The short answer
Yes, spray foam can lead to rot in roof timbers, though it does not always. The RICS consumer guide warns that foam can trap moisture against timbers and make the roof hard to inspect. If water gets in from a leak, or condensation forms because the roof can no longer ventilate, the wood cannot dry through the foam, and prolonged dampness is what allows fungal decay to start.
How foam traps damp against wood
A traditional loft is a cold roof. The insulation sits on the loft floor, air moves under the tiles, and the rafters and battens dry out after any wetting. Spray foam changes that by sealing the underside of the roof covering, which turns it into a warm roof. If the roof is dry and the foam is well applied, the timber can sit happily within it. The trouble starts when moisture finds a way in, because the foam stops it leaving.
Moisture arrives in two ways. One is a leak: a slipped tile, a cracked slate or a failed flashing lets rain onto the felt and down onto the rafter, and the foam holds it there. The other is condensation. Warm, humid air from the house gets into the roof space through gaps, meets the cold underlay and condenses. With no air movement behind the covering, the water stays. Trading Standards guidance has noted that foam can reduce roof-space ventilation, which raises the risk of condensation, mould and potentially timber rot.
Why you cannot see it
The RICS guide makes a point that surprises owners: timber can look dry on its visible face and be wet behind the foam. A damp rafter is only dangerous if the dampness is sustained, but you cannot tell whether yours is, because the foam covers the face you would normally inspect. A surveyor with a moisture meter can read the exposed edges. Probes pushed through the foam give some indication, though they only sample a few points.
That uncertainty is why some buyers, surveyors and lenders react strongly to foam in a loft. Our page on spray foam and mortgages covers how that plays out when a house is sold or remortgaged.
Not every foamed roof is a problem. A roof that was dry and well ventilated before the foam went on, with a covering in good order, is in a very different position from a leaky roof with an existing damp patch sprayed over. The age and type of timber matter too: old, dense softwood rafters tolerate occasional damp better than thin modern ones, and a rafter that has already been attacked by woodworm or old rot starts from a weaker place. The question for any individual roof is whether the timbers are sound now and whether anything is feeding water into them.
Warning signs to look for
From inside the loft, look for a musty smell, dark mould spots on the foam surface or on any exposed timber, a damp or crumbling patch at the eaves, and nails or fixings showing orange rust. From downstairs, look for stains on the bedroom ceiling that appear in cold weather, condensation on the loft hatch and mould in the corners of upstairs rooms. On the outside, slipped or cracked tiles above the stain are a sign that water is getting in. None of these proves rot, but several together justify a proper survey, and our guide to spray foam insulation problems lists the others.
Soft rafters that crumble when pressed, a spongy feel under the foam or a sour smell are signs of advanced decay, and a surveyor should look at them promptly.
Do not try to strip foam yourself to look behind it. Cured foam sticks hard to timber, tiles and felt, and pulling it away can tear the underlay and damage the surface of sound wood. It also generates a lot of dust and offcuts that need proper disposal.
What a survey and a fix look like
Our free survey begins with photographs of what is visible: the tiles, the flashings, the foam surface and any exposed timber ends. Where we need to know what is behind the foam, we can test with a moisture meter and open a small inspection area, and we show you the result. If the timbers are dry and the roof is sound, we tell you so and leave the foam alone. If they are damp, the fix starts with the source of the water, then removing the foam from the affected area, drying and treating the timber, replacing anything that has lost its strength, and adding ventilation or alternative insulation.
Removal and a ventilation check are part of the usual remedy, according to the RICS guidance. The price is fixed and written down once we have seen the roof. You can read about the wider service on our spray foam insulation removal in Basingstoke page.
Every roof is different. A free survey with photos settles it for yours.
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