Can You Put a Flat Roof on an Extension?
Flat roofs are one of the most popular choices for single-storey extensions — they're cost-effective, they allow big rooflights for natural light, and with single-ply they look superb. Here's what to consider before you build.
Building Regulations & warm roofs
A new extension roof will generally need a warm-roof build-up with enough insulation to meet the required U-value. This keeps the new room comfortable year-round and designs out condensation. It's worth getting the spec right at the planning stage rather than retrofitting later.
Let the light in
Flat roofs are ideal for roof lanterns and skylights, which transform an extension with daylight. The key is detailing the kerb properly so it's completely watertight — we set ours up ready for glazing on a Sprotbrough extension with two large skylights.
Drainage matters
"Flat" roofs aren't truly flat — they're built with a slight fall so water runs to the outlets. Getting the falls and outlet positions right is essential to avoid ponding, which is the enemy of any flat roof's lifespan.
The right finish
A single-ply PVC membrane gives a clean, flat, architectural finish and a 30–40+ year life — ideal for an investment like an extension. We supplied and fitted exactly this on a Doncaster extension in fully-adhered Sika Sarnafil.
Get expert advice on your roof
JLH are single-ply flat roofing specialists covering Rotherham and across South Yorkshire, backed by a 20-year workmanship warranty. Book a free, no-obligation survey and we'll give you honest advice and a clear, fixed quote.
Frequently asked questions
Is a flat roof cheaper than a pitched roof for an extension? +
Usually yes — a flat roof has a simpler structure and is quicker to build, and it allows large rooflights that a pitched roof can’t.
Can I have a lantern or skylight on an extension flat roof? +
Absolutely — flat roofs are perfect for them. The critical part is a properly built, watertight kerb, which is exactly the kind of detailing we specialise in.
Do I need Building Regulations approval? +
Extensions generally require Building Regs sign-off, which covers the roof’s insulation and structure. We build to the required standard and can liaise on the roofing detail.
Need a flat roof done properly?
Book a free, no-obligation survey across Rotherham & South Yorkshire.