The short answer
A flat roof inspection works through the membrane, the deck beneath it, the falls that move water towards the outlets, the upstands where the roof meets walls, and the outlets and gutters themselves. We check from above for blisters, splits, ponding and lifted edges, and from below, where access allows, for stains or soft boarding. The result is a photographed record of each part, which is the basis for any repair quote.
The membrane: what the surface tells us
The membrane is the waterproof skin, and on most flat roofs it is the first thing to wear. Single-ply PVC, bitumen felt, GRP (glass-reinforced plastic, also called fibreglass) and EPDM (a synthetic rubber sheet) all age in different ways, so the first task is to identify what is up there.
We then look for splits, open seams, blisters, cracking at folds, bare patches where a mineral finish has worn away, and signs that someone has patched the roof before. On felt we check the laps and the edges. On EPDM we check the seams and the corners where the sheet has been folded. On GRP we look for hairline cracks, especially along joints in the deck. Typical lifespans vary widely between sources, from 15 to 30 years for flat roofs in MyBuilder's 2026 guide, and they depend on workmanship and upkeep, so the condition we find counts for more than the date.
The deck and the falls
Below the membrane sits the deck, usually boards or plywood on joists. We press along the surface to feel for softness, which suggests rot, and look for dips between joists. A deck that has sagged leaves hollows where water collects.
That brings us to falls, the gentle slope that sends water towards the outlets. BS 6229 sets out the design: IKO's guidance, dated 2021, says the recommended minimum finished fall is 1:80, with a design fall of 1:40 to cover tolerances and deflection. Water still lying a day or two after rain suggests the roof is not achieving that, and standing water speeds up wear on the membrane. We look for ponding, stains where it has dried, and algae or silt lines marking its edge.
Ponding is not always a leak
A pond does not mean the roof is leaking, but it tells us where the next failure is likely to occur. We record it and say whether the fix is local or needs the falls reworked.
Upstands, flashings and edges
Where the membrane climbs a wall, a rooflight kerb or a parapet, that vertical return is the upstand, so that rain cannot creep behind the roof edge. It is where flat roofs fail most often, because movement between roof and wall strains the joint. We check that the upstand is high enough, that it is stuck down firmly, and that the flashing or cover at the top is sealed and not lifting.
At the edges we check trims and drip details, which throw water clear of the fascia. Failed edge trims let water track back under the membrane. Around rooflights we look at the kerb, the seal between kerb and membrane, and any condensation or staining on the glass that might suggest a leak in the frame. Where a flat roof butts up to a chimney or a pitched slope, we study the lead that joins them. That junction takes a lot of movement on an extension or porch, and it is easy to overlook from the ground.
Outlets, gutters and drainage
Water leaves a flat roof through an outlet into a gutter or a hopper and downpipe. We check that outlets are clear, that leaves and silt are not building up around them, that the outlet is joined to the membrane without gaps, and that gutters are falling the right way. Guidance from makers such as FloPlast and Brett Martin suggests a slight tilt towards the outlet, from 1:600 to 1:350 depending on the system, and we look for runs that sag or hold water.
Overflow positions are worth checking on a roof with a parapet or an upstand all round. If the main outlet blocks, water has nowhere to go but through the wall or the membrane. Inside, a stain near a downpipe or the wall plate can show a drainage problem that looks like a roof leak. We also lift any loose debris, leaf litter and moss from the surface and note how quickly it has built up, because a roof that fills with leaves within a season needs a regular clearing routine, not just a repair. A pitched roof draining onto the flat one, or a neighbouring downpipe discharging across it, can also overload the outlets, so we trace where the water arrives from.
From below, and what you get afterwards
If there is access, we look at the flat roof from underneath, through a loft hatch, a cut-out or the ceiling void. Staining, damp insulation, rotten joist ends and daylight are all signs of what the membrane is failing to hide. A moisture meter or thermal camera helps trace water that has travelled, as described in our guide to the roof damp survey.
For access, a flat roof can be walked in many cases, but we may use a drone or pole camera where the roof is fragile or hard to reach, as compared in our guide to inspection methods. Our page on inspecting without climbing says when that is enough.
Stacks and flashings on the roof are covered in our guide to chimney inspection, and the whole range is on our roof inspections and surveys page.
The report comes with pictures taken on your roof and, if work is needed, a quote in writing covering that work. Repairs carry our 10-year workmanship guarantee in writing. Call 01453 933613 to book.