New Flat Roofs for Extensions, Dormers & Outbuildings

The short answer: A new flat roof is designed before it is built: the fall, the deck, the insulation position, upstand heights at every abutment and the outlet positions. Agreeing those on drawings prevents the compromises that follow a roof worked out on site.

New build

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. On a new build there is nothing there to dispute; bringing an existing cold roof up to the same standard is a conversion, and a different job with different constraints.

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.

Comparison

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. Where the finished roof is meant to be walked on rather than merely watertight, it becomes a roof terrace and the surface is designed with the structure. Where the outbuilding already has a covering and the question is whether it can be saved, that is a re-covering rather than a build, and it turns on the boarding rather than the drawings.

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.

On site

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, as structural and decking work. 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.