How to Join EPDM Using 3” Seam Tape

In this installation guide, we will demonstrate how to join two pieces of EPDM together using the 3-inch seam tape.

Most domestic EPDM system installations can be completed with the membrane in a single sheet up to 15.24m x 60m. If you do need to make a seam join, this is a fast and straightforward way to do it.

Joining EPDM Using 3” Seam Tape

What you will need:

EPDM 3“ Seam Tape
Silicone Roller
White Crayon or Chalk
Straight Edge (a timber batten is ideal)
QuickPrime Plus primer
QuickScrubber Pad and Handle kit
QuickSeam FormFlash patch if creating a 'T-section'

Buy 3-inch seam tape and find all your install essentials at PermaRoof UK Store. Fast UK delivery.


Step 1

Ensure the leading edge overlaps the lower sheet by at least 100mm, and has a straight edge along the entire length of the seam.



Step 2 

Position and mark the sheet with a white crayon. The line should 18mm from the leading edge of the overlap, all the way down the seam. An 18mm timber batten is a useful tool for this.



Step 3

Flap the leading edge away from the line. It may be necessary to keep it secured with various items so it will not flap back onto your working area.



Step 4

Apply Quickprime Plus with a scrubber pad and handle, from the marked line and at least 100mm wide. Apply Quickprime Plus to the overlapping membrane also. Apply all the way down the seam avoiding puddles by spreading it out evenly.




Notes: Wait for the Quickprime Plus to go tacky. This is especially important, as the tape will not adhere to it if it is in a wet state. Use the “pushed finger” test, and if the finger moves through the Quickprime Plus or is stringy in any way, it is not ready.



Step 5

Once the Quickprime Plus is dry, apply the tape to the primed area. Lay the edge of the roll of tape parallel to the marked line, keeping the tape taut to prevent air bubbles being caught underneath it.

Notes: If the tape veers off at an angle, cut it, peel the backing paper and overlap the tape by at least 50mm. Continue to lay the tape. This ensures you stick to the marked line. This also applies if you run out of tape, and have to start a new roll. Use the silicon roller to ensure the tape has adhered properly to the membrane.




Step 6

Flap the leading edge back, and check the distance to the brown paper edge of the tape. This should be around 18mm. Trim the top leading edge if necessary.



Step 7

Lift up the leading edge, and starting from one end of the tape, peel the backing paper  towards you, and let the primed leading edge fall onto the tape, flattening down with your hand whilst peeling. 



Step 8

Using a silicon hand roller, roll the entire seam thus eliminating any air pockets. 



The finished seam will look like this.



Notes: A T-piece detail is required whenever the end of a seam needs to join an adjacent membrane as shown below. 




An excess of membrane from underneath the original seam will need to be cut back at a 45º angle. If this is not done, it will leave an unsealed entrance point for water ingress. 

Step 9

Cut away the excess membrane as shown.





Step 10

Do the seam join as usual and use the rollers to firmly press down the seam.




Step 11

This area now needs to be reinforced with a patch of Quickseam Formflash, of size 150mm x 225mm. This covers the necessary 75mm from the weak points. Mark the area to be primed.



Step 12

Apply Quickprime Plus using the pad, exceeding just beyond the crayon lines.



Step 13

Apply the Quickseam Formflash, and use the rollers as normal.



The finished T-piece should look like this.




More guides from PermaRoof UK

PermaRoof UK offers support for EPDM application, whether for trade roofers or for the DIY market. The resource library contains an extensive range of EPDM installation guides in downloadable PDF format and video tutorials with step-by-step instructions given by qualified and experienced EPDM specialists.

Visit the Knowledge Base >

Become an expert with our EPDM training days

We run accredited EPDM training days for both sheet membrane application and liquid waterproofing. EPDM application training days are held weekly at the Permaroof headquarters in Derby at our purpose-built, approved training centre. Suitable for both trade and DIY flat roofers, the one-day courses offer industry-leading training and certification.

Visit our corporate site to find out more about training >