New flat roofs
New Flat Roofs for Extensions, Garages & Dormers
Building one is a different job from replacing one. There is nothing to strip, nothing hidden and no verdict to wait for: the roof is drawn before it is built, so the decisions that matter get made on paper, with whoever is putting up the structure, well before anyone reaches for a torch.
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Building a flat roof from nothing
The advantage of a new roof is that everything can be right by design instead of corrected later. Falls are the clearest example: the slope is drawn in before a sheet goes down, by firrings on the joists or tapered insulation above the deck, and the outlet sits where the fall sends the water rather than where a previous century put a hopper. Getting that right costs nothing now and is a puddle for the life of the building if it is missed.
The build-up, in order
A warm roof is the modern default, and the name describes where the insulation goes: above the structure, so the timber underneath stays at room temperature. Upward from the room that means the deck, then a vapour control layer sealed over it so moist indoor air never reaches the cold side, then insulation boards, then the waterproofing across the whole lot. Cold build-ups, with the insulation down between the joists, still have their uses when the design cannot spare the extra height, and our guide to warm roofs and cold roofs explains what each one asks of the design.
What goes on top
Most new roofs we build are finished in torch-on felt: tough, repairable, and forgiving of the foot traffic a building site puts on it before the job ends. Where the geometry is fiddly, or where the roof runs up against a neighbour's boundary and a naked flame has no business being there, the answer is cold-applied liquid waterproofing instead, laid as the HYDRONYLON system whose assessment we name in the specification rather than leaving you to guess. Plenty of roofs get both: sheet across the open area, liquid at the awkward end.
Three jobs, and what actually differs
A rear extension tying into the house
The roof itself is usually the easy part; the junction with the existing house is the job. The covering has to turn up the main wall high enough to survive driving rain, tuck into brickwork that was never built with this roof in mind, and meet whatever the existing rainwater goods are doing. Where bifold or patio doors fix the finished floor level, the build-up has to work back from that height rather than the other way round.
A garage or outbuilding
Simple geometry, and the questions move to the structure. Old outbuildings were built with a different lifespan in mind: sound walls over tired timbers, or a perimeter rebuilt more than once. What the building can carry, and whether its edges are square enough to detail cleanly, decides the specification.
A dormer
More detailing per square metre than any other flat roof we build: a small top, two cheeks, a junction with the pitched roof either side, a window head below and often a trim along the front, all within a couple of metres. The field is trivial and the edges are everything, which is why dormers are where cheap work shows first.
Working alongside your builder
Most new roofs land in the middle of somebody else's programme, so three things are agreed in writing before we price anything.
- Who lays the deck. Usually the builder, boarding to our specification and to the falls we set out; sometimes us. Either is fine. What is not fine is each side assuming the other one is setting the slope.
- When we come. After the structure and the boarding, before anything is loaded onto the roof and before the internal finishes go in. Roofs that go on too early get walked over by every following trade; roofs that go on too late hold up the plasterer.
- What is decided. The build-up and the insulation depth, because they set finished heights; where the water leaves the roof; the covering; and who supplies what. A roof cannot be priced honestly while any of those is still open.
If new fascias, soffits or gutters are part of the same job, they are roofline work and belong in the same quotation rather than as a surprise at the end.
Building control, in plain terms
A new flat roof is building work, so it falls under the Building Regulations, and that includes requirements about insulation as well as the construction around it. What applies to your project depends on the project: what it is attached to, what it is for, and what your building control body wants to see. We build to the approved details for the job and provide what the inspector needs on the roof build-up.
What you will not find here is a regulation number or a target figure: those belong in your project's approved documents, not on a page that cannot see your drawings.
What moves the price
Six things, in roughly the order they matter.
- Size and shape: corners and changes of direction cost more than area.
- Access: what can be carried in, and what needs scaffold.
- The deck, and whether it is ours to lay or arrives finished.
- Insulation depth, which follows the design rather than the budget.
- The count of abutments, upstands and penetrations, which is the real measure of a roof's difficulty.
- Roofline work, if the job includes it.
No rate goes on this page for the same reason no timescale does. The figure is written for your drawings, and it is written before anything is ordered.
Working the other way round, with a tired roof already up there? That is flat roof replacement, and if the question is how much life it has left, a condition survey answers that first.
Frequently asked questions
Do you take on work for builders as well as homeowners?
Both, and the arrangement differs. With a builder we usually supply and fix the roof against their drawings and their programme, taking the deck as they leave it once the specification is agreed. With a homeowner we take the roof end to end and deal with the builder directly, so nobody in the middle is relaying decisions they cannot judge.
How long does a new flat roof take?
No number belongs on a web page: the size, the access, the weather windows and the builder's own sequence all move it, and a roof that waits three weeks for a structural opening is not a slow roof. What you get instead is the programme in writing, and a fresh one whenever the site moves it.
Warm roof or cold roof on a new extension?
Warm is the modern default: the insulation sits above the deck, the structure stays at room temperature and condensation is designed out rather than ventilated away. Cold build-ups still make sense where height is genuinely trapped, by a threshold or a planning line, and the design settles which applies before anything is ordered.
Who lays the deck, you or the builder?
Either, but it is agreed in writing before the roof is priced. Most often the builder frames and boards it to our specification and we take it from there; where it suits the programme better, we do the boarding ourselves. What never works is both parties assuming the other set the falls.
Can you put a new roof on an old garage?
Usually, once we have looked at what is carrying it. Old outbuildings often have sound walls and a tired structure, or the reverse, and the perimeter matters as much as the span. If the building cannot take a modern build-up without work first, we say so before quoting rather than after starting.
Got drawings, or just an idea?
Send what you have, even if it is a sketch and a postcode. We will tell you what the roof needs and what it will cost to build properly.
Talk about a new roof