The short answer
Fix the roof first, then judge the insulation by type and by how long it was wet. Mineral wool that was briefly damp can often dry out if the loft is ventilated, while compressed, stained or mouldy material, and anything soaked for weeks, should be replaced. Boarding and timber need drying and checking too. Insulation laid over a leak that is still active will only get wet again.
Why the roof comes before the insulation
It is natural to want the soggy material out of the loft straight away. Resist laying anything new until the cause has been found and repaired. Fresh insulation under an active leak just soaks up water, and the timber beneath stays wet behind it, out of sight.
The order is simple. Stop the water entering, either with a lasting repair or a safe temporary cover. Let the loft air out. Then decide what stays and what goes. That decision belongs after the roof work because the roofer will need to see the underside of the covering, the rafters and the felt, which loose insulation may be hiding.
If you are not sure where the water came in, our guide to leaks and ceiling damage covers the plaster and the rooms below, while this answer deals with what sits above them.
What happens to wet insulation
Mineral wool, the common glass or rock fibre quilt, is made of fibres that do not rot in themselves. Wet, though, it slumps and loses its trapped air, so it insulates poorly until it is dry again. Briefly damp quilt on a warm, airy loft often recovers. Quilt that has been wet for weeks, or soaked and then squashed under boards, tends to stay flattened and may hold stains, smells or mould.
Loose-fill materials such as blown fibre or cellulose behave differently. Cellulose is made from paper and absorbs a lot of water, can clump, and takes much longer to dry, so we treat any wetted patch of it as a candidate for removal. Rigid foam boards shrug off surface water but let it pool in the joints and behind the board.
Spray foam is a separate case. Where it coats the underside of the rafters it cannot simply be lifted, and it can hide moisture in the timber behind it. Ask your surveyor and insurer before deciding, and see the RICS consumer guide of March 2023.
When it can dry out
You have the best chance if the leak was small, found quickly and fixed soon after. In that case lift the quilt away from the wet area, shake it out and lay it somewhere airy, or at least spread it so that air reaches both faces. A dry, ventilated loft helps; a sealed loft full of damp air does not. Ventilation at the eaves matters, and our work follows the guidance of BS 5250 on keeping a cold roof space aired.
Check by touch and by sight. It should feel dry through its full thickness, spring back when pressed, show no brown or tide marks and smell of nothing in particular. If it fails any one of these, it is a poor candidate for reuse. A moisture meter on the timber beneath tells you more than a glance at the surface.
Do not use a heater to hurry it, and take care with any cable running through damp material.
When it must be replaced
Replace insulation that is mouldy, discoloured, foul-smelling, thoroughly compacted or soaked for a long time. Replace anything contaminated by dirty water, for example from a tank overflow, a bird or squirrel nest, or sooty chimney run-off. Treat any wet cellulose patch with caution as above.
Wear gloves, a long-sleeved top and a mask when handling old mineral wool and mouldy material, bag it and take it to the tip, as it is household waste. We clear it away when we carry out the work. Where asbestos might be present, say in old sprayed or lagging material or in cement sheeting, stop and have it identified first; HSE guidance applies and it should not be disturbed.
The replacement is a good chance to bring the loft up to a better thickness. Loft insulation put into a home escapes VAT until 31 March 2027, but the repair work around it is charged at 20%, and the quote makes that clear.
Boarding, rafters and the timber underneath
Boarding can trap damp against the joists. Lift a few boards near the wet area and look at the timber beneath. Dark staining, soft spots you can press a screwdriver into, white fungal strands or a musty smell mean the wood has been wet long enough to be a concern. Chipboard loft boarding swells and breaks down when it gets wet, and usually has to be replaced, while solid timber boards can be dried and reused.
Look at the rafters near the leak, especially where they meet the wall plate, as that is where water collects. Surface mould can be cleaned off dry timber. Rot that has softened the timber is for a surveyor or carpenter to assess, and may call for cutting out and splicing in new wood. We note anything of that kind when we survey, and photograph it.
It is one more reason not to leave a small leak alone for long; our answer on whether roof repairs are worth it sets out how early repairs compare with waiting.
Finding the cause and sorting the order of work
The cause can be a slipped tile, failed lead, a blocked valley, an overflowing gutter or condensation, so it pays to establish which. Our page on what damages roofs lists the usual suspects. A survey typically costs £150 to £500 according to MyBuilder in 2026, and ours records the covering, the underlay and the loft with photographs.
Once the survey is done we put the repair, any scaffold, the insulation and the clearance into one written quote. If you want a plan, it runs: repair the roof, ventilate the loft, remove what cannot be saved, check timber and boards, then re-insulate.
To talk it through, ring 01453 933613, or send a WhatsApp with pictures of the loft, or use the enquiry form. Our roof repairs page covers the work we do, and our workmanship comes with a written guarantee of 10 years.