The short answer
Roll roofing is a covering made of felt soaked in bitumen (a tar-like waterproofing material), often topped with a layer of mineral granules, and sold in rolls. In the UK it is called mineral felt or shed felt. It is laid in overlapping strips over a timber deck on sheds, garages and outbuildings. It is cheap and quick to fit, and works well on a small, simple roof. It lasts for a shorter time than rubber or GRP, and wears faster where water stands.
What roll roofing is made of
A roll of roofing felt has a base layer, usually polyester or glass fibre, which is saturated with bitumen. The top is coated with more bitumen and covered with mineral granules, typically slate grey, green or red. The granules protect the bitumen from sunlight, which would otherwise dry it out and crack it. The result is a flexible sheet, 1 metre wide on most rolls, and light enough for one person to lift.
The same product is sold under several names. In the UK you will see mineral felt, shed felt, roofing felt and bitumen felt. Higher grades are reinforced and sometimes labelled as a cap sheet. In America the same type of material is called asphalt roll roofing, because bitumen is called asphalt there. It is quite different from the asphalt shingles seen on American houses, and the UK equivalent for a garden building is often a felt shingle, covered on our page on felt roof shingles.
How it is laid on a garage or shed
Felt goes onto a dry, sound timber deck, which is usually OSB or plywood on an outbuilding. The deck must be flat, with no sagging, no loose boards and no protruding nail heads. The first strip is cut to length, laid at the eaves, and fixed with galvanised clout nails at the edges. Each following strip overlaps the last by about 75 to 100 mm, and the laps may be bonded with bitumen adhesive or heat. At the edges the felt turns over the drip trim, and where it meets a wall it is lapped up the brickwork and covered with a flashing.
A felt roof for a garden building is a DIY job for many people, which explains the product's popularity. Done on a small, flat roof with good laps and edge detail, it keeps water out for some years.
Grades differ. Cheap shed felt is a thin sheet that tears when you look at it hard. Heavier reinforced mineral felt, sometimes sold in 2 or 3 layer systems, is far tougher, and a built-up system with a base sheet and a cap sheet is a different roof altogether, closer to the torch-on felt used on house flat roofs (layers of bitumen bonded with a gas torch). The legacy name roll roofing covers all of these, which is one reason advice online is confusing.
How long it lasts, and where it fails
Single-layer mineral felt is a short-life covering. Sun dries the bitumen, frost cracks it, and standing water speeds the decline. Corners, laps and the line where the felt meets a wall are the usual weak points: nails lift, the edge curls, water runs under the lap and finds the deck. A felt roof on a shed in a sheltered spot can last longer than one on an exposed garage in a windy spot. Our page on how long shed felt lasts deals with the lifespan in more detail.
Felt is not a good choice for a roof that holds a pond, a roof with many penetrations or a roof you want to forget about. Moss and leaves hold moisture against it, and foot traffic wears the granules. A leak in a felt roof is often fixed by patching, and a worn one is a candidate for stripping and replacing.
Preparation decides the result more than the product does. Sweep the deck, fix loose boards, and use a primer where the maker asks for one. Lay in dry weather, since felt laid on a damp board traps moisture. Seal every nail head at the edges, and take care at the corner, where two strips meet and water collects. A sloppy corner is the first place a cheap felt roof lets go.
Choosing between roll roofing and the alternatives
For a small shed, mineral felt is often the right answer on price. For a garage you value, a single-sheet rubber (EPDM) or a fibreglass (GRP) roof will usually outlast it, with fewer seams to fail. Our page on whether EPDM is good for a garage roof explains the main alternative, and garage roof materials compares the rest.
When we look at an outbuilding we say straight away whether felt is enough or a false saving. A roof that can be repaired is repaired. Our garage roof replacement page covers the full service.
Every roof is different. A free survey with photos settles it for yours.
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