Why the build-up matters
Each survey method reads a roof in a different way, so what works depends on how the roof is built. Electronic leak detection needs a non-conductive membrane with a conductive layer directly beneath it. Thermography picks up the heat held by wet insulation, so it reads best through a single layer. Moisture mapping takes readings through the build-up from the surface, and a core sample shows every layer from the membrane down to the deck.
Matching the method to the roof is what makes the results clear enough to act on. That is why every survey starts by reviewing the drawings and the build-up, so the method suits the roof.
Start with the table for a quick answer, then go to your roof type for how it is built and what each method will show.
At a glance
Which survey methods work on each roof type. Select a roof to see how it is built and what each method will show.
- Suits most roofs of this type
- Depends on the build-up or covering
- Not used on this roof type
| Roof type | Leak detection | Moisture mapping | Thermography | Core sampling | Condition survey | Drone survey | Measured survey |
|---|---|---|---|---|---|---|---|
| Flat roofs | |||||||
| Single ply membranes | Depends on the build-up or covering | Suits most roofs of this type | Depends on the build-up or covering | Suits most roofs of this type | Suits most roofs of this type | Suits most roofs of this type | Suits most roofs of this type |
| Built-up felt | Depends on the build-up or covering | Suits most roofs of this type | Depends on the build-up or covering | Suits most roofs of this type | Suits most roofs of this type | Suits most roofs of this type | Suits most roofs of this type |
| Mastic asphalt | Depends on the build-up or covering | Suits most roofs of this type | Depends on the build-up or covering | Suits most roofs of this type | Suits most roofs of this type | Suits most roofs of this type | Suits most roofs of this type |
| Liquid applied | Depends on the build-up or covering | Suits most roofs of this type | Depends on the build-up or covering | Suits most roofs of this type | Suits most roofs of this type | Suits most roofs of this type | Suits most roofs of this type |
| Hot melt | Depends on the build-up or covering | Depends on the build-up or covering | Not used on this roof type | Depends on the build-up or covering | Depends on the build-up or covering | Suits most roofs of this type | Suits most roofs of this type |
| Green, ballasted and paved roofs | Depends on the build-up or covering | Depends on the build-up or covering | Not used on this roof type | Depends on the build-up or covering | Depends on the build-up or covering | Suits most roofs of this type | Suits most roofs of this type |
| Metal and sheeted roofs | |||||||
| Profiled metal | Not used on this roof type | Not used on this roof type | Depends on the build-up or covering | Not used on this roof type | Suits most roofs of this type | Suits most roofs of this type | Suits most roofs of this type |
| Standing seam | Not used on this roof type | Not used on this roof type | Depends on the build-up or covering | Not used on this roof type | Suits most roofs of this type | Suits most roofs of this type | Suits most roofs of this type |
| Fibre cement | Not used on this roof type | Not used on this roof type | Depends on the build-up or covering | Not used on this roof type | Depends on the build-up or covering | Suits most roofs of this type | Suits most roofs of this type |
| Pitched roofs | |||||||
| Tiled and slate roofs | Not used on this roof type | Not used on this roof type | Depends on the build-up or covering | Not used on this roof type | Suits most roofs of this type | Suits most roofs of this type | Suits most roofs of this type |
Flat roofs
Single ply membranes
Single sheets of PVC, TPO or EPDM, mechanically fixed, bonded or held down by ballast, usually over a layer of insulation.
- Single ply membrane
- Insulation
- Vapour control layer
- Profiled metal deck
Survey methods
- Depends on the build-up or coveringLeak detectionTraces punctures, failed seams and breaches at upstands. It needs a conductive layer directly beneath the membrane, and black EPDM needs a specialist method.
- Suits most roofs of this typeMoisture mappingShows how far water has spread through the insulation.
- Depends on the build-up or coveringThermographyReads best with a single layer of insulation beneath, although highly reflective membranes can be harder to read.
- Suits most roofs of this typeCore samplingConfirms the build-up and whether the insulation is wet.
- Suits most roofs of this typeCondition surveyRecords seams, laps, upstands, flashings and outlets, and the quality of earlier repairs.
Worth knowing
Leak detection finds the breach itself, while moisture mapping or thermography shows how far the water has spread, so the two are often used together.
Built-up felt
Layers of bitumen felt bonded together over the insulation or straight onto the deck. Older felt roofs often carry several layers of insulation and earlier waterproofing.
- Cap sheet
- Underlayer
- Earlier felt, on overlaid roofs
- Insulation
- Vapour control layer
- Concrete deck
Survey methods
- Suits most roofs of this typeMoisture mappingTests through several layers of insulation and earlier waterproofing, which suits roofs that have been overlaid.
- Suits most roofs of this typeCore samplingShows how many layers there are and whether the insulation is wet.
- Suits most roofs of this typeCondition surveyRecords the felt, laps, upstands, flashings and outlets, and the quality of earlier repairs.
- Depends on the build-up or coveringThermographyHarder to read where there are several layers of insulation.
- Depends on the build-up or coveringLeak detectionWorks where there is a conductive layer directly beneath the membrane.
Worth knowing
Because older felt roofs often carry several layers, the build-up is confirmed first so the right method is chosen.
Mastic asphalt
Asphalt laid hot in coats to form a seamless waterproof finish.
- Asphalt, top coat
- Asphalt, first coat
- Separating layer
- Concrete deck
Survey methods
- Suits most roofs of this typeMoisture mappingReads through the asphalt into the insulation and, on some roofs, the deck.
- Suits most roofs of this typeCore samplingConfirms what lies beneath the asphalt and whether it is wet.
- Suits most roofs of this typeCondition surveyRecords the finish, upstands, flashings and outlets.
- Depends on the build-up or coveringLeak detectionWorks where the build-up gives the test a conductive layer beneath the asphalt.
- Depends on the build-up or coveringThermographyReads best where there is a single layer of insulation beneath.
Worth knowing
On uninsulated roofs, moisture can sit within the deck itself, which moisture mapping can pick up on some roofs.
Liquid applied
Coatings applied as a liquid that cure into a seamless membrane, on new roofs or over an existing one to extend its life.
- Liquid applied coating
- Existing felt
- Insulation
- Vapour control layer
- Deck
Survey methods
- Suits most roofs of this typeCore samplingConfirms the build-up and whether the insulation beneath is wet.
- Suits most roofs of this typeMoisture mappingMaps wet areas across the whole roof, so cores are taken from the right places.
- Suits most roofs of this typeCondition surveyRecords the coating, upstands, flashings and outlets, and any earlier repairs.
- Depends on the build-up or coveringLeak detectionConfirms a new coating is watertight, where the build-up gives the test a conductive layer.
- Depends on the build-up or coveringThermographyHarder to read where the coating sits over earlier layers.
Worth knowing
Wet insulation generally has to come out before a roof can be overlaid, so the survey comes before the coating.
Hot melt
Hot applied rubberised bitumen bonded to a concrete deck, usually protected by insulation, paving or planting laid on top.
- Paving
- Filter layer
- Insulation, laid above
- Hot melt membrane
- Concrete deck
Survey methods
- Depends on the build-up or coveringLeak detectionBonded straight to the concrete, it gives the test a conductive layer. It is best tested before the covering goes on, or later through a trace wire where one was installed.
- Depends on the build-up or coveringCondition surveyCovers the upstands, outlets and drainage, and the state of anything laid on top. The membrane itself is hidden.
- Suits most roofs of this typeDrone surveyCaptures the whole roof and its covering from above.
Worth knowing
Thermography does not work through paving or insulation laid on top of the membrane.
Green, ballasted and paved roofs
A membrane protected by planting, gravel or paving, including inverted roofs with the insulation laid above the membrane.
- Planting or gravel ballast
- Filter layer
- Drainage layer, on green roofs
- Insulation, laid above
- Waterproofing membrane
- Concrete deck
Survey methods
- Depends on the build-up or coveringLeak detectionCan test through the covering, although results are less conclusive than on an exposed membrane.
- Depends on the build-up or coveringMoisture mappingSurface scanning cannot read through ballast or planting, so a nuclear moisture gauge is used where mapping is needed.
- Depends on the build-up or coveringCondition surveyCovers the outlets, drainage, upstands and the state of the covering.
- Suits most roofs of this typeDrone surveyCaptures the whole roof from above, including areas that are hard to reach.
Worth knowing
The best time to test the membrane is before the covering goes on. A trace wire installed during construction lets it be tested later. Thermography is not used on ballasted, paved or inverted roofs.
Metal and sheeted roofs
Profiled metal
Steel or aluminium sheets as a single skin, built up on site with insulation and a liner, or as factory-made composite panels.
- Outer profiled sheet
- Insulation
- Liner sheet
- Purlin
Survey methods
- Suits most roofs of this typeCondition surveyCovers the sheeting, laps, flashings, rooflights and gutters.
- Suits most roofs of this typeDrone surveyCovers roof edges, gutters and any areas that cannot be walked safely.
- Suits most roofs of this typeMeasured surveyRecords the ridges, rooflights and rainwater goods as CAD plans.
- Depends on the build-up or coveringThermographyMetal reflects infrared, so readings from above are harder to take.
Worth knowing
HSE guidance lists corroded metal sheets and rooflights among the surfaces likely to be fragile.
Standing seam
Long aluminium or steel sheets joined by raised seams that run from ridge to eaves.
- Raised seam
- Standing seam sheet
- Insulation
- Vapour control layer
- Liner deck
Survey methods
- Suits most roofs of this typeCondition surveyCovers the seams, flashings, penetrations, rooflights and gutters.
- Suits most roofs of this typeDrone surveyCovers the whole roof from above, including high-level and curved areas.
- Suits most roofs of this typeMeasured surveyRecords the levels, ridges, valleys and rainwater goods.
- Depends on the build-up or coveringThermographyMetal reflects infrared, so readings from above are harder to take.
Fibre cement
Corrugated or profiled cement-based sheets, found on many older industrial buildings.
- Fibre cement sheet
- Rooflight
- Purlin
Survey methods
- Suits most roofs of this typeDrone surveySurveys the whole roof from above, with no one walking on it.
- Depends on the build-up or coveringCondition surveyCarried out from safe positions, with a drone covering whatever cannot be reached safely.
- Suits most roofs of this typeMeasured surveyDrone capture records the roof without anyone on it.
- Depends on the build-up or coveringThermographyUsed mainly to show heat loss, on a cold day with the building heated.
Worth knowing
HSE guidance lists non-reinforced fibre cement sheets as likely to be fragile, whatever their profile.
Pitched roofs
Tiled and slate roofs
Clay or concrete tiles, or natural or man-made slate, fixed to battens on a pitched roof.
- Slates or tiles
- Battens
- Underlay
- Rafter
Survey methods
- Suits most roofs of this typeCondition surveyRecords damaged or displaced coverings, ridges, valleys, flashings and gutters.
- Suits most roofs of this typeDrone surveyCovers the roof from above wherever it cannot be reached safely.
- Suits most roofs of this typeMeasured surveyRecords the ridges, valleys and rainwater goods as CAD plans.
- Depends on the build-up or coveringThermographyUsed mainly to show heat loss, on a cold day with the building heated.
Worth knowing
HSE guidance lists slates and tiles among the surfaces likely to be fragile.
Not sure how your roof is built?
Drawings and O&M information are reviewed before any survey. Where they are missing or out of date, a core sample confirms the waterproofing layers, insulation, vapour control layer and deck, so the survey is matched to the roof as it is, not as it was drawn.
Questions about commercial roof types
Can you survey a roof if you do not know how it is built?
Yes. Drawings and O&M information are reviewed first, and core samples confirm the build-up where they are missing or out of date.
Which survey finds a leak in a flat roof membrane?
Electronic leak detection finds the breach itself, including pinholes too small to see. Moisture mapping or thermography then shows how far water has spread through the build-up.
Can a leak be traced under a green roof or ballast?
Results through a covering are less conclusive, so the membrane is best tested before it goes on. On some roofs a trace wire is installed during construction so the membrane can be tested later.
Can you test a black EPDM roof?
Standard leak detection needs a non-conductive membrane, which rules out black EPDM. Specialist methods can test it where the build-up allows, and your quote confirms the approach.
Is thermography suitable for every roof?
It works best on flat, non-metallic roofs with a single layer of insulation beneath the membrane. Roofs with several layers, and reflective or metal roofs, are harder to read, and it is not used on ballasted, paved or inverted roofs.
Should a roof be surveyed before it is coated or overlaid?
Yes. Core samples confirm the existing build-up and whether the insulation is wet. Wet insulation generally has to come out before a roof can be overlaid.
Can fragile roofs be surveyed?
Yes. HSE guidance treats every fixed roof as fragile until a competent person confirms otherwise, and lists fibre cement sheets and rooflights among those likely to be fragile. Roofs like these are surveyed by drone, with no one working on them.
Sources
- ASTM C1153 guidance as summarised by Roofing Contractor, FacilitiesNet and The Snell Group
- Vector Projects, flat roof leak detection methods
- IIBEC summary of SPRI standards
- HSE, Fragile surfaces