Rubber underlay epoxy issues

Question

We have a concrete floor with underfloor heating. The concrete is dry. We are having issues when laying the glue down rubber underlay as it expands when bonding with the epoxy or curls up unexpectedly/unevenly in places and the exposure the drains into the space and makes ridges. How do we stop this from happening? We are installing a hardwood glue down floor over it.

Ibrahim · Answer

This usually happens because of either excessive epoxy spread, substrate temperature variation from the underfloor heating, or the rubber underlay not being fully acclimatised before installation.

To prevent this:

• Switch off underfloor heating at least 48 hours before installation
• Ensure the rubber underlay is fully acclimatised in the room for 24–48 hours
• Apply the epoxy evenly with the correct notched trowel and avoid over-application
• Roll the underlay immediately after laying to remove trapped air and ensure full contact
• Check the concrete is perfectly level and free from low spots around drains, as excess adhesive will naturally pool there and create ridges
• If needed, use a skim/self-levelling compound first to eliminate uneven areas before bonding

The curling/expansion is typically caused by adhesive reaction combined with temperature/moisture imbalance or uneven adhesive distribution.

Aj · Answer

Dear Sir/madam

On glue down products a specially on hardwood 
Underlay is not suitable 
You should be advised to purchase the correct glue 
Use good quality Primer first 
Let it set 
Spread your glue accordingly then start laying 
Insure that you use tape over the board to stop them from moving as you lay the floor
Best wishes 
AG 
Kevin’s floors

İlya · Answer

This is a rubber underlay/adhesive compatibility issue.

Here’s how to fix it:

1. **Check adhesive compatibility** — not all epoxy adhesives are suitable for rubber underlays. Use a pressure-sensitive adhesive (PSA) specifically rated for rubber.

2. **Acclimate the rubber** — let it lay flat and unrolled in the room for 24–48 hours before gluing. This releases built-in tension and prevents curling.

3. **Roll it properly** — after laying, use a 100kg floor roller to press the entire sheet down firmly and evenly.

4. **Work in sections** — apply adhesive in manageable areas, let it flash off to the correct tack level before laying the rubber (don’t lay into wet adhesive).

5. **Consider skipping the underlay entirely** — with a glue-down hardwood floor over a dry, flat concrete slab with UFH, many manufacturers actually recommend gluing **directly to the concrete** with a flexible wood flooring adhesive. The underlay can cause instability and is often unnecessary.

**The most likely culprit** is either laying into wet adhesive before it’s flashed off, or the rubber not being acclimated — causing it to move as it relaxes.

Milan · Answer

The rubber underlay is most likely reacting to either moisture, too much adhesive, heat from the underfloor heating, or an uneven floor.
To stop it happening:
Turn the underfloor heating OFF before laying.
Make sure the concrete is fully level and smooth first.
Use less adhesive and the correct trowel size.
Use a primer/moisture barrier on the concrete before gluing.
Let the rubber underlay acclimatise in the room for 24–48 hours before installing.
Roll the underlay flat with a heavy roller immediately after laying.

Bobby · Answer

you are not meant to glue down LVT flooring as it already has underlay built into it , the glue and the built in underlay do not stick together

Andrew · Answer

Hi I would suggest a screed a high temp reinforced screed this should help with your issues.
Many thanks Andy.

Rubber underlay epoxy issues

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