If you are extending, the roof decision usually comes down to cost, headroom, planning constraints and how the extension looks against the existing house. Both options work well when they are specified and built properly. Both fail when they are not.
Where flat roofs make sense
Flat roofs suit single storey rear extensions, garages, dormers and porches, particularly where a pitched roof would block light to windows above or breach a height restriction. They are usually cheaper and quicker to build, and modern systems are far better than the felt roofs that gave flat roofing a poor reputation decades ago.
Felt
Modern torch on felt, laid in layers, is cost effective and reliable when the falls and edge details are right.
Fibreglass, GRP
A seamless rigid surface with no joints to fail. It handles foot traffic well, so it is a good choice for a balcony or any roof you will actually walk on.
Rubber, EPDM
A single sheet membrane with very few seams, flexible in cold weather and quick to fit with little disruption.
Where pitched roofs make sense
A pitched roof sheds water naturally, generally lasts longer, and usually ties in more sympathetically with an existing tiled house. On larger or two storey extensions it is often the obvious choice, and in conservation areas it may be the only one the planners will accept.
The thing that actually determines success
Falls and detailing. A flat roof is never truly flat, it has a slight fall so water runs to an outlet. If the fall is wrong, water sits in a pond and the covering eventually fails whatever material it is. The other common failure point is the upstands and edge trims, not the middle of the roof. Get those two things right and a flat roof will serve you well for many years.
Ask before you commit
If you are weighing this up, we are happy to look at the plans and give you an honest view, along with a written quote within 48 hours of the visit.