Flat Roofing & Roof Lanterns in London

A flat roof is never actually flat. It is laid to a fall so that water runs off it, and when that fall is too shallow or is interrupted, water sits. Standing water finds every weakness in a covering and works at it through each freeze and thaw. Most flat roof failures trace back to falls that were never set correctly rather than to the covering material itself.

The second common failure is at the edges and upstands, not in the middle of the field. Where a flat roof meets a wall, a parapet or a rooflight kerb, the covering has to be dressed up and terminated so that water cannot track behind it. These junctions take the longest to detail and are the first thing to check on a roof that leaks.

GRP fibreglass is laid wet and cures into a single seamless layer over the whole roof, including upstands and trims. Because there are no joints, there is nothing to lift or split at a seam. It needs dry conditions and a suitable temperature to cure properly, which means it is weather-dependent to install.

Felt systems are cheaper and applied in layers, with the seams as the weak point. Single-ply membranes are welded rather than bonded at seams and perform well over larger areas. For a domestic extension roof of typical size, a seamless GRP layer is generally the most durable choice for the money.

Daylight from above reaches significantly further into a room than daylight from a window in a wall, which is why a lantern transforms a deep extension in a way that additional glazed doors do not. The kerb the lantern sits on is part of the roof structure, and it has to be built, insulated and weathered as such — a lantern is only as watertight as the upstand beneath it.

Glazing specification matters for comfort as much as for heat loss. A large area of overhead glass without a solar control coating turns a north-facing room pleasant and a south-facing room unusable in July.