On a commercial flat roof, water that gets in almost never stays where it entered. It travels under the membrane, soaks the insulation, freezes and thaws, and eventually shows up metres away, often months after the problem started. By the time a stain appears on the ceiling, part of the insulation is already saturated. Infrared thermography exists precisely to see that hidden moisture before it turns into a full replacement.
What a roof thermography survey actually detects
An infrared camera does not see water. It sees temperature differences. The principle is thermal inertia: during the day, the sun heats the roof surface and the insulation beneath it. After sunset, dry insulation cools quickly while water-laden insulation holds heat longer. That lag creates a contrast the camera picks up. Wet zones show up as warm patches against a cold background.
In practical terms, a thermographic survey maps the moisture present in the insulation across the whole surface: the percentage of roof affected, the location and extent of water pockets, the areas where moisture is spreading. It is a reading of the internal condition of the waterproofing system, not just its visible surface. The method applies to low-slope roofs where the membrane sits directly on the insulation, which covers the large majority of commercial, industrial and institutional flat roofs.
ASTM C1153: why the standard governs the survey
The recognized reference is ASTM C1153, the Standard Practice for Location of Wet Insulation in Roofing Systems Using Infrared Imaging. It defines the conditions under which an infrared survey gives a reliable result, rather than pretty pictures that prove nothing.
Three requirements are decisive:
- No appreciable precipitation for at least 24 hours before the survey. Surface water completely distorts the reading.
- A nighttime survey, usually one to two hours after sunset, following a day with enough solar gain. It is that heat-and-cool cycle that makes the contrast visible.
- A sufficient temperature differential between dry and wet insulation at the time of the survey.
These constraints are why a serious survey is scheduled around the weather and is not improvised in the middle of a daytime visit.
The step no one should skip: verification
This is the most important point, and the most often ignored. ASTM C1153 requires that every anomaly spotted by the camera be confirmed by an invasive method: coring (removing a small sample of the roof system) or a combination of cores and moisture-meter probes. Thermography tells you where to sample. It does not replace the sample.
A report that shows thermal images with no core verification does not comply with recognized practice. The correct sequence is always the same: infrared survey first, then targeted cores at the flagged locations, to confirm the presence of water and assess the real condition of the insulation. That verification turns an observation into evidence you can use for a decision, a warranty claim or an insurance file.
When to run a thermographic survey
Several situations justify a survey, beyond the simple presence of a leak:
Deciding between repair and replacement
Before committing to a replacement, knowing what percentage of the insulation is actually wet changes everything. If water is confined to a few zones, a targeted repair is enough. If moisture is widespread, repairing only delays the inevitable. The survey quantifies the scale of the problem instead of guessing it.
Acceptance of a new roof
On new construction or a recent re-roof, a survey before the warranty period ends confirms that no moisture was trapped during the work. It is concrete protection: documenting the roof’s condition while it is still under warranty.
Budget planning and the contingency fund
For a manager or a co-ownership syndicate, knowing the internal condition of the roof makes it possible to plan the expense at the right time rather than in an emergency. It is the kind of data that feeds a credible contingency fund study and a well-founded building condition assessment.
What a thermographic survey does not do
Thermography has limits worth naming. It detects moisture in the insulation, not defects in the membrane itself while they are still dry. It depends on precise weather conditions. And without core verification, it remains a hypothesis. It is a powerful diagnostic tool within an expert process, not a verdict on its own. In the field, it is combined with a visual examination of the membrane, flashings, drains and penetration points.
In short
Commercial roof thermography, done to ASTM C1153, answers a precise and costly question: how much of my insulation is wet, and where. Done well, at night, with no recent rain and with core verification, it avoids two symmetrical mistakes: replacing a roof that is still sound, or repairing one that is already lost. Done badly, it is just pretty pictures.
Boreon carries out roof surveys as part of its roof inspection and expertise and building thermography mandates. To understand the life cycle of a flat roof, see also our article on the lifespan of a commercial flat roof.
Sources
- ASTM International, C1153 Standard Practice for Location of Wet Insulation in Roofing Systems Using Infrared Imaging — store.astm.org
- Cx Associates, ASTM C1153: Standard Practice for Location of Wet Insulation in Roofing Systems Using Infrared Imaging — cx-associates.com
- The Snell Group, Conducting a Roof Moisture Inspection using Infrared Thermography — thesnellgroup.com
- Professional Roofing, Michael T. Williams, “Surveying roof moisture” — professionalroofing.net
