How Long Does Shed Felt Last?

What shed felt is

Shed felt is a roll of fabric or fibre soaked in bitumen, sold in different weights. The cheapest is thin and light, made for temporary covers and low-use sheds. Heavy-duty grades use a stronger base and more bitumen, and mineral felt has a gritty surface of crushed stone that shields the bitumen from the sun. Self-adhesive felt has a sticky back and saves nailing. Polyester-reinforced felts are tougher still, and are used on garages and larger outbuildings.

The weight and the surface matter because ageing comes from sunlight, which dries the bitumen until it cracks, and from movement, as the felt expands and contracts day and night. A thin felt with no mineral surface suffers both quickly.

What shortens its life

Poor fixing is the first cause. Felt laid with too few nails, or nails that are too short or rusty, lifts in wind and tears around the heads. Laps that are too narrow let water through. Felt laid on an uneven or damp deck, or over a surface with sharp nail points, wears early. Felt stretched tight in cold weather can pull apart in the heat, and felt left loose can flap and fatigue.

Exposure counts. A south-facing roof in full sun cooks the bitumen, while a roof under trees stays wet and grows moss. Wind at an open site works at the edges. Snow loads, and water pooling on a roof with too little fall, speed up the damage. Our page on felt roof shingles covers a different felt product for sheds and garden rooms.

Getting more from a felt roof

The deck should be dry, flat and free of sharp points. The roof needs a fall, and the felt should overhang the edge and be dressed into a drip so water runs clear of the walls. Laps should be generous and nailed through at close spacing, with the nails driven flat and not through the fold. Edges can be finished with a timber batten and clout nails, or with a trim. Adhesive laps and mastic round edges help in exposed spots.

Choose the grade for how long you want the roof to last. Saving a little on the roll often means recovering a few years sooner. For a garage, heavy polyester-reinforced felt or a proper flat roof system is more sensible than shed grade. Our comparison of garage roof materials weighs the choices.

Signs the felt is finished

Look for cracks across the surface, edges that curl or lift, bare patches where the mineral surface has gone, splits along the folds or laps, blisters and bubbles, and soft or stained boards beneath. A damp smell in the shed or marks on the underside of the boards tell you water is getting in. Moss and standing water point to a poor fall or a sagging deck.

Small splits can be patched with a strip of new felt and adhesive. Where the felt is brittle across the whole roof, patches fail, and a new covering is the sensible answer. Recovering over rotten boards just hides the problem, so soft boards should be replaced first. For flat garage roofs the better route is often a membrane such as rubber, covered in our page on whether EPDM is good for a garage roof.

Fitting felt in the right weather

Timing helps. Felt is stiff and cracks if unrolled in frost, and it is soft and marks easily in strong heat. A mild, dry, still day is best. Unroll the felt and let it relax flat for a while before fixing, so that it does not buckle after it is nailed. Start at the eaves and work up, with each higher sheet lapping over the one below, so water runs over joins and not into them. Work from the lower edge upward and keep your weight off the freshly laid felt, as it can scuff and tear while it is warm.

When to call a roofer

A small shed is a sensible DIY job for many people. A larger outbuilding, a roof you cannot reach safely, a garage or a roof with a rotten deck is worth handing over. If the existing covering might be asbestos cement, do not touch it, and have it tested first.

Our free survey includes photographs of your roof, so you can see what needs doing, and the fixed written quote names the covering and the work. New roofs come with ten years of written workmanship cover. More choices are on the garage roof replacement page.