How to Flash Inside and Outside Corners on an EPDM Roof
Corners are where rubber has to be folded, stretched or patched. Pre-formed pieces and uncured flashing do the work.
- Advanced
- 20 to 40 minutes per corner
- Updated
The short answer
Flash an EPDM inside corner by adhering the sheet up the wall with slack at the corner, folding the extra rubber into a neat flap, and covering it with a pre-formed inside corner or an uncured flashing patch laid with primer and splice tape. Flash an outside corner with a formed piece or a stretched uncured patch. Run flashings at least 8 inches above the roof and terminate the top.
A flat sheet of rubber can cover a flat plane, but it cannot simply turn a corner. When the membrane runs up a wall, a curb or a parapet, the extra material at the corner has to go somewhere. The answer is either a fold, which leaves a thickness and a gap, or a separate piece, either pre-formed or made from uncured flashing, that fits over the corner. Corners are the most common place for an EPDM roof to leak, along with pipes and edges.
There are two kinds of corner. An inside corner is where the roof meets two walls, and the membrane has extra material to fold. An outside corner, such as at a curb or a parapet end, needs the membrane to stretch around the convex shape. Both are handled with the same ideas: no tension, good lap, primed surfaces and splice tape or self-adhering flashing over the top.
Corner flashing follows the field membrane in how to install a fully adhered EPDM roof and the seams in how to seam EPDM with seam tape. The top edges are covered in how to terminate EPDM at edges and walls.
Tools
- Scissors or utility knife
- Hand roller
- Clean rags and primer brush
- Heat gun where the manufacturer allows
- Tape measure
- Marker
Materials
- Pre-formed inside and outside corners
- Uncured EPDM flashing
- Splice tape and primer
- Bonding adhesive
- Termination bar and sealant
- EPDM patch membrane
Safety first
- Adhesives and primers are flammable. No flames, sparks or cigarettes near the work, and ventilate.
- Corners are often at walls and parapets. Keep fall protection in place, as WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall of any height could cause serious injury.
- Uncured flashing is soft and can be stretched. Wear gloves and avoid contact with hot surfaces.
- A heat gun can ignite combustible materials and overheat the rubber, so use it only where the manufacturer allows and keep it away from wood and insulation.
Step by Step
Step 1: Adhere the membrane up the wall with slack at the corner
The membrane is adhered up the vertical surface with bonding adhesive, to a height of at least 8 inches above the finished roof, or as the manufacturer and code specify. Leave a little slack at the corner so the sheet sits on the surface without tension. A sheet that is pulled tight at a corner tears or lifts when the roof moves.
Do not apply adhesive to the last few inches where the membrane will be covered by the corner patch, because tape must bond to clean rubber.
Step 2: Make the inside corner fold
At an inside corner, the extra rubber forms a flap. Fold it in a triangular fold, flatten it against one wall, and adhere it. Some installers cut a relief slit in the flap, but the safest approach is a fold that keeps the rubber continuous, then a cover.
The fold creates a step in thickness, so it is covered by a patch. Pre-formed inside corners have this shape already and are laid over the corner with no folding.
Step 3: Clean and prime the corner area
Wipe the area with the cleaner the manufacturer names, then apply primer to the membrane and to the back of the corner piece where tape will go, in a band wide enough for the tape. Let it dry to a slight tack.
Do not touch primed surfaces, and keep dirt and dust off them. Primer on the wall that will be covered by the membrane is applied as the adhesive manufacturer directs.
Step 4: Install a pre-formed inside or outside corner
Pre-formed corners come in inside and outside shapes, and in sizes for common membrane thicknesses. Set the piece so the flanges cover the fold and lap at least 2 to 3 inches onto the flat membrane in every direction, then seal every edge with splice tape or the method the manufacturer specifies, pressing with a hand roller.
Orient the piece so its laps face downslope, and press out air before the tape sets.
Tip: Dry-fit the corner first. A corner that does not sit flat will not seal well.
Step 5: Use uncured flashing where no pre-formed piece fits
Uncured EPDM flashing is a soft, unvulcanized rubber sheet that moulds to shapes. Cut a patch with rounded corners that extends at least 3 inches past the fold in every direction, work it over the corner without stretching it more than the manufacturer allows, and seal it with primer and splice tape or the self-adhering surface it has.
Uncured flashing is not as durable as cured membrane in the long term, so it is used for small details, not for large areas. It is not for places with heavy movement or foot traffic.
Step 6: Flash curbs, walls and outside corners
An outside corner wraps around a convex shape, so the membrane is cut and the gap is covered by a formed outside corner or an uncured flashing patch that wraps around it. Round the corners of every patch to about 1/2 inch radius, because square corners lift.
On curbs, flash each side with a separate piece, overlapping at the corners, and cover the corner with a formed piece. See how to flash a curb on a flat roof.
Step 7: Terminate the top and inspect
The top of the wall flashing is secured with a termination bar fastened every 6 to 12 inches, depending on the product, with sealant along the top edge, and covered by siding, counter flashing or coping. See how to terminate EPDM at edges and walls.
Run a blunt probe around each corner. Any lifted edge is reprimed, retaped and rolled.
Pre-Formed Versus Uncured
- Pre-formed corners: made of cured rubber, durable and consistent, available in inside and outside shapes. The preferred choice where they fit.
- Uncured flashing: soft and mouldable, good for odd shapes, but less durable over the long term. Use for small details.
- Field folds only: acceptable on some inside corners when covered by a patch, but not as a stand-alone seal.
Fraser Valley Notes
Corners at walls get the most driven rain in winter, and they sit in shaded spots where moss and needles collect. A corner that is well made on a dry day still has to survive months of wet. Keep the tops terminated and sealed, and clear debris from corners every autumn.
Common Mistakes
- Pulling the membrane tight at corners. Tension tears the sheet or lifts the adhesive when the roof moves.
- Cutting the sheet at the corner. An open cut is a leak path unless it is covered.
- Patches with square corners. They lift at the corners first.
- Patches under 3 inches of lap. The tape has no margin.
- Using uncured flashing over large areas. It ages faster than cured membrane.
- No termination at the top. Water runs behind the membrane at the wall.
When to Call a Professional
Corners where walls, curbs and parapets meet are the hardest details on a flat roof. If you have many corners, tight spaces, or a structure with movement joints, call a roofer who works with the products every day.
If an existing corner is leaking, see how to repair an EPDM roof for how a patch is made.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Flat Roofing and Torch On service or the towns we cover.
How to Flash Inside and Outside Corners on EPDM: Questions
How do you flash an inside corner on an EPDM roof?
Adhere the membrane up the walls with slack at the corner, fold the extra rubber into a flat flap, clean and prime the area, then lay a pre-formed inside corner over it and seal every edge with splice tape. If no pre-formed piece fits, a patch of uncured flashing is worked over the corner and sealed the same way.
What is uncured EPDM flashing?
Uncured EPDM flashing is a soft, unvulcanized rubber sheet that can be moulded to corners, pipes and odd shapes. It is used for small details where a pre-formed piece does not fit. It is less durable in the long term than cured membrane, so it is used in small areas and inspected regularly.
How high should EPDM flashing go up a wall?
Commonly at least 8 inches above the finished roof surface, with the top terminated by a bar, counter flashing or coping. Some manufacturers and codes specify different minimums, and higher is better where water or snow can pool. Confirm the requirement in the membrane manufacturer's detail for your roof.
Why do EPDM corners leak?
Common causes are membrane pulled tight at the corner, a cut or fold left uncovered, patches with too little lap or square corners that lift, dirt or oil under the tape, and a top edge that was never terminated. Movement and ponding then open the weak spot. Reprime, retape with a larger rounded patch and terminate the top.
Related Guides
All EPDM Rubber Roofing guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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