Most home extensions have a flat roof. Rear kitchen extensions, side returns, wrap-arounds, garage conversions, most of them use a flat or low-pitched roof because it keeps the ceiling height sensible and doesn't overshadow the garden. But the roof is often the last thing anyone thinks about, right up until it starts to fail. Getting the choice right at the specification stage costs the same as getting it wrong, and saves you tens of thousands of pounds and a lot of hassle down the line.
What actually matters on an extension roof
Before we get into materials, four things are worth prioritising. Not all quotes make these obvious, so ask about them.
- Seamlessness. Extensions have lots of junctions where the new roof meets the existing house, upstands, parapets, walls, window heads. Every seam or overlap is a potential leak point. Fewer joints = fewer future problems.
- The deck underneath. A good roof on a rubbish deck will fail early. The deck should be at least 18mm OSB3 with proper falls, so water actually runs off rather than sitting.
- The lifespan of the covering. Your extension is designed to be there for 50+ years. A roof that needs replacing every 10-15 years means doing this job three or four times over its life.
- Warranty backed by the fitter, not just the manufacturer. A materials warranty is worthless if the installer disappears. Ask who guarantees the workmanship, and for how long.
The three main systems
We've covered this in more depth in GRP vs felt vs EPDM, but the short version for extension work:
- Felt is the cheapest way to cover a roof, and it looks fine on day one. It's laid in overlapping layers, and those overlaps are where it eventually fails. Expect 10 to 15 years and rising annual attention as it ages. Fine for a shed, hard to recommend on an extension you're building today.
- EPDM rubber is a step up: a single flexible membrane, usually bonded down. It handles large, plain roofs well and can last around 20 years. The membrane itself is one piece, but the joints at corners and upstands are bonded and taped, and those are the spots that need attention over time.
- GRP fibreglass is what we fit, so we're upfront about the bias, but the reasons are practical. It's built up wet, in place, into one continuous sheet with no joints anywhere, then finished with a hard, coloured topcoat. A properly laid GRP roof runs to 30 to 40 years with a written 25-year guarantee.
Extension-specific things to get right
Extensions have their own quirks that a general "flat roof" quote can miss.
- The join to the existing house. This is the single most common leak point on any extension: where the new roof meets the old brickwork. It should be properly upstanded and flashed, not just butted up and sealed.
- Rooflights. Skylights and lantern roofs are common on extensions and they're brilliant for light, but they're extra junctions that need detailing well. Cheap kerbs and quick trims cause leaks; well-fitted upstands don't.
- Bifolds and window heads. The roof edge above a run of bifold doors is a common weak point. It needs to shed water clear of the frame, not drip onto it.
- Are you standing on it? If the roof is going to be a terrace, or even just accessible for cleaning windows above, say so at the quote stage. A proper walk-on finish is different to a standard one, more on that in can you walk on a GRP flat roof.
- Insulation. Building regs may require the roof build-up to include insulation. A good roofer works with your builder or architect on this, not against them.
Planning and building regs
An extension itself will usually need building regulations approval and sometimes planning permission, depending on the size, height and where you live. The roof itself normally rides on top of that approval rather than needing its own. If you're not sure where you stand, our guide on planning permission for a flat roof covers the basics, and your local authority website has the definitive answer for your postcode.
When to book your roofer
Book the roofing in early, and specify it in writing. On a building project the roof usually goes on right after the deck is finished, so you need someone lined up ready to slot in. Because GRP cures best in dry, mild weather, summer bookings fill up fast. Getting your date locked in a few weeks ahead beats scrabbling around at the last minute.
Getting an honest opinion
If you're planning or mid-way through an extension and want a straight view on what the roof should be, we're happy to come out, look at the drawings and give a written, fixed-price quote. No pressure, no sales calls, just a real answer.
Free, fixed-price quote. Call 07976 730433 or request one here.
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