Thermal imaging has a reputation for being a magic eye that sees straight through a roof. It is not. A thermal camera records heat radiating from a surface and paints it as a colour map. Warm and cool patches then have to be interpreted by someone who knows how the roof is built and what the weather and the heating have been doing. The picture is a clue, not a diagnosis.
Still, it earns its place. On a flat roof, in a loft with suspect insulation, or across a large commercial deck, a thermal survey can show in minutes what a hands-on search would take a long time to find. This guide covers what the camera shows, the conditions it needs, how we use it on flat and pitched roofs, and the points where it stops being useful. It deals with the thermal method alone. For finding hidden moisture by any means, read our answer on roof damp surveys. The full service is on our roof inspections and surveys page.
What an infrared camera actually records
Every surface gives off infrared radiation in proportion to its temperature. A thermal camera reads that radiation and turns it into an image, usually with warm areas in reds and yellows and cool areas in blues and purples. It measures the surface only. It does not see inside the build-up, and it does not see water. What it shows is that one patch of roof is behaving differently from the patch beside it.
That difference is useful because materials hold and lose heat at different rates. Dry insulation resists the flow of heat. Wet insulation conducts it and also stores it. A gap where the insulation is missing lets warmth from the room below escape faster. Each of these leaves a signature in the image, and an experienced operator learns to tell them apart from the way the pattern is shaped. A neat rectangle can mean a missing quilt between joists. A blotchy pool can mean saturated insulation.
Pictures need a reading
The camera shows what is warm. It does not tell you why. A warm patch could be a hot-water pipe, a recessed light, a flue, a patch of sun-warmed surface, or a wet area that has held the day's heat. This is why a thermal image on its own is not a report. We pair it with ordinary photographs and a look at the construction before drawing any conclusion.
Heat loss and missing insulation in a pitched roof
The simplest use of the camera is checking where warmth escapes from a heated house. On a cold, still day, with the heating on, we can scan the outside of a pitched roof and the ceilings inside. Areas with thin or missing loft insulation show as warmer lines, often following the rafters, which conduct heat more readily than the insulated bays between them. Gaps around the loft hatch, at the eaves, and where cables and pipes pass through the ceiling show up as bright spots.
What this is good for
It helps a homeowner judge where a loft needs topping up before spending money, and it helps a roofer understand why snow melts early on one slope, or why the same ceiling stains each winter. It can also expose warm air leaking into the roof space, which is a common route for condensation. Warm, moist air that escapes through a ceiling meets the cold underside of the roof covering and turns to water. The camera finds the leak path; the remedy is often sealing it, not just adding more insulation.
Conditions it needs
Heat-loss scans work best when there is a clear temperature difference between inside and outside, and when the roof has not been warmed by direct sun. Wind and rain cool the surface and blur the picture. We choose the day, and where conditions are poor we reschedule rather than hand over a muddy image. Our seasonal inspection guide explains how timing fits into a year of roof checks.
If a scan leads to new loft insulation, the VAT treatment differs from ordinary roofing work: loft insulation fitted in a home is zero-rated until 31 March 2027. Any quote from us shows the VAT, and a scan of the finished ceilings afterwards can confirm the gaps are closed.
Wet insulation and trapped moisture in a flat roof
This is where thermal surveys do their best work. A flat roof is a sandwich: a deck, usually insulation, and a waterproof covering on top. If the covering is breached, water can enter and spread sideways through the insulation, well away from where it first entered. The ceiling stain may appear a metre or more from the real defect, and tracking it by eye is slow guesswork.
Why wet shows up at dusk
Wet insulation holds heat longer than dry. After a sunny day the whole roof is warm. As evening cools the surface, dry areas lose their heat quickly while saturated areas release theirs more slowly. A scan in the hours after sunset therefore shows the damp zones as warmer patches against a cooler background. A scan in daytime, with the sun heating the surface, hides the effect.
Marking the damp area
Once the camera has outlined a suspect area, we can mark it on the roof and take moisture readings or cut a small test sample to confirm. This turns a vague worry into a defined area, which makes the repair smaller and the quote more exact. If the wet patch is limited, the repair may be to cut out and replace the saturated section and patch the covering, rather than relaying the whole roof. If saturation is widespread, a replacement becomes more likely. Our guides to the flat roof materials we fit, such as EPDM rubber and GRP, explain what the new covering would be.
Renewing the waterproof layer on a flat roof can trigger a thermal upgrade under Part L of the Building Regulations, so a survey that maps the insulation also helps you plan for that cost before it surprises you.
What a thermal survey cannot tell you
Limits matter as much as strengths, and a survey that oversells them wastes money. First, the camera sees surfaces. A thick covering, a ballast layer of stone, or a reflective finish can hide the pattern underneath. Shiny metal surfaces reflect heat from their surroundings rather than showing their own, which can mislead an unwary reader. Zinc and some lead-covered areas fall into this category.
No cause, no age
The camera does not locate the hole in the covering, does not say how long water has been getting in, and does not separate wet insulation from an area that is warm for another reason. Water can come from a failed seam, a cracked flashing, condensation or a leaking pipe. A thermal scan narrows the search; it does not close it. We always follow it with a hands-on check of the area, including the covering, the details and the drainage.
Weather and access
A scan needs the right day. Rain, strong wind, a wet surface or thick cloud change the picture. Access also matters: to read the underside of a pitched roof, we need to get into the loft and see the ceilings clearly. A cluttered loft hides a lot.
Treat any thermal result as a map of where to look next. The repair decision comes from what we find when we open the roof or take readings.
If the pattern is ambiguous, the report admits it and recommends other checks, such as moisture meter readings or a small core sample, before you commit to work.
Where it fits next to other inspection methods
Thermal imaging is one tool among several. A ladder and a close look find slipped tiles and cracked flashings. A pole camera reaches awkward spots. A drone gives a view of high or steep slopes without anyone climbing on them. None of these sees heat. The camera adds a different kind of information, so on larger or more complicated roofs they are often combined.
On a commercial or large flat roof
On a big flat roof, a hands-on search for a single leak can take days of probing. A thermal scan from the roof or from a drone can reduce that to a list of areas worth checking. It is also a useful baseline for a building with a long roof history, because repeat scans show whether a suspect area is growing.
On a house
For an ordinary house, a thermal survey is not always necessary. A tiled roof leaking at a known flashing does not need one. A flat extension roof with unexplained ceiling stains, a loft that grows damp every winter, or a roof you are about to insulate might. Our survey before a re-roof describes where scanning fits into scoping a full replacement.
As a guide to price, MyBuilder's 2026 inspection guide lists thermal surveys at £300 to £500 in the UK, against £150 to £250 for a standard survey and £200 to £400 for a drone survey. Those are national ranges. The price we quote follows our own visit, in writing.
What you receive and what happens next
A thermal survey report from us is written plainly. It starts with the conditions on the day: outside and inside temperatures as read on site, whether the sun had been on the roof, and whether it was dry. It then shows the thermal images beside ordinary photographs of the same areas, with each warm or cool zone labelled and explained. A thermal image without a normal photograph beside it is hard to read, so you get both.
Findings and priorities
We separate what the images suggest from what we have confirmed. A suspected wet area is marked as suspected until readings or a sample support it. Heat-loss findings are listed by location, so you can see whether the cause is thin insulation, a gap or an air leak. For each finding the report gives a plain recommendation: repair, monitor, re-check after a certain season, or investigate further.
From survey to quote
The written quote that follows covers access, stripping or opening up, materials, labour, making good and waste taken away. Photographs of your own roof support the quote. Where one damp area needs a modest repair, the quote shows exactly that. If the roof is saturated across most of its area, the report says so and explains why replacement is the sounder choice.
Where work is notifiable under the Building Regulations, such as a larger replacement covering, we explain the consent route and file the notification for you. The work carries our 10-year workmanship guarantee in writing.