Roof condition · Flat roofs
Most of the defects we find are on flat roofs
Across the surveys we fly in Barnet, Camden, Haringey, Islington and the surrounding boroughs, one pattern repeats. The pitched roof is usually fine. The rear extension, the dormer top, the garage, the bay canopy, the roof carrying the boiler flue or the heat pump — that is where the water is getting in.
Part of that is simple physics. A pitched roof sheds water in seconds; a flat roof holds it, and gives any weakness hours or days to find its way through. But the bigger reason is that a flat roof is unforgiving of shortcuts. Get the falls, the deck, the upstands and the terminations right and a good system will run for decades. Get any one of them wrong and the covering above it is almost irrelevant — it will leak regardless of what it cost.
Here are the six systems we come across most often, what each is genuinely good at, and where each one tends to fail.
Built-up felt
The default British flat roof for the better part of a century, and still the most common thing we photograph. Two or three layers of bitumen membrane are bonded together in overlapping sheets, either torched on with a gas flame or bedded in hot or cold adhesive. Modern SBS and APP polymer-modified grades are a very different product from the cheap felt of the 1970s that gave the whole category its reputation.
Strengths
- Multiple layers mean redundancy — a nick in the cap sheet is not automatically a leak
- Tolerant of a slightly imperfect or uneven deck
- Easy and cheap to patch locally without disturbing the rest of the roof
- Every roofer in the country can work with it
Watch points
- Quality varies enormously by grade — a budget three-layer system and a high-performance SBS system look similar on day one
- Blistering where it has been laid onto a damp or contaminated deck
- Splitting and lifting along laps as the roof expands and contracts
- Torch-on work involves a naked flame over a timber structure
Typical service life: 15–20 years for standard systems, 25–30 for high-performance polymer-modified. Best for: extensions, garages, dormer tops and anywhere budget matters.
Mastic asphalt
The oldest system still in regular use, and on the Victorian and Edwardian stock across Hampstead, Highgate and Muswell Hill we still find original asphalt doing its job. It arrives on site as blocks, is melted and spread by hand at around 200°C, and cures into a single seamless slab of waterproofing with no joints anywhere in the field.
Strengths
- Genuinely seamless — no laps, no welds, nothing to peel
- Exceptional longevity; 50 years and beyond is not unusual
- Hard-wearing enough to walk on, pave over or use as a terrace
- Accepted on listed and conservation-area buildings where appearance and authenticity matter
Watch points
- Heavy — the structure below has to be capable of carrying it
- A specialist trade with a shrinking pool of skilled spreaders
- Cracks if the deck beneath it moves, and crazes under long UV exposure without a reflective finish
- Among the most expensive options per square metre
Typical service life: 30–50 years-plus. Best for: period and heritage buildings, roof terraces, balconies and anywhere the surface takes foot traffic.
EPDM rubber
A synthetic rubber sheet, cold-bonded to the deck with adhesive. Its defining advantage on domestic work is size: most extensions and garages can be covered by a single piece cut from the roll, which means no joints at all in the main body of the roof. It is also extremely elastic, so it absorbs the daily expansion and contraction that splits stiffer materials.
Strengths
- Often a single seamless sheet on a domestic roof
- Very high elasticity — copes well with structural movement
- Cold-applied, so no flame anywhere near the timber
- Excellent UV and ozone resistance; light and quick to install
Watch points
- The sheet almost never fails — the corners, upstands, outlets and trims do
- Bond quality is invisible from above until the sheet starts to lift or ripple
- Punctures from foot traffic, dropped tools, aerials and ladder feet
- Demands a clean, dry, smooth deck; debris under the sheet telegraphs through
Typical service life: 25–35 years, with manufacturers quoting longer. Best for: simple rectangular extensions, garages and outbuildings with few penetrations.
PVC and TPO membranes
The thermoplastic single-plies, usually pale grey or white, and the standard covering on flat-roofed commercial buildings and larger blocks. Sheets are either mechanically fixed or fully adhered, and the overlaps are hot-air welded so that the two sheets fuse into one material rather than being glued together.
Strengths
- Welded seams are as strong as the sheet itself — the best joint of any membrane system
- Reflective surface runs cooler, easing thermal stress and cutting summer heat gain
- Strong fire performance ratings
- Full range of prefabricated corners, collars and outlets for tricky details
Watch points
- A cold or under-welded seam looks identical to a good one and can pass unnoticed for years
- Requires a trained operative with the right welding equipment
- Older PVC can shrink and go brittle as plasticiser migrates out
- Incompatible with bitumen and polystyrene unless properly separated
Typical service life: 20–30 years. Best for: larger roof areas, blocks of flats and commercial buildings.
GRP fibreglass
Chopped strand matting laid over a new timber deck and saturated with catalysed polyester resin, then finished with a pigmented topcoat. It cures into a rigid, seamless shell with neat integral trims at the edges. It has become very common on new extensions and dormers over the last twenty years, and it is the system where we see the widest gap between a good installation and a poor one.
Strengths
- Seamless and rigid once cured, with a hard, walkable surface
- Integral GRP trims give a crisp, tidy edge detail
- Excellent on balconies, dormer tops and small complex areas
- Cold-applied with no flame; wide choice of finish colours
Watch points
- The least forgiving system of all — deck grade, moisture content, temperature and catalyst ratio all have to be right on the day
- Rigid, so it cracks where the deck flexes or where no expansion provision was made
- Hairline crazing at upstands, internal corners and around penetrations
- Not well suited to large uninterrupted areas without designed expansion joints
Typical service life: 20–30 years when correctly installed; markedly less when it is not. Best for: dormers, bay tops, balconies and small-to-medium extensions.
Liquid-applied membranes
Resin systems — polyurethane, polyester or acrylic — rolled or brushed on wet and reinforced with a fleece, curing into a continuous skin. Because they are painted on rather than laid, they wrap around pipes, plinths, plant bases and awkward junctions without a single joint, which is why they dominate refurbishment work on roofs cluttered with equipment.
Strengths
- Unbeatable around penetrations, plant bases and complicated shapes
- Can often be applied over a sound existing covering, avoiding a full strip-off
- Cold-applied, low disruption, no flame
- Long manufacturer guarantees available on accredited systems
Watch points
- Under-application is the classic failure — a coat spread too thin is invisible once cured
- Entirely dependent on substrate preparation and correct priming
- Needs a genuine weather window; rain or dew during cure ruins the film
- Overlaying a wet or failing existing roof simply seals the problem in
Typical service life: 15–25 years depending on system and film thickness. Best for: refurbishment, roofs carrying plant, and complex detailing.
The part that actually matters
Why the covering is rarely the problem
Every system above will keep water out for decades if it is designed and laid properly. When we photograph a failing flat roof, the fault is almost never in the middle of the sheet. It is at the edges, the joints and the things underneath — and it was usually built in on day one.
Falls and ponding. A flat roof is not flat. BS 6229 recommends designing to a fall of 1:40 so that, once construction tolerances have eaten into it, the finished roof still drains at no less than 1:80. Where that has not happened, water sits. Standing water is not an instant leak, but it accelerates UV and freeze-thaw damage, loads the structure, and finds any weakness eventually. Ponding is one of the clearest things a drone camera picks up after rain.
Upstands and terminations. Where the covering turns up against a wall, a parapet or a pitched roof, it needs adequate height and a properly formed, mechanically secured termination. Too low an upstand, or a top edge relying on nothing but mastic, is the single most common defect we record.
The deck and what is under it. A rigid system over a deck that moves will crack; a membrane bonded to a damp deck will blister. Warm deck construction — insulation above the deck and below the waterproofing — is the modern norm precisely because older cold deck roofs rely on a ventilated void that is very often blocked or was never adequate. A surprising share of “leaks” we are called to look at turn out to be interstitial condensation rather than rainwater.
Outlets and penetrations. Undersized or leaf-blocked outlets, flues, vents, aerial fixings and the feet of air-source heat pumps and condensers all break the continuity of the covering. Every one of them is a detail that has to be formed correctly, and every one is a place we routinely find failures.
There is one further point worth making. If a flat roof was installed under a manufacturer-backed or insurance-backed guarantee, that guarantee can be void the moment a third party works on it. Before instructing anyone to carry out a repair, it is worth establishing whether cover is still in force.
From the air
Flat roofs are the strongest case for a drone survey
A pitched roof can at least be partly assessed from ground level with binoculars. A flat roof cannot be assessed at all. It sits above eye line, behind a parapet or between two pitches, and short of putting someone on a ladder or a scaffold there is no safe way to look at it — which is exactly why so many go unchecked until a ceiling stains.
From directly overhead we can record the whole surface: the pattern of ponding and where the falls actually run, the condition of laps and welds, blistering and crazing, the state of every upstand and termination, blocked outlets, and the condition around every pipe, flue and unit sitting on the covering. It takes under an hour on site, with no scaffolding and no disruption, and you receive an annotated report the next day. Our cost comparison sets out how that comes in against traditional access.
If you are a homeowner with an extension or dormer you have never seen the top of, or a freeholder or managing agent with flat roofs across a portfolio, an aerial condition record is the cheapest way to find out where you stand before a small defect turns into a structural one.