Can spray foam be removed from a breathable roof membrane?

Beneath the covering of tiles or slates, a second layer is laid across the rafters. That underlay is there for the rain and snow that blows in past the covering. On older roofs it is usually bitumen felt. Newer roofs, and plenty of re-roofs, use a breathable membrane, also called a breather or low-resistance underlay: a fabric that sheds liquid water yet lets water vapour from the house escape through it.

The vapour part is the whole point. Warm, damp air rises from kitchens, bathrooms and bedrooms into the loft, and a breather gives it somewhere to go, out through the batten space and the covering, so the rafters stay drier. Marley's summary of BS 5250 treats a breather underlay over a well-sealed ceiling as needing only modest eaves ventilation. That design assumes the fabric is free to do its job, which is exactly what a layer of foam interferes with.

Foam sprayed from inside the loft lands on the underside of the membrane and on the rafters. It expands into every gap and sets hard or spongy depending on the type. Once cured, it holds the fabric across its whole face and closes off most of the vapour route through it. The membrane stays in place but stops behaving like a breather.

Two sources explain why that worries surveyors. The Energy Saving Trust warns that rafters may rot if water gets past the covering or moisture condenses on cold timber, while calling the danger far smaller with the softer, open-cell product. RICS, in its consumer guide, links badly installed insulation to dampness such as uncontrolled condensation. Neither says every foamed roof is failing. They do explain the close look a membrane stuck to foam gets in a survey.

The method follows the product that was used. Rigid closed-cell foam bonds firmly and is cut away in blocks and then scraped. Softer open-cell foam can often be pulled or shaved back, although it leaves a fibrous residue on the fabric and on the timber beneath.

Either way the membrane is the weaker partner. Fabric glued to foam stretches, splits or peels away with it. Even when it looks whole from below, the fibres are usually clogged with residue, and a patch never quite matches the original. We therefore plan on renewing the underlay across the affected area, not on saving it.

Renewal means taking off the tiles or slates over that area, laying new membrane over the rafters, adding fresh battens where the old ones are damaged and relaying the covering. Where the roof is already tired, that visit is a sensible moment to judge the whole covering. Our page on spray foam and a new roof explains when the faults run beyond the underlay.

A membrane job is priced after the survey and never before. We go up into the loft, photograph the foam, the rafters and any spot where the underlay shows or has started to lift, then check the covering, ridge and eaves from outside, because those show where air and water were meant to travel.

• How much of the roof is foamed, and whether the layer is continuous or in patches.
• Which kind of foam was used, the soft open-cell type or the dense closed-cell type.
• The state of the rafters and battens, including dark staining or soft spots.
• The age and type of the tiles or slates, which decides whether they can be reused.
• Access for scaffolding, which often forms a big part of the bill.

For orientation, MyBuilder's spray foam removal guide, updated August 2026, gives typical national figures of £20 to £50 per square metre for removal, £500 to £1,500 for scaffolding and £500 to £3,000 for repairs and new insulation. Those describe the country, not your roof.

For anyone buying or selling, the membrane question turns up beside a lender or surveyor query. RICS notes that foam may influence a valuer's figure, and it tells owners to put the question to their lender and insurer. A November 2024 press report named several lenders that declined such homes, and others that weighed each case on its own facts. Positions shift, so ask the lender directly for its current one.

Our guide buying a house with spray foam lists the questions to put. Owners thinking of releasing equity will find the same issue covered in spray foam and equity release. Foam on a flat roof behaves differently again, and spray foam on a flat roof explains why.

Cutting foam off a membrane makes dust and debris, and the loft and the route to it need protecting. Our page on how messy spray foam removal is explains how we protect the house and clear as we go. The wider service sits on our spray foam insulation removal page.

You receive photographs of your own loft and roof, and a written quote follows them. If the membrane is sound in the places the foam never reached, we say that and leave it alone, because we only want to do what the fault demands. Afterwards we sweep out and remove every bag of waste, and the finished work carries a 10-year workmanship guarantee in writing. Ring 01793 626096, message us on WhatsApp, or send a photograph of the underside of the roof through the enquiry form.