Why choose a flat roof extension?
A flat roof is the most common choice for a single storey rear extension, and for good reasons. It keeps the extension low, which helps it fit under first-floor windows and within permitted development height limits near a boundary. It gives a clean, modern outline, it is straightforward to add a roof lantern or rooflights, and the structure is simpler than a pitched roof with tiles.
A pitched or lean-to roof can suit some houses better, especially where the extension needs to match a traditional building or where a vaulted ceiling is wanted inside. The right answer depends on the house, the height available and the look you want.
A flat roof is never quite flat
A so-called flat roof has a gentle fall so rainwater runs to a gutter or outlet. That fall is built in using tapered timber firrings or tapered insulation. Getting the falls and the outlets right is the single most important part of a flat roof; standing water is what shortens a roof's life.
Where the extension meets the house, the roof has to be joined to the existing wall with flashings and, usually, a cavity tray so water running down the wall above cannot get behind the new roof.
Warm roof construction
Most new flat roofs are built as warm roofs, with the insulation above the timber deck and the waterproof covering on top. This keeps the structure warm and avoids condensation problems in the roof void. The amount of insulation is set by Building Regulations and affects the total roof thickness, which matters if height is tight under a window above.
Roof coverings
There are three main types of covering. EPDM is a synthetic rubber membrane laid in a single sheet, with few joints. GRP, or fibreglass, is laid wet and cures to a hard, seamless finish that can be walked on for maintenance. Modern torch-on or cold-applied felt systems are built up in layers and are very different from the old felt roofs many people remember. Each has its own installers and details, and the right one depends on the shape of the roof, the number of upstands and lanterns, and access.
Rooflights, lanterns and parapets
Light is often the main reason to choose a flat roof. A roof lantern over a dining or kitchen area brings daylight deep into the room; flat rooflights give a more minimal look. Each needs a properly built upstand and careful waterproofing around it.
A parapet, where the walls rise a little above the roof edge, hides the roof and gutters and gives a crisp brick outline. It needs a coping and a careful outlet design so water does not collect behind it. A simple fascia and gutter is the more traditional, lower-maintenance finish.
Planning and Building Regulations
Under permitted development, a single storey rear extension has height limits, including a lower limit for any part within two metres of a boundary. A flat roof often makes those limits easier to meet. Larger extensions, or those in conservation areas or on houses where permitted development rights have been removed, need planning permission. All extensions need Building Regulations approval covering structure, insulation, ventilation and drainage. Our extension planning guide explains the routes.
Cost and timing factors
The size of the roof, the covering chosen, lanterns and rooflights, parapets, insulation thickness and access for materials all affect cost and programme. Roof coverings also need dry weather to be laid, so timing can depend on the forecast. Our house extension cost guide covers the wider factors.
Our house extensions page explains how we build extensions from foundations to finish, and we offer free estimates.
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Questions people ask
Is a flat roof good for an extension?
Yes, a well-built flat roof with correct falls, a warm roof build-up and a good covering suits most single storey rear extensions and makes rooflights and lanterns easy to add.
Flat roof or pitched roof extension: which is better?
Flat roofs are lower and simpler and suit modern designs and height limits. Pitched roofs suit traditional houses and vaulted ceilings. The house and the height available usually decide.
What affects the cost of a flat roof extension?
The size, the roof covering, insulation, lanterns or rooflights, parapets, the steelwork into the house and access for materials.
How long does a flat roof extension take?
It depends on the size of the extension, foundations, glazing lead times and the weather for laying the roof covering. A timescale comes with the free estimate.