Flat roof repair in Lewisham
Almost every Victorian and Edwardian house in this borough has a flat roof on the back — the rear back-addition or outrigger. It is usually the oldest, least-inspected and hardest-working roof on the building, and it produces a disproportionate share of the leaks we are called to.
We repair and re-cover flat roofs across Lewisham: back additions, rear extensions, dormers, bay roofs, porches, garages and the parapet and box gutters that go with the larger period properties.
Why the back addition is where the leaks are
The rear back-addition is a standard feature of the terraces across Brockley, Ladywell, Hither Green, Catford and Forest Hill. It creates three problems at once:
- A junction between two different roofs. The main pitched slope meets a flat roof, and the two move differently. That junction is the single most productive place to look for a leak on this kind of house.
- Abutments on two or three sides. A back addition roof usually meets brickwork on multiple sides — your own rear wall and often the neighbour's flank. Every one of those upstands is a potential entry point.
- Nobody can see it. It is not visible from the street or usually from the garden. Coverings routinely reach the end of their life unnoticed until water appears inside.
The single most useful thing to know about flat roofs: they very rarely leak in the middle. Almost all failures are at the edges — upstands where the roof meets a wall, the junction with the main roof, outlets, and the perimeter detail. If someone proposes patching the middle of your flat roof, ask what they found at the edges.
What actually goes wrong
- Splits and cracks in the covering, usually at stress points and laps
- Blistering where moisture is trapped between layers and expands in the sun
- Failed upstands where the covering turns up against a wall and the flashing or chase above it has let go
- Ponding — standing water where falls are inadequate or the deck has sagged
- Blocked or undersized outlets, so water backs up above the covering level
- Perished laps on older felt, opening along the joins
- Deck deterioration where water has been getting into the timber below, often for years
- Failed junction with the main pitched roof
The coverings you are likely to have
| Covering | What it looks like | Typical behaviour |
|---|---|---|
| Built-up felt | Layers of bitumen felt, often with green mineral chippings | The most common on older Lewisham back additions. Becomes brittle, splits at laps and upstands. Repairable, but at the end of its life it is end of life. |
| Torch-on felt | Modern multi-layer bitumen, usually a mineral top sheet | The mainstream replacement. Good life when properly laid with correct detailing at upstands. |
| EPDM rubber | Single black rubber membrane, few or no joins | Long-lived and flexible. Failures are almost always at edges and terminations rather than the sheet itself. |
| GRP fibreglass | Hard, seamless, usually grey | Durable but rigid. Cracks where the deck moves, particularly on timber decks with any flex. |
| Lead or asphalt | Traditional, found on older and larger properties | Very long-lived. Repairs need matching craft rather than a modern patch over the top. |
Repair or re-cover?
Repair is the right answer when the covering is generally sound and the failure is localised — a split, one failed upstand, a blocked outlet. That is a lot of the work we do, and it is far cheaper than a re-cover.
Re-covering becomes the sensible answer when:
- The covering is brittle and cracking in multiple places rather than one
- There are already several patch repairs across it
- Water has been getting in long enough that the deck below is soft
- Ponding is structural — the falls are wrong or the deck has sagged, which patching cannot fix
- The perimeter and upstand details are fundamentally wrong rather than worn
What drives the price either way — access, scope, system and deck condition — is set out on the roof repair cost page.
Be wary of liquid coatings sold as an alternative to re-covering. Painting a coating over a failing felt roof with a wet deck underneath seals moisture in and hides the deterioration. There are legitimate liquid systems, properly specified, on a sound substrate — but as a way of avoiding a re-cover on a roof that needs one, it is money spent to delay.
Parapet and box gutters
The larger period properties in the borough — and much of the older stock around Blackheath — have gutters concealed behind a parapet wall or set between two slopes, rather than a visible gutter at the eaves. These are effectively narrow flat roofs, usually lined in lead, felt or asphalt.
They cause serious leaks for specific reasons:
- They are invisible. Nobody looks at them, and they collect leaves and debris.
- When they overflow, water goes inside. An eaves gutter that blocks spills harmlessly down the outside. A blocked parapet gutter backs up over the upstand and into the building.
- Outlets are small and easily blocked, often discharging through a parapet wall into a hopper.
- Lead work fatigues where sections were made too long, and splits along the fatigue line.
Clearing them is cheap maintenance that prevents very expensive damage. If you have a parapet and you have never had the gutter behind it looked at, it is worth doing. See also lead flashing and leadwork.
Ventilation and condensation
Not every damp patch under a flat roof is a leak. A cold-deck flat roof with insulation at ceiling level needs a ventilated air gap above that insulation. Where a roof has been insulated at some point without maintaining that ventilation path, moisture condenses on the underside of the deck and drips — producing damp that looks exactly like a leak but does not track rainfall.
The tell is the same as for chimneys: a leak follows rain, condensation follows cold weather. See how to tell damp causes apart.
Do not walk on an old flat roof to inspect it. Where water has been getting in, the deck beneath a sound-looking covering can be rotten, and it gives way without warning. This is a genuine cause of serious falls.
Leak on a flat roof?
Tell us where it shows inside and roughly how old the covering is. Photographs from an upstairs window overlooking the back addition are ideal.
Flat roof questions
How long should a flat roof last?
It depends on the system and, more than anything, on the detailing. Older built-up felt commonly reaches the end of its useful life somewhere around the fifteen to twenty-five year mark; modern torch-on and EPDM systems are longer-lived. But a well-laid roof with badly detailed upstands will fail long before a modest covering that was detailed properly.
Is ponding water a problem?
Shallow ponding that clears within a day or two after rain is common and not necessarily urgent. Persistent standing water is a different matter: it accelerates degradation, adds weight, and usually indicates either inadequate falls or a deck that has deflected. If it never dries, it is worth investigating rather than tolerating.
Can a flat roof be repaired in winter?
Temporary protection, yes. Torch-on and most permanent systems need a dry substrate and reasonable temperatures to bond properly, so a permanent repair generally waits for a dry spell. Laying a new covering onto a wet deck is a guaranteed failure.
My flat roof leaks but only in heavy rain. Why?
Usually an outlet or drainage capacity issue rather than a hole. In ordinary rain the water drains fast enough; in heavy rain it backs up above the level of an upstand or a lap and finds a way in. That points at outlets, falls or the perimeter detail rather than the field of the roof.
Do I need building regulations approval?
Re-covering more than a set proportion of a roof generally brings thermal upgrade requirements under Building Regulations, so a full re-cover is not always a like-for-like swap. A localised repair does not. We will tell you which side of that line your job falls on.