the L-shaped roofs guide.
Parapet junctions
Where two parapet walls meet you get an internal corner with height — form the pig's ear at the base exactly as above, and keep the fold's open edge pointing down the roof's fall.
Trim corners
Edge trims have their own moulded corner pieces, supplied with our kits — these are separate from membrane corner patches and clip the trim system around the corner over the finished rubber.
Mistakes that cause corner leaks
Cutting out the excess
The most common and the worst. A cut corner has edges in the highest-stress spot on the roof. Fold and patch instead; if a cut has already been made, patch over it generously.
Bridging the angle
Rubber spanning the corner instead of being pressed into it leaves an unsupported drum skin that punctures under foot traffic or snow. Press into the angle first, then fold.
Skipping the patch
A neatly folded corner looks finished. It is not — thermal movement works folds open over a few seasons. Every corner gets its patch.
Under-rolling the patch edges
Patches fail at their edges, not their centres. Roll from the middle outwards, hard, on every face the patch touches.
Working cold or damp
Contact adhesive and uncured tape both want a dry surface and a mild day. Below about 5°C the bond is compromised — warm the patch in a pocket and pick your moment.
Folding against the fall
A fold whose open edge faces uphill collects the water running towards it. Always flatten the pleat so its open edge faces downhill.
Frequently asked questions
Do I need to cut the EPDM membrane at a corner?
No — and you should not. Internal corners are folded into a pig's ear pleat and flattened; external corners are wrapped. Both are then covered with a moulded corner patch. The membrane stays continuous.
What is a pig's ear fold?
The standard internal-corner fold: the surplus rubber at the corner is gathered into a single neat vertical pleat, bonded, and folded flat against one upstand face, keeping the membrane in one piece.
What is the difference between internal and external corner patches?
They are moulded to opposite shapes. An internal patch is formed to sit into a corner the membrane folds into; an external patch is formed to wrap around a corner. They are not interchangeable — fit the one that matches the corner.
Can I use flashing tape instead of moulded corner patches?
Yes — uncured flashing tape can be cut and formed into corner patches on site, and is the right choice for odd angles and repairs. Round the patch corners and roll every edge firmly.
How many corner patches do I need?
Count every corner: a rooflight kerb has four external corners, an L-shaped roof has at least one internal corner, and each parapet junction adds one. Our roof kit calculator counts them for you from the edge types and skylights you set on the plan.
My corner has already been cut — can it be saved?
Yes. Clean the area, bond the cut edges down flat, and cover the whole corner with an oversized patch (moulded or uncured tape) lapping at least 75mm past the cut in every direction, rolled hard.
Why has my corner opened up after a few years?
Almost always an unpatched fold or an under-rolled patch edge working loose with thermal movement. Clean the area, re-bond anything lifted, and fit a new patch over the top.
Shop related products
- Internal corner patches
- External corner patches
- Contact adhesive
- Flashing, trims and detailing products
- EPDM seam tape 75mm
- Professional installation kit
- Price a complete kit — roof kit calculator