Spray Foam on Old Bitumen Felt: What Happens?

Bitumen felt is the black sheet that lies under the battens, the thin timber strips your tiles or slates hang from. It is the roof's second line of defence. If wind pushes rain past a tile, the felt carries that water down the slope to the gutter without it reaching the rafters, the inclined timbers of the roof frame.

Spray foam is applied wet, then swells and hardens in place. Sprayed from inside the loft, it fills the space between the rafters and presses against the underside of the felt. It sets into the rough bitumen surface and into the laps, the overlaps where one sheet runs over the next.

Is the felt still sound once foam has covered it?

You cannot tell from below. Old felt is often brittle, split at the laps or sagging between battens long before any foam arrives, and a layer of set foam hides all of that. The felt may have been doing its job quietly for decades, or it may have been failing and now be held up by the foam itself. We only know once the foam is off and the felt is in view.

Taking foam off a surface it has bonded to is slow, careful work. Large areas can sometimes be cut away in sections, while the last layer on the felt is scraped or shaved back by hand. Felt that was already tired does not stand up to that treatment.

Will the felt tear when the foam comes off?

Often in places, yes. Wherever the foam has keyed into the surface, the bitumen coating can lift away with it, and the thin fibrous core underneath can tear along a lap or round a nail. A few small tears are the usual result. A heavily bonded slope can lose a good deal more, and a slope that was brittle to begin with can lose the lot.

So we watch the felt while the foam is cleared and pause to photograph any damage. You see pictures of your own felt, and nothing is guessed. Small surface scuffs can stay. Anything split or lifted has to be treated, because a hole in the felt behind a cracked tile is exactly where a stain on the ceiling begins.

If the roof leaked before the foam was fitted, the felt is the first suspect, and our page on spray foam and roof leaks covers how a leak hides behind foam.

Once the felt is exposed, there are two sensible routes, and the survey photos decide between them.

Can damaged felt be patched?

Sometimes. If only a few laps are torn, or one short length of slope has failed, a new strip of underlay can be slid in under the course above and lapped over the old felt, so water runs over the joints instead of through them. This is partial re-felting. It works when the surrounding felt is still flexible and the battens are sound.

It does not work when the felt is brittle across the whole slope, or when the battens have gone soft. Then the better buy is to lift the tiles on that slope, lay new underlay and new battens, and refit the covering. We would rather tell you that after the survey than patch a roof that will fail again in the next gale.

If a re-roof is already planned, removing the foam first makes the work simpler, and our answer on spray foam and a re-roof sets out the order of work.

Wherever the foam allows, we inspect the felt from below with a torch and a camera and read the tell-tale signs. Daylight showing at a lap, dark water tracking along a rafter, rusty nail heads and a stiff, crumbly texture all point to felt that will not survive removal.

How do you know how much felt will need replacing?

We cannot know it exactly until the foam is off, and any roofer who says otherwise is guessing. What the survey gives is a sound estimate of the likely range, from a few patches to a whole slope. We show you the photos, explain what each one means and the written price comes after the visit. If the felt turns out better than we feared, the work is simply smaller.

Some loft spaces hold foam on the underside of the tiles as well as the felt. That is a harder case, covered in our answer on foam stuck to roof tiles. For the full picture of how removal runs from start to finish, start at the spray foam removal page.