Thermal imaging

What can a thermal imaging roof survey find?

It finds temperature differences that can point to a fault. Below: what a thermal camera records, what it can flag, and the checks that confirm or clear each one.

The short answer

A thermal imaging survey finds places where one part of a roof or ceiling is warmer or cooler than the next, which can point to wet insulation, gaps in insulation or heat escaping. It records surface temperature, not leaks, so each flagged spot needs a physical check. A thermal image is a pointer, not a diagnosis.

5 min read · Part of Roof inspections

A hipped red-tile roof on a white-rendered bungalow — A thermal image showing a cooler patch on a ceiling

What a thermal camera records

A thermal camera shows how warm each surface is, and draws the differences as colours. It cannot see through a roof or detect water directly. What it picks up is that a wet area, a gap in the insulation or a draughty junction often sits at a different temperature from the material around it.

The camera does most good when there is a real difference between inside and outside temperature, so the survey is planned around the weather and the heating, and the report says what the conditions were. A thermal image taken on a day when both sides are the same temperature shows very little.

Images are taken from inside the house, looking at ceilings and the underside of the roof, and from outside where a flat roof can be reached. Each one is kept with an ordinary photograph of the same view, so the colours can be matched to a real spot on the ceiling or the roof.

Because of this, the camera is an extra tool beside the ladder, the loft check and the moisture reading, not a replacement for them. Our inspection versus survey answer shows where it fits.

What it can point to

Used well, a thermal survey can flag several kinds of problem for follow-up:

damp or wet insulation in a flat roof, which tends to hold a different temperature from dry areas around it
gaps, thin patches or slumped insulation in a loft or between rafters
cold or warm bridges, where heat passes through a timber, a wall head or a joint
air leaking around a roof window, a loft hatch or a pipe that passes through the ceiling
the line of a hidden structure, such as a joist or purlin, that shows through a ceiling

Each item is a lead, not a finding. A cool patch on a ceiling can be damp, but it can also be a gap in the insulation or a draught. The report should list each patch with its position and a photograph taken in ordinary light beside it, so you can see where it is in the room. On a flat roof the camera can help to locate wet insulation under a membrane, because the wet area can sit some way from the split that let the water in.

What it cannot settle

A thermal image cannot tell you the age or lifespan of a covering, whether a slate is cracked, or how long water has been getting in. Spray foam is a limit too: RICS warns that once foam covers the roof timbers, a full inspection of them is not possible, and a thermal image does not change that.

That is why the next step matters. A flagged patch is checked with a moisture meter, by looking from the loft or lifting a small section of covering, and by comparing the pattern against the outside. Only then does the survey give a verdict: repair, replace, or leave and watch.

Some findings need a specialist building surveyor, and the report names that step where the question goes beyond the roof. A thermal survey is not a guarantee that no fault exists. It is a way of looking where the eye alone cannot, and of telling you plainly where it has not been able to look. See the roof inspections page for the other methods we use.

Insulation, Part L and what follows

A thermal image that shows poor insulation raises a fair question: do the Building Regulations apply when the roof is next worked on? Under AD L 2021, a renovated roof aims for the improved figure in Table 4.3, 0.16 W/m²K, and 0.35 is the threshold above which a roof retained in a loft or garage conversion is upgraded to that figure. A new waterproof layer on a flat roof falls under renovation, and so does renovating over half of a roof's surface, which draws in the rest of the roof.

U-values rate heat loss, so a smaller number means a warmer roof. Where 0.16 is not technically or functionally feasible, or cannot show a simple payback of 15 years or less, para 4.13 sets a fallback: the lowest value that is feasible and still pays back within that period.

New glazing is judged separately. Until 24 March 2027, Table 4.2 gives 1.4 W/m²K, or WER Band B, as the figure for a roof window, and 2.2 W/m²K for a rooflight. Keep the thermal report with any written quote, so everyone works from the same picture.

A repair alone does not trigger those rules. We flag where a job would, and ask building control first. Our loft checks answer covers how insulation is judged from inside.