A rafter is a sloping beam that carries the roof covering, and the roof boards or battens are the timbers fixed across them. When foam has been sprayed over these, nobody can see or probe the wood. An inspection of roof timbers under spray foam is the process of finding out what is going on beneath the foam, in a way that tells the owner something solid without tearing the whole roof apart.
This guide describes the stages we follow in homes around Stroud, the Five Valleys and the villages beyond: a look from the loft, careful sample openings in the foam, readings of the moisture in the wood, and a written report with photographs. It also explains what an inspection cannot prove. The first steps after a surveyor raises the issue are covered separately in our answer on a surveyor flagging spray foam; this page is about the inspection itself. It forms part of our spray foam insulation removal service.
What an inspection has to answer
The question behind every inspection is plain: is the structural timber under this foam sound, and is it dry? Everything we do is aimed at answering that with evidence rather than a guess.
It matters because of how foam can fail. RICS warns that badly fitted foam may cause damp, uncontrolled condensation included, and asks for a qualified person to run the heat-loss sums and the condensation risk check ahead of installation. If nobody did those checks, or the paperwork has gone missing, the roof has never been tested against the risk. An inspection supplies the missing test after the event.
Three possible findings
Broadly, an inspection leads to one of three conclusions. The timbers are dry and sound, and the foam can be left in place with the evidence on file. The timbers are damp but undamaged, which means the cause is found and fixed, and perhaps the foam is taken off the affected area. Or the timber shows decay, which means repair work and probably removal of the foam around it. The report should say which of these applies and where, so that you can decide what to do and your buyer, lender or insurer can see the reasoning.
An inspection is also useful when nothing looks wrong. Foam does not announce itself when it begins to cause trouble, so a clean result on a roof of unknown history is real information, and it can be kept with the deeds.
Starting in the loft, before anything is cut
Our first visit is observation only. We climb into the roof space with a torch, a camera and a probe, and we look at the foam as a whole before touching the surface.
Clues in the foam
We look for dark staining that spreads past the edge of the foam, sagging or ballooning sections, foam that has pulled away from the wood to leave a gap, and any patch that has gone soft or crumbly. A musty or sour smell tells us moisture has been present. Rusted nail tips poking through, or nails with a dark halo, suggest damp has reached the roof boards.
Clues in the building
The roof tells us where water is likely to enter. Chimney stacks, valleys, abutments with a neighbouring wall, roof windows and the area behind a bathroom extract fan all get special attention. At the eaves we check whether the foam runs right down onto the wall plate, the horizontal timber that sits on top of the wall, because that is a common place for hidden damp.
From the outside in
We also look at the roof from outside, or by drone where access is awkward, because a slipped slate or a cracked flashing above the damp patch is often the story. The survey photographs go with the report. MyBuilder's 2026 inspection guide lists general roof inspections at £150 to £500, with drone surveys at £200 to £400; foam assessment is quoted on its own terms after we see the roof.
Sample openings: how a window is cut in the foam
Once the loft survey has told us where to look, we open the foam in selected spots. These sample openings are small, usually the size of a hand or a little larger, and each one gives a view of the timber surface and a place to take a reading.
Choosing the spots
The locations come from the survey, not from a grid. We tend to open the foam at the eaves, around the base of a chimney, beside a valley, below a roof window and at the ridge or apex. We include a spot that looks entirely normal as a control, since a good result on healthy ground gives the bad result somewhere else its meaning.
How it is done
The foam is cut back with a blade and lifted off the wood, working slowly to avoid scoring the timber. In soft foam this is quick; in rigid foam it is a patient job with a scraper. Dust is collected, and we protect the floor below. Each opening is photographed before and after, and its position is recorded on a rough sketch of the roof.
Closing the window
Every opening is closed again before we leave, with a patch of insulation or sealed foam appropriate to the roof, so the roof is not left weaker than we found it. If the sample shows decay, we stop and discuss the next step with you before widening anything. The type of foam affects how awkward the openings are; our guide to open-cell and closed-cell foam explains why.
Moisture readings and what they can and cannot tell us
A moisture meter measures how wet the wood is. The type we use for roof timbers has two pins that press into the surface and read the electrical resistance between them, which falls as the wood holds more water. It is quick, cheap and widely used, and it has real limits.
Reading in context
A single number means little by itself. Readings vary with the species of timber, the temperature of the wood, the depth the pins reach and whether the surface has dried while the core is still damp. So we take several readings along a rafter and across its width, compare them with a control reading from a dry stretch of the same roof, and look for a pattern, not a figure. We do not publish a threshold for pass or fail, because a published figure would suggest a certainty the method does not have.
Probing the wood
Alongside the meter, we press a pointed tool into the timber. Healthy wood resists; decayed wood is soft, can be indented easily and may break away in fibres or small cubes. This simple test often says more than the meter. A low reading on soft wood can mean timber that was once wet and has since dried, and the damage remains.
Why one visit may not be enough
Moisture changes with the weather and the heating. A roof read in a dry week can look fine and read badly after a wet one. Where readings are borderline, we suggest a second visit under different conditions before reaching a conclusion.
Recognising decay and the damage that follows
Timber decay is caused by fungi that grow in wood kept damp over time. The word that matters there is time: a damp patch seen today does not mean rot has set in, and a roof that has stayed wet for years may well be affected. This is why dates and history are part of the report.
What decay looks like
Typical signs include darkened, crumbly or cracked wood, a surface that flakes under a thumbnail, fine white or grey strands on the surface, and a distinct mushroomy smell. We also look at the ends of rafters where they bear on the wall plate, because that is where water collects first. Metal fixings can corrode, so we check straps, brackets and nails.
When a structural engineer is needed
If rafters or the wall plate look weakened, we do not make a judgement on strength ourselves. A structural engineer should assess any timber whose load-carrying role may have been reduced, and our report says so. Our job is to open the roof, record what is there and carry out the repair the engineer or surveyor specifies.
Where only the surface of a timber is affected, repair may involve cutting out the damaged section and fixing a new length alongside, known as sistering. Where the damage is deep, replacement may be needed. Either way, work on the structure has to be planned alongside the roof covering, so the timbers are repaired before the underlay and the tiles or slates go back.
What the written report contains
You should come away with a document that a lender, a buyer or an insurer can read without having been in the loft. Ours leans on pictures taken in your loft, with a plain description beside each finding.
The usual contents
- A summary of the property, the roof covering and the type and thickness of foam.
- A sketch showing where each sample opening was made.
- Photographs of the foam, the openings and the timber beneath, with captions.
- The moisture readings taken at each opening, with the control reading.
- Notes on the condition of the wood, including any probing results.
- Observations about leaks, flashings, ventilation and the covering above.
- Recommendations in plain language, ranked by how urgent they are.
- The limits of the inspection, for example areas that could not be reached.
What the report does not claim
It describes what was seen at the spots we opened. It cannot prove that unopened areas are in the same condition, and it is not a structural engineer's report or a valuation. We state those limits in the document itself, so nobody relies on it for something it cannot do.
Pairing the report with the original installer's paperwork, where it exists, gives the fullest picture. If the findings point to a moisture source, the next topic is how and why spray foam and condensation interact, which is the subject of its own guide.
After the inspection: leaving, partly removing or removing the foam
With the report in hand, the choices become clearer. If the wood is sound and dry, many owners choose to leave the foam alone and keep the report. If there are local problems, a partial removal around the affected area may be enough. If the timbers are generally damp or decayed, taking the foam off, repairing and re-insulating is the likelier outcome.
Re-insulating
Whichever route you choose, the roof needs a plan for insulation and ventilation. Our guide to re-insulating after spray foam removal explains the options, including Part L, which is the part of the Building Regulations that deals with fuel and power.
Costs and paperwork
MyBuilder's 2026 guide puts spray foam assessment at £100 to £200 nationally, and repairs plus new insulation at £500 to £3,000. Those are national ranges and say nothing about what your roof will cost. Once we have surveyed the roof, you get a written quote that runs from scaffold and access through foam removal and timber repair to insulation, waste and making good. Should the work be notifiable, we deal with the Building Regulations notification and explain what it means.
Our workmanship is guaranteed for 10 years on paper, and we leave the loft clear of debris. A survey report, a lender's letter or the installer's paperwork helps if you can attach it to your enquiry, because we read it before we visit.