Can Spray Foam Be Applied Over Roof Felt?

What roof felt does under the tiles

Underlay is the second line of defence on a pitched roof. The tiles or slates shed most of the rain, and the felt beneath catches wind-blown spray and any water that gets past. Traditional underlay is bitumen-based felt, laid over the rafters in horizontal strips with overlaps, then held down by the battens. It often has a service life shorter than the tiles above it. On roofs from the mid twentieth century, felt may be cracked, torn at the eaves, or missing in patches, because it was never meant to last as long as the covering.

That condition is the point. Many owners are told that foam will "seal" an old roof, but it can only seal what is already there. Torn felt means water already reaches the rafters, and the foam holds it against the timber. Our page on re-roofing a spray foam roof shows what that leads to later.

Felt that is not breathable

Bitumen felt blocks vapour as well as liquid water. In a cold loft this is dealt with by leaving gaps and ventilation at the eaves and ridge, so air carries the moisture away. Foam applied hard against bitumen felt takes away the air and leaves a vapour-tight layer on the cold side of the insulation. Any warm humid air that reaches the foam surface cannot dry out into the roof space, and any condensation on the felt's underside has nowhere to go. The Trading Standards guidance on spray foam notes that reduced roof-space ventilation raises the risk of condensation, mould and potentially timber rot.

Breathable membranes are a different product, and they raise a different question, covered on our page about spray foam and breathable membranes. If you do not know which kind you have, you cannot judge the risk. The only safe approach is to find out.

Checking before foam goes on

Before spraying, a competent installer or roofer inspects the felt from inside the loft. Is it intact, or torn and sagging? Are there stains on the rafters that suggest water has been coming in? Are the battens sound? Are there light gaps where tiles have slipped or are missing? Are the eaves open, or has the felt dropped into the gutter? Any of these should be repaired first. Spraying is permanent, and an unrepaired leak is the most common reason foam goes wrong.

It is worth asking for moisture readings on the rafters before the foam is applied, and a record of them. If the timber is wet, it should be dried or replaced, and the leak found. That record can also be useful later if a surveyor or lender asks questions. Our page on spray foam paperwork lists what to keep.

If foam is already on top of old felt

Many homes have exactly this. The foam is stuck to the felt, and the felt is stuck to the rafters, and the whole thing is hidden. You cannot lift the felt to look, and a tile that slips or a flashing that fails can feed water into the layers for years. Signs are the same as for any foamed roof: a musty smell, stains on ceilings, mould at the eaves, slipped tiles outside.

We survey from outside and inside, photograph the covering, flashings and any exposed felt at the eaves or verge, and read the timber with a meter where we can reach it. If the roof is dry and the covering sound, the foam can stay. If the felt is perished and the timber damp, the repair usually means removing the foam, replacing the felt and battens, fixing the covering and re-insulating, as we describe on our page about spray foam removal.

Some owners ask about spraying foam over the top of the felt, onto the outside of the roof before the tiles go back. That is a different arrangement and not one we see done well. Foam above the felt sits where water arrives, bonds the tiles in place so they cannot be lifted, and gives no way to inspect or replace anything. A conventional rigid board above the rafters, under a new membrane and battens, is the sounder way to build a warm roof when a roof is stripped.

Timing helps in a re-roof. The felt is replaced as a matter of course when tiles come off, so there is a natural moment to put right whatever the old one hid. It is far cheaper to do that once than to open a foamed roof later.

Our advice

Do not foam over old felt without repairing it. If the roof covering is at the end of its life, deal with the roof and then choose the insulation, because both together are far cheaper than one after the other. The wider service is described on the spray foam page, and a survey gives you a fixed price for whatever the roof needs.