Roofing Materials

The roof is the first line of defence against water ingress. A poorly specified or incorrectly installed roof covering will cause problems that are expensive and disruptive to fix, often long after your builder has moved on to the next job.

These guides cover the roofing materials used on domestic extensions: the tiles and coverings that shed water, the membranes and flashings that prevent it getting underneath, and the flat roof systems that work where pitched roofing cannot. Each page explains what the material does, how it is installed, and the inspection points your building control officer will want to see before issuing a completion certificate.

Where to start

The first question is whether your extension has a pitched roof or a flat one, because the two share very few materials. Your drawings will say which. After that, the details that fail most often are where the new roof meets the existing house: lead flashing, lead soakers and the lead wedges that hold it in the chase. Read those whichever roof you have.

Pitched roofs

Under the tiles. Breathable roof membrane, roofing battens, counter-battens, membrane lap tape, the eaves support tray and the underfelt collar for pipes coming through. This layer is what keeps the roof dry if a tile slips, and it is hidden within days, so photograph it before the tiles go on.

The covering. Concrete roof tiles or clay roof tiles, finished with ridge tiles, hip tiles, a dry ridge system and a dry verge system. Matching the tile to the existing house is usually a planning point as well as a looks one.

Rooflights. A roof window flashing kit comes matched to the window and the tile profile. A generic one is a common source of leaks.

Flat roofs

EPDM rubber. The EPDM membrane itself, with bonding adhesive for the main area, contact adhesive for perimeters and upstands, seam tape and a termination bar where it meets a wall.

GRP fibreglass. Start with the GRP overview, then the layers in order: edge trim, chopped strand mat, polyester resin with its MEKP catalyst, and the topcoat. GRP depends heavily on weather and mixing, which is why it goes wrong more often than EPDM.

Flashing and roofline

For flashing, choose between lead flashing (finished with patination oil) and lead-free flashing. Around the edge of the roof you have uPVC fascia, ventilated soffit board, bargeboards, guttering and downpipes.

Where these fit in the build

The timbers go up in roof structure, and these materials are fitted in roof covering and roof windows and rooflights. Once the roof is weathertight, the trades inside can start.