Nobody builds a roof that is truly flat. The word describes a roof with a low pitch, laid to a slight fall so that rain runs to an outlet. The waterproof layer on top does the keeping out, but it depends on everything under it: a deck that is level in the right way, insulation that does not trap moisture, and edges and details that are finished properly.
This post walks through a flat roof from the joists up, in the order the layers go on. It explains the choice between warm and cold construction, the falls that British Standards set for flexible membranes, the common coverings and what the Building Regulations say when a deck is replaced. It is written for homeowners who want to understand what a roofer is describing in a quote, not as a set of instructions for building one yourself.
That last point matters. A flat roof has several places where a small mistake becomes a leak a year later, and the work involves access at height. Our flat roofing page explains what we do, and the last section here says when to call a roofer.
One thing runs through every layer: moisture has to be kept out from above and not allowed to build up from inside.
The layers from the joists up
A typical flat roof is built from the structure up, in a set order. Each layer has a job, and leaving one out causes trouble somewhere else.
Structure
Timber joists, or in some buildings concrete or steel, span between the walls and carry everything above. The joists are what give the roof its falls, if the falls are built into the structure, so the way they are laid out shapes everything that follows.
Deck
The deck is the surface that carries the waterproof layer. It has to be firm, flat in the sense of free from bumps and dips, and dry. A soft or uneven deck shows through the covering and holds water.
Vapour control layer
Warm moist air from inside the building tries to rise into the roof. A vapour control layer under the insulation limits how much gets in, so that it does not condense within the build-up. Approved Document C says a roof should be designed so that interstitial condensation does not affect its structure or its insulation.
Insulation
Insulation reduces heat loss and sits either above or below the deck, which is the warm and cold deck question taken up next.
Waterproof layer
The membrane or covering keeps rain out. It can be felt, EPDM, GRP, lead or another material, each with its own method.
Finish and edges
Edge trims, upstands, outlets and gutters complete the roof, and they are where a rushed flat roof is most likely to fail. A trim that is not fixed firmly, or an outlet that is set too high, can undo the work of every layer beneath.
Warm deck and cold deck construction
The position of the insulation sets the construction type, and it changes how the roof handles moisture. Our guide to warm roof versus cold roof sets out the comparison; here is the outline.
Cold deck
In a cold deck roof the insulation sits below the deck, between or beneath the joists, and the space above it is cold. That space has to be ventilated so that moisture can escape. For cold deck roofs, Approved Document C looks to ventilation, though it accepts that very small roofs, a porch cover for example, may be excused it. A roof is also taken to comply on condensation if it is designed to BS 5250 and BS EN ISO 13788.
Warm deck
In a warm deck roof the insulation sits above the deck, under the waterproof layer, so the structure stays on the warm side. Because the deck stays warm, the risk of condensation in the joists is lower. A continuous vapour control layer under the insulation still matters.
Why it matters
Mixing the two, or getting the vapour control layer wrong, can lead to condensation and rot over the following years, long after the roofer has left. A roof with insulation added over the years may have moved from one type to another without anyone noticing, which is something a survey from inside the roof void can identify.
The 2021 edition of BS 5250, the moisture-management standard for buildings, is the current one. A designer should state which construction is intended and show how moisture is controlled.
Falls: getting the water off the roof
The fall is the single most important geometry on a flat roof. Without it, water sits, and standing water speeds up wear and loads the structure.
What the standard says
BS 6229:2025 governs flat roofs whose waterproof covering is flexible and supported along its whole length, with a pitch of up to 10 degrees. It took over from the 2018 edition, withdrawn on 16 December 2025. Its finished-fall minimum is 1:80, for the open roof and internal gutters alike.
Design fall and finished fall
The design fall is steeper than the finished fall, to allow for deflection and building tolerances. IKO advises designing for a fall of 1:40, which is twice the finished minimum. A roof designed to 1:80 may finish flatter than intended, and then hold ponds.
How falls are made
Falls can be built into the structure, laid with tapered insulation boards, or formed with timber firrings fixed to the deck. Each has its own consequences for the build-up.
Not every roof is covered by BS 6229
The standard covers flexible membranes only, so a roof of another type needs its own guidance. Lead flat roofs follow BS 6915, which Approved Document C cites, and they have their own rules on falls and detailing.
Gutters
Gutters have their own fall, and a small one, which is easy to get wrong by eye. BS EN 12056-3 is the current drainage standard, and Brett Martin gives 1 mm for every 600 mm along the run.
Coverings and the details that matter
The covering is the visible part, but the details around it decide whether it stays watertight.
The common coverings
- Felt: bitumen felt laid in layers. HomeBuilding gives 10-15 years and Less 10 to 20.
- EPDM: a synthetic rubber sheet, glued to the deck. HomeBuilding gives 25-30 years, Less 30 to 50.
- GRP: fibreglass in resin, as one skin. HomeBuilding gives 25-30 years, Less 25 to 40.
- Lead: sheet metal on a lead flat roof. HomeBuilding gives 60-100 years.
Treat those as indicative ranges, not promises.
Upstands and edges
Where a flat roof meets a wall, the covering is carried up the brickwork as an upstand, with a flashing over it. At the edge, a trim or drip carries water clear of the wall. Approved Document L gives a maximum U-value of 0.35 for an upstand built on site, so the insulation at the upstand matters too.
Outlets and penetrations
Outlets, rooflights and pipes each pass through the roof, and each needs a watertight detail. They are also where leaves and debris collect, so they have to be reachable. A roof with several penetrations needs more time and more care than a plain rectangle, which is one reason two flat roofs of the same area can be priced differently.
Fixing
Approved Document C expects fixings that suit each part of the roof, which on a flat roof means bonding or fixing the covering the way its system requires.
When Building Regulations apply to a new deck
Renewing a deck goes further than renewing the covering, and the rules treat it that way.
Renovation and new work
In the 2021 edition of Approved Document L, with its 2023 amendments, putting a new waterproof layer over a flat roof counts as renovation. A new roof element in an existing dwelling has a figure of 0.15 for the roof, from Table 4.2. For renovation of an existing roof, Table 4.3 gives a threshold of 0.35 and an improved figure of 0.16, and para 4.13 lets the roof be taken as far as a 15-year payback allows where the full figure cannot practically be reached.
The 2026 edition
A 2026 edition takes over on 24 March 2027 for ordinary building work, and its tables are numbered 3.2 and 3.3. Until then, the 2021 tables apply.
Approval and the Planning Portal
The Planning Portal puts the line at 25 per cent of the envelope and 50 per cent of the roof, below which no application is normally needed. A deck replacement can involve structure, so building control is worth asking. For Farnborough, that is the Hart and Rushmoor Building Control Partnership, which also serves Hart. Our guide to flat roof building regulations covers the detail.
Planning
A like-for-like re-cover rarely raises a planning question, but flats have no permitted development rights, and listed buildings need listed building consent as well. Where a conservation area applies, ask the council before changing the look of the roof edge or adding upstands that show from the street.
A roofer's job, and what a survey records
You can understand how a flat roof is built without building one. For a homeowner, the useful role is to ask good questions of the person who will.
Why this is a roofer's job
A flat roof involves working at height, handling hot or adhesive materials, forming details that cannot be seen once covered and meeting standards for fall, insulation and ventilation. A small error, such as a lap that is not bonded or a fall that is too shallow, shows up as a leak or a pond after the first winter.
Older roofs and asbestos
If the garage or outbuilding is old, the roof may be asbestos cement sheeting. HSE advises presuming asbestos in anything dating from before 2000, and treats asbestos cement roofs as fragile. Do not walk on one, and do not begin stripping an old roof without identifying what it is.
What a survey records
The surveyor examines the outlets, upstands, deck and covering, along with the fall, looking from the garden and from within the roof void, photographing defects and measuring the area. The findings lead to a recommendation, a repair or a new roof, and one written price follows.
Questions to ask
- Is the construction warm or cold, and how is moisture controlled?
- What falls will be achieved, and how?
- Which covering, and what is its typical life?
- Does the job need building control, and who arranges it?
If you are weighing up the drawbacks, our post on the main drawbacks of a flat roof covers what to watch for.
