The short answer
Roof timbers need a specialist look when you find soft or crumbling wood, widespread flight holes with fresh dust, fungal growth, or damp that keeps returning. A roofer can spot the signs and photograph them from the loft, but identifying the cause and treating it is a specialist's job. Most timber is sound, and a few flight holes in old wood are not always active.
What the timbers do and why they fail
The roof structure is a frame of timber. Rafters run from the ridge to the wall plate along the eaves, purlins and collars brace them, ceiling joists tie the feet, and battens carry the slates or tiles. On an older house those timbers may be hand-cut oak or softwood that has carried the roof for generations. On a newer house they are machine-cut trusses.
Timber fails when it stays damp. Wet wood invites fungal decay, commonly called rot, and it also becomes attractive to wood-boring insects. The usual source of that damp is a leak: a slipped slate, a failed flashing, a blocked valley or a gutter that has overflowed against the wall plate. Condensation in a poorly ventilated loft can do the same, more slowly.
So the first rule of timber is to look for water. Dry timber that has never been wet tends to last a very long time, and the cure for most timber trouble is fixing the leak that caused it.
Signs a roofer can see from the loft
Many warning signs are visible with a torch. Darkened or stained rafters near a chimney or valley, a musty smell, white or grey strands on wood, and cracks running along the grain are all worth recording. A blunt probe eased into the surface shows whether the timber is firm; soft, spongy timber that crumbles into small cubes points to decay.
Insect damage shows as small round flight holes, and sometimes as fine, pale dust on or beneath the timber. A scatter of holes in old wood does not prove a current problem, since the insects may have died out long ago. Fresh, bright dust and clean-edged holes suggest recent activity and deserve a closer look.
We photograph what we find and note where it sits, so you can follow the report. Our page on roof inspection access explains why the loft hatch matters so much to this part of a survey.
When timbers need a specialist look
Some findings go beyond a roofer's remit. If the decay seems extensive, if it affects load-bearing members such as a purlin or the ends of rafters, or if the cause is unclear, a specialist is the right next step. Likewise if you see strong signs of active insect infestation across many timbers, or a fungal growth you cannot identify, someone who treats timber and damp for a living, or a structural engineer, can say what it is and what to do.
That specialist can identify the species of insect or fungus, test moisture levels, judge whether a beam is still structurally sound and advise on treatment or replacement. A roofer is not the best person for that job, and a good one will say so.
The same applies to old houses where timbers have been altered, notched for pipes or cut for a loft conversion. Whether the structure is still carrying its load is an engineering question, and the cost of getting it checked is small compared with a failure.
Spray foam, insulation and timber
Insulation can hide timber. Foam sprayed onto the underside of a roof coats the rafters and boards, so a later inspection struggles to see anything. The Energy Saving Trust, in guidance last updated 8 May 2026, notes that roof timbers can rot if the roof leaks or condensation occurs, and advises against foaming a damaged roof before the fault is mended. RICS also issued a consumer guide in March 2023 that covers condensation checks before installation.
Lenders have taken different positions too: reporting in November 2024 found some refusing homes with spray foam while others judged each case separately. If your loft has been foamed, mention it before the survey, because it limits what can be seen. Our guide to what to do when a survey flags your roof covers the wider question of how to respond to a report.
Blown mineral wool or quilt laid over ceiling joists is easier. It can be lifted aside in places to check the timbers beneath.
Flat roofs, leaseholds and shared structures
Flat roofs carry their own timber risk, because the joists and decking sit directly beneath a waterproof layer. If the membrane fails, water gets in and has nowhere to go. A soft patch underfoot, a sagging ceiling below or staining at the edge of a deck are all clues. The covering may have to come off to check the structure.
Leasehold flats complicate matters, because the roof structure usually belongs to the freeholder and any timber work may be a shared cost. Our page on who is responsible for the roof of a leasehold flat explains where responsibility normally lies, and who to approach first.
Terraced houses can have timbers that run through the party wall, so a problem at one end can affect the neighbour. That is another reason to document what you find and talk to the people next door early.
What happens after the findings
Once timber trouble is found, the sequence matters. First stop the water, since there is little point treating wood that will get wet again. Then have the timbers assessed, and only then decide whether to treat, repair with new sections or replace. Our page on what to do after a roof report helps you sort the findings into a sensible order.
Our survey pictures go out with a written quote. For timber, matching the job to the fault often means mending the leak and cutting out a damaged section, not rebuilding the roof, and a 10-year workmanship guarantee, in writing, covers what we carry out.
If you have seen signs in your loft and want them looked at, ring 01793 626096 or send a WhatsApp saying what you noticed and where. The wider service is set out under roof inspections and surveys.