How to re-felt a garage roof

Inspired by Dalton Roofing. This guide draws on the "Refelting a garage roof" tutorial from Dalton Roofing, a family-run roofing company based in Sheffield with over three decades of experience. Their walkthrough covers high-performance mineral felt in a way that is clear and practical for a capable DIYer.

1. Pick the right day and the right felt

Felt goes on in dry weather and above about 10°C. Cold or damp conditions stop the adhesive bonding properly. Give yourself a full dry day — roofing is not a job to rush in patchy weather.

For the felt itself, green mineral felt is the traditional choice for garage roofs and is widely available. High-performance torch-on felt lasts considerably longer, but requires a gas torch and more care around edges. Both can work well. Cold-applied adhesive systems exist too — these suit DIYers who do not want to use open-flame equipment.

2. Strip the old felt completely

Start at the ridge and work down. A bolster chisel, claw hammer, and flat pry bar get under the felt fairly quickly. Pull up the tacks or staples as you go — leaving old fixings in the boarding causes lumps under the new felt.

Clear everything back to bare boarding. Trying to felt over existing layers is tempting but usually a false economy. The old felt traps moisture, the new layer cannot bond flat, and the whole thing fails again within a year or two.

3. Check and repair the boarding

This is the critical step that most people skip past too quickly. Inspect every board once the felt is off. Soft, spongy, or visibly rotted sections must come out. Press down on each board firmly — a bit of flex is normal, but anything that gives noticeably under load needs replacing.

That said, do not rip out boards that are structurally sound just because they look a bit tired. Solid boarding that takes a nail cleanly is fine to work with. Replace only what is genuinely rotten.

4. Set the fall and fix any drip edge

A flat roof needs a fall — ideally at least 1 in 40 — to get water off it. If the existing structure drains properly, you do not need to change anything. If water clearly ponds in the centre, tapered insulation boards or firring pieces can introduce a fall without structural changes.

Check the drip edge or drip trim at the eaves. If it is rusted through or missing sections, replace it before the felt goes on. The edge detail is where flat roofs most often fail.

5. Lay the first layer of felt

Cut the first layer of felt to length. Start at the low end of the roof and work upwards so each course overlaps the one below. A minimum 75 mm side lap and 150 mm end lap is the standard guidance — do not skimp on the overlaps.

Nail or staple the felt at roughly 150 mm centres along the edges and about 300 mm across the field. Do not pull it tight — felt needs just a small amount of slack to move without splitting in cold weather.

6. Apply the cap layer

The cap or surface layer is the one that gets the mineral finish and protects everything below. Apply felt adhesive to the first layer, leave it to become tacky, then roll out the cap layer from the low end upwards, pressing it firmly as you go. Roll the overlaps down well and brush extra adhesive into any raised edges.

Turn the felt over the fascia at the eaves and fold it neatly at the verges. Bonded edges are far less likely to lift in wind than ones that are simply cut flush.

7. Seal the abutments and flashings

Where the roof meets a wall — at the back of a garage, for example — the felt needs to turn up at least 150 mm and be tucked under a lead or aluminium flashing. If the existing flashing is sound, re-bed it in mastic after you have dressed the felt up behind it. If it is cracked or missing, replace it.

Run a bead of roofing mastic along any joined edges and around any pipes or fittings that pass through the roof. Water will find any gap, given enough time.

8. Check the finished surface after rain

Wait for a decent rainfall and then look at the roof from below if possible, and check for drips or damp patches at the boarding joints. Walk around the outside and check that the drip edge is shedding water cleanly away from the fascia. Most problems show up quickly if they exist.

When to call a handyman

Call Richard if the boarding is extensively rotten, the roof structure itself has moved, or you are not comfortable working at height. Torch-on felt is also a job best left to someone with the right equipment and experience. A garage roof done properly should last 15 to 20 years — it is worth getting right.