Flat roof survey report: what to check and how to word it

A flat roof report is not a pitched roof report with the word "flat" swapped in. On a pitched roof you are mostly looking at a lot of small overlapping units and the things that hold them down. On a flat roof you are looking at one continuous skin, every joint in it, and whether water can get off it before the skin gives up.

That changes what you photograph, what you measure and how you word the findings. Below is how we approach it, covering by covering, with report wording you can lift and adapt.

Start with the deck and the build-up, not the surface

Before you write a word about the membrane, work out what is under it. A split in a felt lap on a warm deck with 120mm PIR is a different job from the same split over a cold deck with a rotten joist end.

On site, look for: the ceiling below (stains, sag, nail pops), the fascia and eaves detail, any vents, and the feel of the deck underfoot. Soft, springy or dipping patches mean the deck or insulation has taken water. If there is a hatch or an edge trim you can lift safely, say what you actually saw rather than what you assume.

Example wording:

The roof appears to be of warm deck construction over a timber deck. Underfoot, the area approximately 1.5m south of the rooflight is noticeably soft and deflects under load, indicating saturated insulation and probable deck deterioration. This cannot be confirmed without opening up. Allowance should be made for localised deck and insulation replacement. Category 2.

If you did not open up, say so in the same sentence. "No opening up was carried out" protects you and tells the client why the cost is indicative.

Ponding: measure it, do not just mention it

Ponding is the single thing clients argue about most, so be precise. The usual working definition is standing water remaining more than 48 hours after rain. If the roof is dry when you visit, you can still read it from tide marks, silt rings, moss and the dirt pattern on the covering.

Photograph the ring with something for scale, and note the depth if there is water in it. Note the direction of fall and where it is supposed to go.

Example wording:

Dished areas with silt staining approximately 2.5m x 1.5m are present adjacent to the north parapet, consistent with standing water after rainfall. Falls in this area are inadequate and water is not reaching the outlet. Ponding over felt laps accelerates failure and increases load on the deck. Tapered insulation or additional outlet provision should be considered at recovering. Category 2.

Ponding on its own is rarely a Cat 1 unless it sits directly over a defective lap, an upstand or an internal area where water ingress is already occurring. Then say that link out loud in the report.

Coverings: what each one fails at

You will see four main coverings and each has its own tell.

Built-up felt (torch-on) fails at laps, at blisters and at UV-degraded cap sheet where the mineral has gone. Look along every lap line, kneeling, with the sun behind you. Open blisters are Cat 1; closed blisters are a note.

Cap sheet mineral loss is extensive across the central field, exposing the bitumen beneath and leading to crazing. A 400mm length of lap approximately 1m from the rainwater outlet has lifted and is open. Water ingress is likely at this point. Immediate patch repair recommended as an interim measure. Category 1.

EPDM rarely fails in the field; it fails at bonded seams, at corner patches and at terminations. Check the adhesive line, press the seam edge with a thumb, and look for shrinkage pulling the membrane away from upstands.

The EPDM membrane is sound across the open field. The seam running east to west has lifted along approximately 300mm at the corner patch adjacent to the parapet. Membrane shrinkage has drawn the sheet away from the upstand, reducing the effective upstand height to approximately 80mm. Category 2.

GRP fails at cracks over deck joints, at crazing from a thin topcoat, and at trim junctions where the laminate has not been dressed properly. Hairline crazing is cosmetic; a crack running in a straight line over a joint is movement and will recur.

A linear crack approximately 900mm long runs across the laminate, corresponding with a deck board joint beneath. This is consistent with deck movement rather than surface weathering and is likely to reopen if repaired with topcoat alone. Category 2.

Mastic asphalt fails by crazing, blistering, cracking at upstands and by the solar reflective paint washing off. Old asphalt can have decades left in it if the detailing is intact. Do not condemn it on appearance alone.

The asphalt covering shows surface crazing consistent with age and loss of solar reflective treatment. No cracking through the full thickness was observed. Recoating with solar reflective paint is recommended to slow UV degradation. Category 3.

Upstands, abutments and terminations

This is where most flat roofs actually leak. Check upstand height at every abutment, the condition of the cover flashing or termination bar, the mastic seal, and whether anyone has run a bead of silicone over a failed detail.

The usual industry expectation is 150mm of upstand above the finished surface. If you find less, say the figure you measured and say what it means.

The upstand to the rear wall abutment measures approximately 90mm above the roof surface, below the 150mm generally recommended. The lead cover flashing is loose and the mortar joint has failed over approximately 1.2m. Driven rain is likely to pass behind the flashing. Category 1.

Also check: rooflight kerbs, soil vent pipe collars, handrail and aerial fixings through the covering, and any cable or pipe penetration someone has sealed with a tub of black goo. Photograph each one close up and wide, so the client can see where it is.

Outlets, falls and rainwater

Lift the leaf guard if there is one. Note whether the outlet is a proper bonded flange or a hole with a grate dropped in. Check the outlet sits at the low point, not 200mm from it.

The single rainwater outlet to the south-west corner is partially blocked with silt and moss. The outlet flange is not fully bonded to the covering and water is bypassing the dressing. The roof has a single point of discharge with no overflow provision; blockage would cause water to back up over the upstand. Category 1.

No overflow is worth calling out on any roof with parapets. It is cheap to fix and expensive to ignore.

The Cat 1 to 3 calls, kept consistent

Category	Meaning	Typical flat roof example
1	Urgent, active or imminent water ingress	Open lap, failed upstand, blocked sole outlet
2	Needs attention within the planned maintenance cycle	Ponding, shrinkage, soft deck patch, deck movement crack
3	Monitor or routine maintenance	Surface crazing, moss, minor mineral loss

The important thing is that the same defect gets the same category every time you write a report, and that the definitions appear in the document so nobody has to guess. If you want to see how the categories sit alongside photos and costs in a finished document, look at the sample roof survey report.

Pulling it together

Structure the flat roof section by area rather than by defect type: main roof, dormer, porch, each with its own photos and calls. Then one summary with the Cat 1 items first and an indicative cost against each. The general principles of layout, tone and ordering are covered in our guide on how to write a roof survey report, and they apply here too.

One aside: we built Docutrades because typing this up at nine at night is the worst part of the job. Notes and photos go in on the roof, one press turns them into report prose, and the client signs the PDF on their phone. If that sounds useful, there is a 14-day free trial with five reports, no card needed.

Next time you are on a flat roof, measure the upstand, photograph the outlet with the guard lifted, and mark the ponding outline before you come down. Those three photos do more work in a report than a dozen general shots of the covering.