Thermal imaging has a reputation as the tool that sees through a roof. It does not, and the gap between the reputation and the reality is where people waste money. A thermal camera records the heat given off by a surface and shows it as a coloured image, so a warm patch and a cool patch stand out from their surroundings. On a roof, that can reveal insulation that is missing or wet, air leaking out through gaps, and in some cases the damp area under a flat roof membrane. What it cannot do is look through a covering, prove that water is present, or tell you why a patch is warm. A thermal survey of a roof can find some things and not others, it needs the right weather and timing, it costs a fairly predictable amount on typical UK figures, and there are times when a plain inspection with a ladder and a torch does a better job.
What a thermal camera actually records
A thermal camera measures the infrared radiation leaving a surface. It reports temperature on the surface it sees, nothing more. If the outer face of a roof is warm in one patch and cool in the next, the camera shows two colours. Everything else is interpretation.
That interpretation depends on the physics of heat. Materials hold and release heat at different rates, and water holds heat longer than dry insulation does. After a sunny day, a patch of wet insulation under a flat roof may stay warm into the evening while the dry areas around it cool down. In winter, with the house heated, a poorly insulated section of ceiling lets heat through and shows up warm from outside or inside. In both cases the image points at a difference. A surveyor then has to work out what the difference means.
The camera also needs a clear view of the surface. Shiny metal reflects the sky and gives false readings, wet surfaces cool by evaporation, and a surface in direct sun can overwhelm any subtle pattern underneath. Slate and tile are thick, and their outer face tells you little about what sits beneath. This is why an experienced surveyor will often take the pictures from inside the loft, aimed up at the roof's inner face as well as from outside.
It is a tool for locating, not for diagnosing. A good thermal report shows the images, the conditions, the settings used and what was then confirmed by other means. Where a patch is flagged, the confirming step is usually a moisture meter reading, a core sample or lifting a small section of the covering to look.
Finding leaks and wet insulation: where it works best
The strongest use for thermal imaging on a roof is the flat roof with insulation beneath the waterproof layer. When water gets in through a split in the membrane, it often travels sideways through the insulation before it shows anywhere inside. The leak can be metres from the stain. Wet insulation behaves differently from dry insulation in the hours after sunset, and that is when a camera can pick it out.
The aim is to narrow the search. Instead of cutting the roof open at a guess, a thermal survey marks the areas most likely to be wet, and moisture readings or small test cuts then confirm them. On a large commercial roof, that can save a good deal of disruption, because the repair is made to the affected zone and not to the whole membrane.
Pitched roofs are harder. The covering of slates or tiles hides the layers below, and water that gets in often runs down the underlay and out at the eaves without ever soaking anything long enough to show. A camera can help in the loft, where it may reveal damp insulation or a cold patch where a gap is letting air through, but it is rarely the first thing we reach for on a pitched roof. A torch, a careful eye and a look at the timbers usually give a faster answer, which is why checking from inside the loft comes first for most houses.
Thermal imaging is also used to look at the aftermath of a storm, when the camera can help check whether water has reached insulation that cannot be seen. Our page on roof inspections after a storm describes where it fits in that work.
Heat loss: what the pictures can show about insulation
The second use is energy. A heated house loses warmth through its roof, and a thermal image taken from outside on a cold day can show where. Bright patches on a roof slope may be missing or compressed insulation, gaps at the eaves, warm air escaping around a chimney breast or a recessed light, or a pipe running through the loft. Taken from inside the loft, the same camera can show cold bridges, where timber or masonry carries heat past the insulation.
That information is useful if you are planning to upgrade the loft insulation. It tells you where the gaps are before you buy material. It can also help when a re-roof triggers a thermal upgrade. As the Planning Portal explains, a re-roof reaching beyond the exemption limits may require the roof to meet current standards as a thermal element, and a thermal survey can show how the existing roof performs.
But the limits are real here too. A thermal survey is a snapshot of a particular day. The picture changes with wind, with whether the heating has been on, with the sun, and with the house's own warm and cold spots. A single image cannot give you a measured performance figure for the roof. It shows relative differences, and a skilled reader of the images knows which ones matter and which are artefacts.
Spray foam complicates things further. Foam sprayed onto the underside of a roof insulates the covering from the heat of the house, so the outside of the roof may look uniformly cold whatever is going on beneath. If your roof has been foamed, a thermal survey will tell you very little about the timber behind the foam.
Conditions, timing and why a bad day wastes the survey
Thermal imaging is fussy about the weather. The usual requirement is a clear difference between the temperature inside and outside the building, and a dry roof surface. Rain, a wet roof or strong wind will cool the surface and wash out the pattern. Direct sun heats the covering so much that the image shows the sun's work and not the roof's defects.
For flat roofs, the best time is often the hours after sunset on a dry day that followed a sunny one. The roof has been heated and is then cooling, and the wet areas, which hold more heat, glow against the dry ones. For heat-loss work on a pitched roof, a cold, dry, still evening with the heating running gives the clearest contrast between insulated and uninsulated areas.
That timing matters for scheduling. A survey booked weeks ahead may have to move if the weather does not suit, and any provider who promises a fixed slot regardless of conditions is not being straight with you. Winter surveys often work well for heat loss, and our note on roof surveys in winter covers what changes with the season.
You can help the result. Keep the loft clear so the underside of the roof can be seen. Leave the heating on as usual in the days before. Tell the surveyor about any previous repairs, spray foam, or work that has changed the build-up of the roof, since each affects how the image should be read.
The final report should state the conditions on the day. If it does not, you cannot tell whether a pattern in the pictures reflects the roof or the weather.
Limits, false alarms and what it will not tell you
Thermal imaging is often oversold, so here is what it does not do.
It does not see through the covering
A camera reads surface temperature. A cracked slate or a nail that has rusted through will not appear unless it changes the temperature of the surface around it. Thin, local defects are often invisible.
It does not prove moisture
A warm patch can be wet insulation, but it can equally be a thicker area of insulation, a different material, a sun-warmed rafter or a pipe. A cool patch can be a leak, or it can be where air flows through the build-up. Without a second method, a thermal image is a suspicion.
It does not date the problem
The camera cannot say whether a patch has been wet for a week or for years, or whether the leak is still active. It cannot say whether a membrane has just split or has been failing slowly.
It can mislead on certain surfaces
Reflective surfaces, such as lead and bright metal, behave oddly in infrared. Lead flashings can show up cooler or warmer than they are, and that is a common source of false readings around chimneys and parapets. That matters on a Bath roof, where lead is everywhere.
Given all this, a thermal survey is a supplement to an inspection, not a replacement for one. For many pitched roofs, a close-up inspection from a ladder or scaffold, with a drone where access is poor, tells you more for less money. For roofs that are hard to reach, drone roof surveys are the usual alternative.
What it costs and when it is worth paying for
On typical UK ranges reported by MyBuilder (2026), a roof inspection costs between £150 and £500 depending on its type. A standard inspection is typically £150 to £250, a drone survey £200 to £400 and a thermal survey £300 to £500. The thermal figure reflects the equipment, the evening or early-morning timing and the time needed to interpret the images. These are national figures, so use them to judge a quote and not as a price from us. Any repair we recommend is priced separately, after we have been on the roof.
It is worth paying for a thermal survey in a few situations. A flat roof on a commercial building, where the leak is hard to trace and the cost of opening the roof at random is high. A roof where you suspect widespread wet insulation but see no obvious entry point. A house where heat loss is a concern and you want to know where to spend on insulation. Or a roof where the stain inside is in one place and the cause is clearly somewhere else.
It is rarely worth it for a pitched roof with a single stain, where a close look at the area will find the cause. Nor does it help much if the roof is foamed, or heavily shaded, or the weather will not cooperate. And it is no substitute for a survey before major work. If you are heading towards a re-roof, our guide to why a survey comes before a re-roof quote explains what that survey needs to cover. If you are selling and wondering what to deal with first, see fixing a roof before selling.
On our roof inspections we photograph the roof in front of us, state up front what we found and what we could not see, and suggest thermal imaging only where it will add something. For a useful thermal survey, book a cold, dry evening when the heating has been on for some hours, as the contrast is too weak on mild days.