Water-based vs contact adhesive for EPDM: which goes where

A typical EPDM rubber roof uses two adhesives: a water-based deck adhesive and a contact adhesive. They look similar in the tin but they work in completely different ways, and they go in different places. Using the wrong one, or using the right one the wrong way, is one of the most common reasons a DIY rubber roof fails.

At a glance

	Water-based deck adhesive	Contact adhesive
Where	The main flat area of the roof	Perimeter, upstands, walls, trims, corners
Surfaces	Timber decks only (OSB3 or plywood)	Timber, masonry, metal and other surfaces
Applied to	The deck only	Both the deck and the membrane
Lay the membrane	While the adhesive is still wet	Once both coats are touch-dry
Repositionable?	Briefly	No, it grabs on contact

Water-based deck adhesive

This bonds the membrane across the main area of a timber deck. It's applied to the deck only, and the membrane is laid into it while it's still wet. The water then escapes through the timber as it cures, which is why it only works on porous timber decks and not on metal, old GRP or existing roof coverings.

How to use it:

1. Lay the membrane out, let it relax, then fold half of it back.
2. Roller an even coat of adhesive onto the exposed half of the deck. Don't flood it.
3. Fold the membrane back onto the wet adhesive straight away, working from the fold outwards and brushing out air as you go.
4. Repeat for the other half.

It's sensitive to cold and damp. Manufacturers say not to use it below about 4–5°C, on a damp deck, or if frost is forecast while it cures. For very large roofs, manufacturers usually specify contact adhesive throughout, so check the guidance for your membrane.

Contact adhesive

This is used wherever the membrane needs a stronger, faster bond: the perimeter of the roof, upstands, walls, edge trims and corners. It's solvent-based, so it bonds to non-porous surfaces that water-based adhesive can't.

How to use it:

1. Apply a thin, even coat to both surfaces: the deck or wall and the underside of the membrane.
2. Let both coats flash off until they're touch-dry. Press a knuckle on: it should feel tacky but not leave adhesive on your skin.
3. Carefully bring the two surfaces together. It grabs instantly, so line it up first.
4. Roll the area firmly to make full contact.

Apply it in a band around the whole perimeter of the deck, about 150mm wide, as well as on all upstands. This holds the edges firm as the membrane moves with temperature changes, and stops it pulling away from the edges.

The mistakes we see most

• Using deck adhesive on upstands. It needs to dry out through the timber, and vertical surfaces and masonry don't let it. Upstands always take contact adhesive.
• Letting water-based adhesive dry before laying. It's a wet-lay product. If it skins over, the membrane won't bond.
• Not waiting for contact adhesive to flash off. Bond it too wet and the solvent gets trapped, leaving blisters and a weak bond.
• Putting contact adhesive on one side only. It needs both surfaces coated to work.
• Working in the wrong weather. Damp decks, dew and frost spoil both adhesives. See fitting EPDM in autumn or winter.

Always follow the instructions on the tin for the specific product you're using, as coverage, drying times and temperatures vary.

How much do you need?

It depends on the roof area and the length of the perimeter and upstands. Our EPDM roof calculator works out the right quantity of each adhesive for your roof, along with the membrane and trims. Start with our guide to measuring your roof, or see the full EPDM installation guide.

Questions about your roof? Call Tom on 07949 862834 or email hello@supraseal.com.