What happens when spray foam is sprayed onto felt or tiles?

Spray foam goes on as a liquid, then swells and cures where it lands. Sprayed upwards from inside a loft, it pushes into every opening it can find between the rafters. The laps, where one sheet of underlay overlaps the next, are exactly that kind of opening, so foam often ends up on both faces of the felt and fixes it to the timber and to the battens above.

Underlay is meant to stay a loose second layer. It carries away whatever water slips past the tiles and it moves a little with the building. Once foam has set through the laps, that layer is a fixed part of a solid mass. Pulling at one sheet drags the next, and old bitumen felt, which can turn brittle with age, tears instead of peeling.

That is the main reason foam on felt is treated as a separate job from foam on plain timber.

Where the felt is missing, torn or sagging, foam can travel on to the tile or slate undersides. It sticks to clay, concrete or stone, and to the battens carrying them. A tile that slips afterwards cannot be lifted and slid back as normal, because the foam grips it and the tiles around it.

Repairs suffer first. A roofer needing to swap one cracked tile finds it welded to the mass below, and lifting it snaps it, often along with its neighbour. Even routine jobs such as renewing a ridge tile or a hip become slower and riskier when the layer beneath is solid.

Removal suffers too. Foam clinging to a covering rarely lets go cleanly, and some tiles will break as they come up. The survey should therefore say whether replacements to match are easy to buy, because handmade clay and old slate are far harder to source than modern concrete.

In an open loft a leak shows as a stain or a drip, and a roofer can follow it back to its source. Under foam it hides. Water passing a cracked tile runs down the felt and into the foam, which can soak it up, press it against the timber and let it creep sideways long before a ceiling shows anything.

The Energy Saving Trust, in guidance updated on 8 May 2026, advises against foaming a roof that leaks until the roof has been repaired, and notes that the danger should be lower with open-cell foam. That describes a risk, not a clean bill of health for open-cell foam.

If you cannot be sure the roof was sound before the foam went on, it is worth finding out. Our guide on spray foam on a leaking roof explains how a leak that was foamed over is traced and fixed.

An ordinary roof space is ventilated, so humid air rising from the house drifts out at the eaves and ridge. Foam that packs the eaves or seals against the felt shuts those exits. RICS names dampness among the risks of insulation fitted wrongly, uncontrolled condensation in particular, in its March 2023 consumer guide.

The harm falls on timber, not tiles. Rafters, battens and the felt's underside sit behind the foam, out of view, and nobody watches them stay wet. Dark staining, a white bloom of mould or a softened patch of wood shows only when the foam is opened.

That does not make every foamed roof damp. It means neither the street nor the hatch can tell you which sort you own. The test is probing the exposed wood and reading what it says. Our answer on spray foam and condensation explains how the moisture builds.

If the foam sits on bare rafters beside the felt and the underlay is loose, it may come out through the hatch with the covering untouched. Once the foam has gone through the laps or up against the tiles, the roof is opened from outside in stages. Foam and glued felt are cleared, the wood is checked, and fresh underlay and battens go down before the tiles return.

Staging matters because an open roof is a risk in its own right. Each stage is closed before the next begins, and a forecast of driving rain postpones the work.

You see pictures from inside and outside the roof along with the written quote, which carries one total price once the survey is done. They show how far the foam has travelled, so you can understand why one slope is lifted and another left alone. The overview of the service sits on our spray foam insulation removal page.