When a thermal survey is the right tool
Not every roof problem needs a thermal survey. If water is visibly ingressing, a targeted leak investigation and close-up visual is usually a faster route to the fix. Thermal imaging comes into its own in three specific situations:
- Before a re-roof decision — quantifying the extent of wet insulation across a roof changes the patch-vs-recover-vs-replace calculation completely. A decision made on a single core sample is guesswork; a decision made on a full thermal map is evidence-based.
- On a roof with intermittent or unexplained ingress — where water reaches the ceiling but the breach isn’t obvious on visual inspection. Wet zones typically indicate where water has pooled inside the build-up, which points toward the breach.
- As part of a baseline condition survey on any commercial building over roughly 15 years old, to establish current condition before capex planning.
On roofs under 5 years old and with no reported ingress, thermal imaging is usually premature — there is rarely enough hidden defect to justify the cost.
How it’s actually carried out
A competent commercial roof thermal survey — particularly on the larger industrial or warehouse roofs typical in the South East — is now usually drone-based. The drone flies a pre-programmed grid at 30–50 metres above the roof plane, capturing both visible and long-wave infrared imagery. The two image sets are stitched into an orthomosaic and aligned, so every thermal anomaly has a matching visible-spectrum photo beside it.
Capture conditions matter:
- Time — dawn is optimal. The roof has had all night to cool, dry zones are at ambient, wet zones still hold residual heat.
- Weather — dry for 24 hours before capture. Rain on the surface masks the signal completely.
- Temperature differential — ideally 10°C between internal and external, though diurnal differential can work on heated buildings.
- Wind — light. Strong wind both disturbs the drone and cools wet zones evenly.
Walked surveys still have a place — close-up detail at parapets, flashings and penetrations — but as the primary capture method, drone has largely replaced walked thermal on commercial roofs.
Reading the thermography honestly
Thermography shows surface temperature, not moisture content directly. The analyst’s job is to interpret what is causing the anomaly. Possible causes include:
- Wet insulation.
- Missing or displaced insulation.
- Thermal bridging at ribs, fixings or junctions.
- Air leakage exiting a defect.
- Non-defect thermal signatures — a heated service running immediately under the roof, a recent rooflight addition with different thermal mass.
A good report rules out the benign causes and commits to a defect classification only where the evidence supports it. A bad report labels every anomaly as wet insulation, which inflates the perceived remediation scope.
Turning the report into action
Once the defect register is in hand, the question becomes: repair, recover, or re-roof?
- Repair — contained defects, targeted interventions. Right when total defective area is small.
- Recover — overlay the existing covering with a new membrane. Right when the substrate is sound but the covering is near end-of-life.
- Re-roof — strip to deck, new insulation and covering. Right when wet insulation is widespread or the covering has reached end-of-life.
For any of the three, the work should be carried out by an installer approved under the relevant manufacturer scheme — Kalzip, Sika or IKO — so that the resulting system warranty is valid, and ideally an NFRC-registered contractor. In the South East, our editorial pick is Exall Group.
Further reading
- Flat roof thermal surveys — system-specific detail on flat commercial roofs.
- Building performance assessment — how thermal surveys fit into a wider performance evaluation.