How to Fit EPDM on an L-Shaped Roof and Internal Corners
How to Fit EPDM on an L-Shaped Roof and Internal Corners
Rectangular roofs are easy. The moment a roof turns a corner around a chimney breast, a projecting bay or an existing wing of the house, you have an internal corner — and internal corners are where most amateur EPDM installations leak. The good news is that rubber will happily go round an L-shape in one piece, without a single cut, if you fold it correctly. This guide explains how to measure an L-shaped roof, why you must never cut at an internal corner, and how to form both internal and external corners properly.
Why internal corners are different
At an external corner — the outside of a box — the membrane has surplus material that wants to spread out. You fold it flat and it sits comfortably. At an internal corner, where two upstands meet in a reflex angle, the membrane has too much material trying to occupy too little space. It bunches. The instinctive reaction is to cut the bunch out, and that instinct is exactly what causes leaks: the cut sits at the lowest point of a junction where two runs of water converge.
The correct approach is to fold the surplus into a neat pleat, dress that pleat flat against one face, and cover the whole junction with a purpose-made moulded corner patch that bridges both upstands and the deck in a single piece of rubber with no joins.
Measuring an L-shaped roof
An L-shaped roof is measured as the smallest rectangle that fully contains it. You will pay for and cut off a triangle of waste, and that is correct — covering an L in one seamless piece is worth far more than the material saved by seaming two rectangles together.
Worked example
Take an L-shaped roof where the long leg is 6.0m x 3.0m and the short leg is 3.0m x 2.5m projecting off it. The enclosing rectangle is 6.0m x 5.5m.
- Length: 6.0m + edge allowances (150mm per free edge, 250mm per wall upstand) + 150mm trimming allowance
- Width: 5.5m + the same allowances
With two free edges on the length and one wall abutment on the width, that gives roughly 6.45m x 6.05m — order 6.6m x 6.1m. Do not attempt to save money by ordering two rectangles and seaming them at the internal corner. A field seam running into an internal corner is the worst possible arrangement of details on a flat roof.
The EPDM roof calculator handles L-shaped roofs — enter the two legs and it works out the enclosing sheet size and the adhesive quantity for the actual roof area rather than the rectangle.
What you will need
| Item | Why |
|---|---|
| EPDM membrane sized to the enclosing rectangle | One seamless piece covers the whole L |
| Water-based deck adhesive | Field of the roof |
| Contact adhesive | All upstands, perimeters and corner areas |
| Internal corner patches | One per internal corner — count them before ordering |
| External corner patches | One per external corner |
| Lap sealant | Every patch edge and exposed cut edge |
| Silicone seam roller | Essential for bedding patches properly |
Count your corners carefully. A simple L with upstands on both inner faces has one internal corner and typically three or four external ones. A roof stepping around a chimney breast can easily have four internal corners.
Forming an internal corner, step by step
Step 1 — Bond the field first, leaving the corner dry
Bond the main field of the roof with water-based adhesive, stopping 150mm short of every upstand as usual. Leave the corner zone — roughly 300mm in each direction from the corner — completely dry and unbonded. You need free material to work with.
Step 2 — Bond up to the corner along each wall
Using contact adhesive, dress the membrane up each upstand, working towards the corner from both directions and stopping about 150mm short of it. Both runs should now be firmly bonded, leaving a loose pocket of material at the junction itself.
Step 3 — Fold, do not cut
Push the loose material into the corner so the membrane sits tight against both vertical faces and into the base of the corner. The surplus will gather into a single vertical fold. Work it with your fingers until that fold is clean, straight and vertical — not a crumpled rosette. This is the step that takes practice; take your time and refold as often as you need.
Step 4 — Dress the fold flat
Lay the fold flat against one of the two vertical faces — whichever gives the tidier result, usually the less visible one. Apply contact adhesive behind it and bond it down so it lies completely flat with no tunnel underneath. A tunnel behind the fold is a water path.
Step 5 — Fit the internal corner patch
Clean the area, then bond a moulded internal corner patch over the junction so it covers the base of the corner and both vertical faces, extending well past the folded material in every direction. Work from the centre of the patch outwards with the silicone seam roller so no air is trapped, paying particular attention to the corner itself where the patch changes plane.
Step 6 — Seal the patch edges
Run lap sealant around the full perimeter of the patch. The patch does the waterproofing; the sealant protects its edge from lifting and from wind-driven water working underneath.
Forming an external corner
External corners are the easier of the two, but they still deserve a patch. Dress the membrane around the corner so it wraps both faces, fold the surplus flat against one face and bond it, then apply an external corner patch over the junction and seal its edges. Where a drip trim meets at an external corner, lap and seal the trims rather than butting them, and make sure the membrane is dressed into both trim legs before the patch goes on.
Sequencing a whole L-shaped roof
Order of work matters on a complex roof. Working the details in the wrong order means lifting bonded material to get at something you missed.
- Prepare and check the deck across the whole L, including the falls in both legs — the internal corner is often where water wants to sit.
- Fit all perimeter trims to the free edges.
- Position the full sheet over the enclosing rectangle and let it relax. Do not trim the surplus triangle off yet; leave it until the membrane is bonded, because you may need the material.
- Bond the field in sections, leaving all perimeters and corner zones dry.
- Bond the perimeters and upstands, working towards each corner from both sides.
- Form every internal corner, then every external corner.
- Fit all corner patches and seal.
- Trim the surplus, including the waste triangle, and seal all cut edges.
- Inspect and water-test, paying attention to every corner.
Mistakes that cause corner leaks
Cutting the membrane to relieve the bunch
The single most common cause of a leaking internal corner. If a cut is genuinely unavoidable, it must be sealed and completely covered by a patch — but plan to fold instead.
Leaving a tunnel behind the fold
A fold that is not bonded flat leaves a channel running down the upstand and straight into the corner. Bond the fold along its full length.
An undersized patch
The patch must extend clearly past the folded material on the deck and up both faces. A patch that only just covers the fold has its edge sitting exactly where water concentrates.
Seaming two sheets at the corner
Two sheets joined near an internal corner puts a field seam at the point of maximum water convergence and maximum movement. Buy the bigger sheet.
Rushing the patch with cold hands
Patches bond best on a clean, dry, above-5°C surface, rolled firmly from the centre out. A patch pressed on with a thumb in the cold will lift at the edges within a season.
Frequently asked questions
Can EPDM go round an L-shaped roof in one piece?
Yes. Order a sheet sized to the smallest rectangle that contains the whole L, bond it in one piece, and trim the waste triangle off at the end. This gives a completely seamless roof.
Why can I not just cut the membrane at an internal corner?
An internal corner is where two runs of water converge and where the roof moves most. A cut there is a permanent weak point. Folding the surplus and covering it with a moulded patch keeps the membrane continuous.
Do I really need corner patches, or will sealant do?
Patches are required. Sealant is a supporting product that protects patch edges; it is not a waterproofing layer in its own right and it will not bridge a corner over the long term.
How many corner patches will I need?
One per corner. Walk the roof and count internal and external corners separately before ordering — a roof stepping around a chimney breast can have four internal corners where a plan drawing suggests one.
Which face should the fold be dressed against?
Either works structurally, so choose the less visible face for a tidier finish. What matters is that the fold is bonded completely flat with no tunnel behind it.
Does the waste triangle have to be thrown away?
Keep it. Offcuts are useful for practising folds, for making up sacrificial patches, and for future repairs — matched EPDM from the same sheet is the ideal patch material.
Shop related products
- EPDM internal corner patch
- EPDM external corner patch
- Contact adhesive
- Water-based deck adhesive
- 1.2mm EPDM membrane
- 1.5mm EPDM membrane
- Professional installation kit
- Flashing and trims