Where the water comes from
Water gets into a flat roof in two ways: rain through a break in the waterproofing, or vapour from inside the building condensing in the build-up. BS 5250, the British Standard on moisture in buildings, covers both.
Leaks through the membrane
A leak starts where the waterproofing has failed. Tramex names penetrations, plant, kerbs, flashings and edges as places water can get in, and one cause BRE names on felt roofs is felt not isolated from the building’s movement.
- Rain gets in at a split
- Water spreads in the insulation
- Drip, some way from the split
Condensation inside the build-up
Vapour in the air rises through the ceiling and the insulation, and where more reaches the roof than ventilation can carry away, it condenses and runs back down. BRE says this matters most where ventilation is restricted or the rooms below give off a lot of vapour, such as kitchens and swimming pools.
- Condensation under the deck
- Water drips into the room
- Vapour from the rooms below
Why they look the same from below
From the room below, both look like a leak. BRE frequently finds condensation in roofs thought to be leaking, sometimes with drips that coincide with bad weather.
The fixes differ. A leak is fixed where the water gets through, so the point of entry has to be found, which is what electronic leak detection is for. Condensation is fixed by controlling the vapour, with a vapour control layer on the warm side of the insulation or ventilation on its cold side.
How water moves once it is in
A leak and the damp it causes are often yards apart. Once water is through the membrane it travels along the insulation and the deck, soaking a wide area, so the wet area is usually larger than the leak suggests. Moisture mapping is how its full extent is found.
The insulation decides the pattern, which thermography shows. Fluke explains that absorbent boards wick water through a whole board before the next, giving a chequerboard, and non-absorbent boards shed it to their edges and joints, giving a pattern like window frames.
Water also gets between layers: between the plies of a felt roof, Tramex notes, and between the earlier layers of waterproofing on a roof that has been overlaid.
Water that keeps spreading reaches the deck and, trapped under the membrane, can damage it. A core sample shows the deck’s top surface, including corrosion on steel. Insulation that holds water loses its ability to insulate.
What each survey method measures
The non-destructive methods do not see water itself. Each measures something that changes when a roof is wet: an electrical signal, slowed neutrons or surface heat. Cores check all three.
Capacitance and impedance meters
These meters read from the roof surface. Tramex’s impedance scanners pass a low-frequency signal between two electrodes, stronger the more moisture there is, and show it as a comparative value from 0 to 100. Set first over an area known to be dry, the meter has a range for each kind of covering, reaching about 30 to 50 mm, 70 to 80 mm or 150 to 160 mm deep.
They cannot say how much water there is, or read below their range. Some EPDM, butyl and other conductive membranes limit them, and the surface, and any gravel on it, has to be dry.
Nuclear gauges
A nuclear gauge reads hydrogen. Troxler’s gauge sends neutrons from a radioactive source into the roof and counts those slowed by hydrogen. Other roof materials hold hydrogen too, so counts are read against one that marks a dry area. Nuclear gauges measure deeper than surface scanning.
A count is not a moisture content. Troxler’s manual says finding one is normally done with cores, cut and measured for their moisture.
Thermography
A thermal camera reads surface temperature. Water holds more heat than a roof’s materials, so once the sun has warmed the roof, wet insulation cools more slowly and shows as warm patterns after sundown. Fluke’s checklist is clear sun beforehand, a clear night, little or no wind and a dry, clean surface.
A warm pattern is not proof of water. Fluke lists repairs, extra gravel or bitumen, exhausts, heat from walls or from inside, walkway pads, blisters and ponding among things that look like wet insulation, and ASTM C1153 recommends cores to confirm it. A single layer of insulation reads best, and reflective or metal roofs are harder to read.
Core samples
A core sample removes a small section of the roof down to the deck, so every layer can be measured, photographed and described, and the insulation’s moisture measured.
It shows only its own small area, so cores are used alongside moisture mapping or thermography, and each is sealed watertight afterwards.
Why readings are calibrated against cores
Every non-destructive reading is relative, and cores tie it to what is in the roof. Troxler advises cutting them across the widest range of readings or, if only a few, where readings change from dry to wet, and Tramex says suspected wet areas should be confirmed by cores, their moisture measured by weight.
Reading a moisture map
A moisture map starts as a grid of readings, typically around two metres apart, plotted on a scaled drawing of the roof with its drains and plant. Troxler notes there is no rule for the grid’s size, but a finer grid gives a sharper map. Extra readings go at penetrations, plant, kerbs, flashings and edges.
Troxler sorts the readings by charting how often each value comes up, deciding which mean dry roof and which wet, and drawing contour lines round the wet ones. Each outline is a wet zone.
A reading means something only against the dry readings on the same roof. A high number says there is more moisture there, not how much, and a wet zone shows where moisture is held, which is not always where it got in.
The map is what the scope of works is drawn from, so wet areas can be removed and dry areas kept rather than stripping the whole roof. Wet insulation generally has to come out before a roof can be overlaid.
Warm, cold and inverted roofs
Where the insulation sits changes what a reading means. BRE’s names for the three arrangements are the warm deck, the cold deck and the inverted warm deck, and the roof types guide describes the coverings over them.
Warm roofs
In a warm roof the insulation sits on the deck under the waterproofing, with a vapour control layer beneath it on the warm side, as BRE advises. Surface readings pass through the membrane into the insulation, and thermography reads a single layer best. A core shows which layers are wet.
- Waterproofing
- Insulation
- Vapour control layer
- Deck
Cold roofs
In a cold roof the insulation sits below the deck, under a ventilated void. Where vapour reaches the void more than its ventilation can carry away, it condenses, and BRE records wet insulation and a deteriorating roof structure as the result, with converting the roof to a warm deck as the remedy. Readings taken on top meet the waterproofing and the deck first, and where water drips from a cold roof, BRE often finds condensation in the void, not a leak.
- Waterproofing
- Deck
- Ventilated void
- Insulation
- Ceiling
Inverted roofs
In an inverted roof the insulation is laid above the waterproofing, under gravel ballast or paving, so the membrane sits beneath both. BRE found that over a lightweight deck, rainfall can lower an inverted roof’s thermal performance and bring a risk of condensation. Thermography is not used on inverted roofs.
- Ballast
- Insulation
- Waterproofing
- Deck
Questions about moisture in flat roofs
Is it a leak or condensation?
From below they can look the same, and BRE often finds condensation where a leak was suspected, even when the drips coincide with bad weather. A leak needs its point of entry found, and condensation needs the vapour controlled.
Why is the wet area bigger than the leak?
Once through the membrane, water travels along the insulation, the deck and between layers, so it soaks a wider area than the break and can show inside some way from it.
Will a moisture survey find where the water got in?
Not always. A moisture map shows where moisture is held, which is not always where it got in. Where the breach needs locating, moisture mapping is often combined with electronic leak detection.
Do the readings give the moisture content?
No. Meter readings and gauge counts are comparative, read against dry areas of the same roof. Cores cut across the range of readings, and measured, turn them into moisture levels.
Does wet insulation have to come out?
Generally, before a roof is overlaid, as insulation that holds water loses its ability to insulate. A moisture map shows which areas can be kept, so the whole roof need not be stripped.
Sources
- BSI, BS 5250:2021 Management of moisture in buildings, its published scope (also on NSAI’s listing)
- BRE, Avoiding dripping roofs
- BRE Defect Action Sheet 33, Flat roofs: built-up bitumen felt, remedying rain penetration
- BRE Defect Action Sheet 59, Felted cold deck flat roofs: remedying condensation by converting to warm deck
- BRE Information Paper 2/89, Thermal performance of lightweight inverted warm-deck flat roofs
- Tramex, Dec Scanner roofing moisture meter
- Tramex, How to moisture survey flat roofing, as per ASTM D7954
- Troxler, Model 3216 Roof Reader application brief
- Troxler, Model 3216 Roof Moisture Gauge manual
- Fluke, Best infrared cameras for roof inspection
- This site’s survey pages for moisture mapping, thermographic surveys and core sampling, and its guide to commercial roof types