What Is Built-Up Felt Roofing?

What the layers are

A built-up roof is assembled from the deck upwards. On a concrete, timber or metal deck, a vapour control layer goes first, to stop moist air from the building reaching the insulation. Rigid insulation boards follow, and on a commercial roof these are often tapered to build in a fall. Then come the felt layers. A base sheet is fixed or bonded to the insulation, and one or more further layers are bonded over it, finishing with a cap sheet. The cap is often surfaced with mineral chippings or granules, which shield the bitumen from sun and give some fire protection.

Each sheet is a mat of polyester or glass fibre soaked in bitumen, and many are modified with polymers to stay flexible in the cold and firm in the heat. These modified bitumen sheets are far better than the old oxidised bitumen felts, which cracked and split. If you are comparing felt with other options, our guide to commercial flat roof systems sets them side by side.

How the layers are joined

Torch-on is the traditional method. A gas torch heats the underside of the roll until the bitumen melts, and the roll is pressed into place. It makes a strong bond, but it uses an open flame, and on a building with combustible material in the roof zone it brings a risk of fire, so hot work needs careful controls. Hot-poured bitumen is an older route, used less now. Cold-applied systems use adhesive, and self-adhesive sheets peel and stick, which avoid flames and suit occupied or sensitive buildings. Mechanical fixing through the base layer is another option, used with some systems.

All methods depend on the laps. Side and end laps must be wide enough and fully bonded, with the bitumen visibly squeezing out of the edge. Staggering laps from one layer to the next means a gap in one layer is covered by the solid sheet above or below.

Why it is still widely specified

Felt has a long record on flat roofs, and the skills to lay and repair it are widely held. It is tough underfoot when well laid, it can be repaired in small pieces without replacing the whole roof, and it suits awkward shapes with many upstands and penetrations. Modern modified sheets are far more durable than the old types, and a two-layer high-performance system is a sensible choice for many shops, offices and schools. It also takes a green or solar load well if the deck allows, and a ballast of paving or gravel can protect it from sun and wind.

It is not the best fit everywhere. A roof with complicated rooflights, repeated hot-work restrictions or a very light structure may do better with a single-ply or liquid system. The right answer depends on the building, not on habit.

How built-up felt fails

The common faults are well known. Blisters form when moisture trapped beneath the felt expands in the sun. Splits open where the deck moves or where the felt has become brittle. Ponding water speeds up ageing and washes the granules from the cap. Laps open where they were not properly bonded. Details fail at upstands, outlets and parapets, because these are made by hand and the felt has to bend through tight angles. Foot traffic damages the cap if no walkway is used.

The multi-layer build gives some resilience: a defect in the cap may not reach the deck. But once water is under the felt, it can travel along the layers and appear far from the entry point. Moisture scans and core samples show where the insulation is wet. See our page on tapered insulation for why falls matter, and finding a leak in a warehouse roof for tracing leaks.

Repair, overlay or replace

Small defects can be patched with a new piece of felt bonded over the damage, with the surrounding granules removed first so the patch can stick. Blisters are cut open, dried and re-sealed. Where large areas have failed or the insulation is wet, an overlay or a strip and replace is the sound choice. Felt can also be covered with a single-ply membrane or a liquid coating if the deck is dry. Our guide to single-ply roofing describes that alternative.

On a felt roof we photograph the cap, the laps and the upstands, and add moisture checks wherever the surface hints at wet insulation beneath. A repair is quoted where a repair works, and the fixed written quote makes clear what a replacement would add. The overall range of our business work is grouped under commercial roofing in Basingstoke.