Top benefits of flat roofs: why more property owners are choosing them

Flat roofs have a reputation in Ireland, and it was earned in the 1970s and 80s by thin three-layer felt laid over decks that had no fall. Modern flat roofing is a different trade. Done properly, a flat roof on an extension or a dormer will outlast the kitchen underneath it.

Why people choose them

• Space and light. A flat roof lets an extension sit under existing windows without blocking them, and takes rooflights easily — which is usually the whole point of the extension.
• Cost. Less structure, less covering, quicker to build than a pitched roof of the same footprint.
• Simple, clean lines. A flat roof suits a modern extension on an older house far better than a pitched roof grafted onto the back of it.
• Usable area. Balconies, terraces and green roofs are only possible on a flat roof, and fibreglass in particular will take foot traffic.
• Easy maintenance. You can walk it, inspect it and clear the outlet without scaffolding.

The three coverings worth having

Torch-on felt is the workhorse: proven, economical and good on large or awkward shapes. Modern systems are a world away from the old stuff — two or three bonded layers with a mineral cap sheet.

GRP fibreglass is laid wet and cures into one seamless sheet with no joints at all. It is hard-wearing and the right answer for balconies and anything that gets walked on.

EPDM rubber is a single sheet of membrane, often one piece across the whole roof. Very few joints means very few places to fail, and it goes down fast on simple rectangles.

What decides whether it lasts

Not the brand of the material — the details. Insist on these and a flat roof will not trouble you:

• A real fall. Firrings should give at least 1:80 so water leaves rather than sits. Ponding is the single biggest predictor of early failure.
• Proper upstands. The covering should turn up the wall at least 150mm and be dressed in or covered by a flashing — not stopped short and sealed with mastic.
• An outlet that suits the roof. One small outlet on a big roof will block, and then the roof becomes a pond.
• A sound deck. New covering over soft, wet decking is money thrown away.
• Ventilation. A cold-deck roof needs an air gap or it will rot from condensation inside.

If you are pricing a flat roof for an extension in Cork, ask what fall is being built in and how the upstands are being handled. The answers tell you more about the roofer than the quote does.