How to Seam EPDM With Seam Tape: Primer, Lap Width, Rolling and T-Joints
Most EPDM leaks are seams. Clean rubber, dry primer, a full width tape and firm pressure are the whole method.
- Advanced
- About 1 hour per 50 feet of seam
- Updated
The short answer
To seam EPDM, overlap the sheets at least 3 inches, fold the top sheet back, clean both surfaces, apply primer to both and let it dry to a slight tack, lay 3 inch splice tape on the lower sheet, then roll the top sheet down onto it, peel the release film and roll the seam hard with a hand roller. Cover every T-joint with a patch of tape-backed membrane.
On a large roof, the field of an EPDM roof is one sheet, but sheets end, and where two sheets meet a seam has to be made. A seam is a small glued joint, but it takes the load of every rainfall, every freeze-thaw cycle and every foot of water running across it. That is why seams are where most EPDM leaks start, and why the method is strict.
Modern EPDM is seamed with a pressure-sensitive splice tape, typically 3 inches wide, used with a primer. The tape is a factory-made strip of butyl or similar adhesive with a release film, so the thickness and width are consistent, unlike a hand-spread adhesive. The job is to get clean, primed, dry surfaces and apply firm pressure along the full width. This guide covers the method.
Seams follow the field installation in how to install a fully adhered EPDM roof. If an existing seam is open, see how to repair an EPDM roof.
Tools
- Hand roller, 2 inch wide silicone or steel
- Clean rags or white scrubbing pads
- Utility knife or scissors
- Chalk line
- Paint brush or pad for primer
- Hand probe or blunt screwdriver
Materials
- EPDM splice tape, 3 inch wide, butyl or acrylic-based
- Seam primer or splice wash as recommended by the manufacturer
- T-joint cover patches
- Cleaner recommended by the manufacturer
- EPDM lap sealant for edge sealing where specified
Safety first
- Primers and cleaners are flammable and their fumes are harmful. Keep away from flames, sparks and heaters, wear gloves, and ventilate.
- Keep rags and pads in a closed metal container. Solvent rags can ignite.
- Seams run along edges of sheets, often near roof edges. Stay clear of unprotected edges, as WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall of any height could cause serious injury.
- Do not walk on uncured tape and primer. Dirt and footprints stop the bond.
Step by Step
Step 1: Plan the seams so they shed water
Plan the direction of each sheet so the higher sheet laps over the lower, like shingles. Keep seams away from drains and low spots where water collects. Fewer seams mean fewer leak paths, so use the widest sheets that fit.
The overlap needs to be at least 3 inches for 3 inch tape, with the tape centred under the edge of the top sheet. Confirm the lap on the tape manufacturer's instructions.
Step 2: Keep the lap area clean and free of adhesive
When gluing the sheet, the lap area is left unglued, about 3 to 4 inches back from the edge. Tape will not stick to adhesive, dirt or oil. If the area has been contaminated, clean it with the cleaner the manufacturer names, using a clean rag and wiping in one direction.
Weathered or dirty rubber, such as on a repair, needs a stronger clean. See how to repair an EPDM roof.
Step 3: Fold back the top sheet and prime both surfaces
Fold back the top sheet so both surfaces are exposed. Apply primer with a clean pad or brush to both the lower sheet and the underside of the top sheet, in a band at least as wide as the tape, in a thin, even coat. Do not flood it, because pooled primer softens the rubber.
Let the primer flash off until it is dry to the touch, usually 5 to 20 minutes, depending on temperature and humidity. Some products are applied only to the top surface or need no primer at all in warm weather, so follow the tape data sheet.
Step 4: Lay the tape on the lower sheet
Unroll the tape along the lower sheet in a straight line, pressing it down as you go, with the release film still on top. Keep the tape straight and avoid pulling or stretching it. Butt tape ends together and do not overlap them. Leave the release film on until the top sheet is ready.
The tape should land fully under the edge of the top sheet. Some manufacturers want a small margin of tape showing past the edge, so check the instructions.
Tip: Mark the tape line with chalk first. A wandering tape line makes a seam that is narrow in places.
Step 5: Roll down the top sheet and peel the release film
Fold the top sheet back down in short sections. Peel the release film back at a low angle under the sheet, pulling it out as the sheet comes down, and keep the sheet smooth so no wrinkles or air pockets are caught. Do not let the sheet touch the tape before it is aligned, because it cannot be repositioned easily.
Work steadily along the seam, keeping your boots clean so dirt does not reach the tape.
Step 6: Roll the seam firmly over its full width
Roll the seam hard, first across the width of the seam, then along it, to wet out the tape and squeeze the layers together. Pressure activates the adhesive, and a light pass will not do it. Go over the seam edges twice.
Some manufacturers also call for a bead of lap sealant along the exposed edge. Apply it only if the instructions say to.
Step 7: Cover every T-joint
A T-joint has three layers of rubber and a step in thickness, which is a leak path. Cover it with a factory T-joint patch or a cut piece of EPDM with splice tape, extending at least 3 inches beyond the joint in every direction, with rounded corners. Prime the area first, as for a seam.
Keep seams staggered so that T-joints are as few as possible.
Step 8: Inspect every seam
After the tape has had time to set, run a blunt probe, such as the rounded end of a screwdriver, along the edge of every seam. A gap, a fishmouth or an edge that lifts is reopened, re-primed and rolled again, or patched. Check T-joints twice.
Keep a record of the seam layout and inspect again after the first heavy rain.
Common Seam Defects
- Fishmouth: the edge of the top sheet lifts and curls, caused by a wrinkle, poor rolling or contamination.
- Dirty tape: dust or oil on primer or tape prevents bonding and shows as a dull, patchy seam.
- Narrow seam: a wandering tape line or a lap under 3 inches leaves no margin.
- Cold seam: tape applied below its minimum temperature looks fine but peels later.
Temperature and Weather
Splice tape and primer have minimum temperatures, commonly around 5 degrees Celsius, although some products are rated lower with a special primer. Tape that is cold does not wet out properly, so store it warm and use it straight from a warm space. Dew, frost and rain on the seam area stop the bond entirely. In the Fraser Valley wet season, seam days are limited, so plan seams for dry, mild hours.
Common Mistakes
- Skipping the primer. Tape on unprimed rubber may stick at first and peel within a season.
- Touching primed surfaces. Dirt and skin oil stop the bond.
- Lap under 3 inches. A narrow lap leaves no margin for movement or a bad spot.
- Light rolling. Pressure activates the adhesive, and unrolled seams look fine and leak.
- Uncovered T-joints. The stepped thickness is where most seam leaks start.
- Seaming cold or wet. Tape in the wrong conditions never achieves full adhesion.
When to Call a Professional
Seams decide the life of an EPDM roof, and the conditions for making them are narrow. If you have long seams, complex T-joint layouts or a weathered roof that needs seam repair, call a roofer who can match the products and the weather.
If an existing roof has open seams or bubbles along laps, do not patch them with roof cement, because asphalt attacks EPDM. See how to repair an EPDM roof.
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 Seam EPDM With Seam Tape: Questions
How wide should an EPDM seam overlap be?
The sheets are commonly overlapped at least 3 inches, to match 3 inch wide splice tape, with the tape centred under the edge of the top sheet. Some manufacturers call for wider laps with different tape. Confirm the lap and tape width on the manufacturer's installation instructions for your system.
Do you need primer for EPDM seam tape?
Usually yes. Most splice tapes are used with a primer applied to both surfaces of the lap, allowed to dry to a slight tack before the tape goes on. Some newer products need less primer in warm weather, but skipping it on a product that requires it leads to seams that peel within a season. Follow the tape data sheet.
What temperature can you apply EPDM seam tape?
Many tapes and primers have a minimum of about 5 degrees Celsius, and some are rated lower when a special primer is used. Tape should be stored warm, and the seam area must be dry and free of frost. Applying tape below its minimum may look fine but often peels later, so check the product data sheet.
Why is my EPDM seam leaking?
Common causes are dirt or oil on the lap, missing or dried primer, a lap that is too narrow, light rolling, an uncovered T-joint, or tape applied in cold or wet weather. Movement from temperature changes then opens the weak spot. Clean, prime, re-tape and roll the seam, and cover T-joints with a patch.
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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