Flat roofs are popular on extensions because they keep the height low against a boundary, sit well under first floor windows and make it easy to add a large rooflight or lantern. Built properly, a modern flat roof is nothing like the leaky felt roofs many people remember.
Flat does not mean level
Every flat roof is laid to a fall, usually built in with tapered firrings or tapered insulation, so water runs to a gutter or outlet instead of sitting in puddles. Ponding water speeds up the ageing of any membrane and finds its way through the smallest defect, so getting the fall right at the framing stage matters more than the choice of top layer.
Warm roof build-up
For most new extensions we recommend a warm roof. The structural deck goes on the joists, then a vapour control layer, then rigid insulation boards, with the waterproof membrane on top. The timber stays on the warm side, so condensation has nowhere to form inside the roof.
A cold roof, with insulation between the joists and a ventilated gap above, can work but depends on that ventilation staying clear. Our warm roof versus cold roof guide goes into the difference in more detail.
- Joists sized for the span and any lantern opening
- Structural deck boards, glued and screwed
- Vapour control layer sealed at every edge
- Rigid insulation to meet building regulations
- Single ply, EPDM rubber, GRP or built-up felt membrane
Choosing a membrane
EPDM rubber is laid in one sheet where possible, so there are few joins. GRP fibreglass is laid wet and cures hard, which suits roofs with foot traffic. Built-up felt systems are laid in layers and torched or bonded. Each can perform well; the quality of the upstands, outlets and edge trims usually matters more than the brand.
Roof lanterns and rooflights
A lantern or flat rooflight transforms a deep extension, putting daylight into the middle of the room. It needs a properly framed kerb, set high enough above the membrane that water cannot back up over it, and the membrane dressed up the kerb rather than tucked under the frame. Trimming joists around the opening are part of the structural design, not an afterthought.
Where the roof meets the house
The junction with the existing wall is the classic leak point. The membrane turns up the wall, and a lead or matching flashing is cut into a mortar joint above it so rain runs over the joint instead of behind it. On cavity walls a cavity tray may be needed so water inside the old wall is not directed into the new room. Our guide to extension roofs leaking at the join covers what to check if that has already gone wrong.
Planning and building regulations
A flat roof keeps height down, which helps with permitted development limits and with neighbours on the boundary, but whether planning is needed depends on the extension itself, not the roof. Building regulations always apply to a new extension roof: insulation, structure and ventilation are checked by building control.
How we build flat roof extensions
We build extensions and roofs with our own team, and have over 40 years of experience in carpentry and building. Each job starts with a free site visit and a fixed-price written quote that sets out the roof build-up, the membrane and how the junctions will be detailed.
Common questions
Is a flat roof a bad idea on an extension?
No. A modern flat roof with proper falls, a warm roof build-up and good edge details is a durable choice, and it keeps height down against boundaries.
What is the best membrane for a flat roof extension?
EPDM, GRP and built-up felt can all perform well. The falls, upstands and junction details matter more than the product.
Can I add a roof lantern to a flat roof extension?
Yes. It needs a framed opening designed with the joists and a raised kerb so water cannot back up over it.
Why do flat roof extensions leak?
Usually at the edges: the join to the house, around lanterns and outlets, or where water ponds because the fall is too shallow.
Do you give a fixed price?
Yes. After a free site visit we provide a fixed-price written quote.