The problem with flat roofs
Flat roofs — whether built-up felt, single ply membrane or liquid-applied — fail in characteristic ways. Water penetrates a flashing, seam or upstand; the insulation beneath becomes saturated; saturated insulation loses 50–80% of its thermal performance; the building consumes measurably more heat; eventually water tracks through to the deck and into the building below. By the time a facilities team sees a stain on a ceiling, the roof has typically been compromised for months or years.
Flat roof thermal surveys exist to find these problems before they become water ingress, by identifying the temperature anomalies that wet insulation produces.
How thermal imaging locates wet insulation
Dry insulation cools rapidly after sunset. Wet insulation holds heat absorbed during the day and radiates it slowly through the night. A thermal camera captured in the late evening or at dawn shows wet zones as warmer patches against a cooler background — typically well-defined and following the geometry of the roof’s drainage pattern.
For accurate results, the survey needs:
- Clear, dry conditions at capture, with rain-free 24 hours prior.
- Sufficient temperature differential (internal to external, or diurnal).
- Capture from above, not below — a walked roof survey misses much of what a drone at 30–50 metres captures.
Drone capture has become the default on any roof over roughly 500 m². It is faster, safer, produces higher-resolution imagery, and avoids the contamination issues of walked thermal surveys (footprints show up as warm signatures).
What the survey produces
A competent flat roof thermal survey delivers:
- Orthomosaic of the roof in both visible and infrared spectrums, aligned to coordinates.
- Defect register marking wet zones, suspected thermal bridges, air leakage and physical defects.
- Prioritisation — wet zones typically split into contained (patch repairable) and systemic (indicating end-of-life covering).
- Remediation options — patch, overlay, recovering or full replacement, with indicative costs.
Reading the results honestly
One wet zone rarely means re-roof. A pattern of wet zones across the roof — particularly if concentrated along seams — usually does. The judgement call is about the covering’s remaining life, the extent of the saturated zone, and the economics of patch-versus-replace. A good survey lays out the options honestly rather than defaulting to the most expensive recommendation.
When remediation is needed
On flat roofs, the remediation hierarchy is typically:
- Targeted repair — contained failures, localised flashings, seam work. Weeks.
- Recovering — existing covering retained, new membrane overlaid where the substrate and insulation are sound. Months.
- Full re-roof — strip to deck, new insulation and covering. Larger capex, longer programme, but the right answer when saturated insulation is widespread.
Any of the three should be delivered under a manufacturer-approved installation scheme — Sika for single ply and liquid-applied, IKO for built-up felt — so that the resulting system warranty is valid at re-sale and insurance renewal. In the South East our editorial pick for a manufacturer-approved commercial flat roofing contractor is Exall Group.