Ask the Experts: How to Join Two Pieces of EPDM

‘How to join two pieces of EPDM” is a question asked at PermaRoof UK for bigger projects. We supply EPDM membrane in a single sheet up to 15.24m x 30m, which reduces and often negates the need for joins on most domestic installations. As a leading flat roofing supplier, what we don’t know about flat roofs isn’t worth knowing… let’s look at the question in further detail to see how to tackle your projects.

Waterproofing joins in EPDM rubber roofing

Firestone EPDM RubberCover systems are designed to keep joins to a minimum, providing large single sheets that are sufficient for most domestic installations. While this is ideal for flat, bare roofing with no extrusions, for more complex roofing projects, the EPDM may need to be cut and joined at some point.

We would most commonly recommend buying the largest sheet size you can to minimise joins. That said, it is straightforward to join two pieces of EPDM should you need to. PermaRoof UK stocks sheet membrane up to 15.24m x 30m in addition to the seam tapes, primer and tools you need to join them.

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Plan for joins and buy materials in advance

When planning your project and buying your materials, it is important to buy everything you need at the same time, allowing for joins in your estimate. Any joins should be sealed as quickly as possible and not left open to avoid potential water, condensation or damp penetrating at these vulnerable points.

Use our free EPDM Kit Builder App to tailor your materials list by simply adding your basic project details and letting the app do the ready reckoning. If you’re unsure about quantities, you can always give us a call and we’ll help you build one.

How to Join Two Pieces of EPDM Membrane

We have two types of tape and the application methods vary slightly. Follow our step-by-step snapshots into joining EPDM membrane or download the full guides (PDF) below.

Joining EPDM with 6” batten strip tape

Install the EPDM and make sure the two pieces fit together as closely as possible along the join. Take care to apply the adhesive right to the edges to ensure both pieces are fully adhered to the roof deck along the entire seam length.

Lay the seam tape out along the length of the join and mark the edges with a piece of chalk or crayon, making the marks extend beyond the tape by approx. 20mm (use a straight edge or timber batten).

Apply QuickSeam Plus primer with the pad and handle within the marked area along the seam. Apply a thin, even coat and avoid puddles.

Allow to go tacky. Important: the QuickPrime Plus must be tacky and stringy to the touch. If it is still wet, it is not ready for the cover strip tape.

Begin laying the seam tape at one end, keeping it taut to prevent bubbles and parallel with the chalk lines.

Use a silicone roller and even pressure to roll over the seam in both directions.

Note: If you have to join the seam tape, overlap the two ends by 50mm and apply a patch of QuickSeam FormFlash over the join.

Joining Two Pieces of EPDM with 3” seam tape

Ensure the leading edge of your EPDM membrane remains unglued, overlapping the lower sheet by at least 100mm and has a straight edge along the entire length of the seam.

Position and mark with a crayon 18mm from the leading edge of the overlap. Flap back the overlapping edge and weigh down if necessary to keep it back.

Apply QuickSeam Plus primer with the pad and handle within the marked area along the seam to 100mm. Apply a thin, even coat and avoid puddles. Also apply the primer to the back of the overlapping membrane.

Allow to go tacky. Important: the QuickPrime Plus must be tacky and stringy to the touch. If it is still wet, it is not ready for the seam tape.

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. Stay within the marked line. If you need to cut or run out of tape, join by overlapping the two edges by at least 50mm.

Use a silicon roller to ensure the tape has fully adhered to the membrane.

Flap the leading edge back, and make sure there is approximately 18mm distance to the brown paper edge of the tape – trim the membrane where necessary.

Starting from one end, lift the leading edge and peel the backing paper towards you. Let the primed leading edge fall onto the tape, flattening down with your hand whilst peeling.

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

Download your installation guides:

Watch: Video Guide

The video guide below demonstrates how to join two pieces of EPDM membrane using the 6″ cover strip tape.

Here at PermaRoof UK, we support all our customers and answer a variety of flat roofing questions every day. If you have a question, please visit our FAQs page, give us a call on 01773 608 839 or use the live chat for a quick query.

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