Traditional three-layer bituminous felt is still the most common flat roofing material for domestic garages in the UK. While EPDM and GRP are more durable alternatives, felt remains a practical and widely available option that an experienced DIYer can apply to a small roof.

Materials and Tools

For a traditional felt roof you need Type 3 glass fibre felt (three rolls for a three-layer system), roofing felt adhesive or a blowtorch for torch-on felt, a stiff brush for adhesive application, a Stanley knife for cutting, a felt roller for bedding, and appropriate flashing at all edges and upstands. Use only roofing-grade felt; cheaper felt products sold for shed roofs are not suitable for garage flat roofs exposed to full weather loading.

Preparation

The deck must be clean, dry, and sound. Replace any soft or rotten boards. Ensure the deck has the correct fall (minimum 1 in 80). Fix any loose boards. Apply a bitumen primer to the deck surface if using cold-applied adhesive. The deck must be completely dry before any felt is applied: laying felt over damp timber traps moisture that causes rot and blistering.

Laying the Felt

Start at the lowest point of the roof and work upward so each course laps over the one below. Lay the first layer of felt dry across the full deck area, cutting to allow for upstands at parapets and abutments. Bond the second layer to the first using cold adhesive or torch-on technique, ensuring a minimum 75mm side lap and 150mm end lap on all joints. Lay the third (cap sheet) layer similarly, offsetting all joints from the layers below. Roll each layer firmly to bed it fully and eliminate air pockets.

Upstands and Flashings

At all edges, walls, and penetrations, turn the felt up to a minimum 150mm upstand. Form the upstand by cutting the felt and folding it up the vertical surface, bonding firmly. Cover the upstand with a stepped flashing of lead or self-adhesive flashing tape let into the masonry at the top edge and sealed with appropriate sealant. The flashing is the most critical waterproofing detail: more leaks occur at upstands and flashings than anywhere else on a flat roof.

Summary

Lay three layers of Type 3 glass fibre felt from lowest to highest with minimum 75mm side laps and 150mm end laps, offsetting all joints between layers. Bond with cold adhesive or torch-on technique. Form 150mm minimum upstands at all edges and cover with stepped flashings. Work only in dry conditions above 5 degrees Celsius.