How do I re-felt a shed roof?

Shed felt has a life of roughly ten to fifteen years and it fails in a predictable order: the exposed nail heads first, then the edges, then the ridge, and finally the whole sheet goes brittle and cracks along the rafter lines. Once cracking is general, patching is throwing an afternoon away. Re-covering is a straightforward day's work and the whole job is in the detailing at the three edges and the ridge — the middle of a felt roof almost never leaks.

Safety

What you'll need

• Shed roofing felt, enough for both slopes plus a ridge strip
• Felt adhesive
• Galvanised clout nails, 13-20mm
• Sharp knife and a straightedge
• Claw hammer and a nail bar or pincers
• Stiff brush
• Replacement roof boards or ply (optional)
• Drip edge trim or timber fascia battens (optional)

Step by step

1. Pick the weather, not the day.Dry, mild and still. Felt is stiff and cracks when cold, adhesive will not grab below the temperature on the tin, and a half-stripped roof in rain is a disaster. Do not start unless you can finish.
2. Strip the old felt and every fixing.Pull the felt off in sheets, then go over the whole deck and pull every clout nail and staple. One left proud will cut through the new felt within a season.
3. Inspect and repair the deck.Press every board. Soft, delaminated or springy boards come off and are replaced — 12mm exterior or treated ply is a substantial upgrade on the 9mm sheet most sheds are supplied with, and now is the only easy time. Punch any protruding fixings below the surface.
4. Cut the felt to length and let it relax.Cut each piece a little long, roll it out flat in the sun for half an hour and let it lose its curl. Felt fought into place cold will pull back and lift at the edges.
5. Start at the bottom of one slope.Felt sheds water by overlapping downhill, so every course starts at the eaves and works up. Lay the first sheet with a 50mm overhang at the eaves and at each gable, and fix along the top edge only.
6. Nail only where the next lap will cover the nails.Clouts at roughly 100mm centres along the top edge of each sheet, and at 50mm centres along the eaves and gable edges where they will be covered by the trim. Nails in the open field of the felt are the single commonest source of leaks.
7. Lap the next course over the first by at least 100mm.Brush a band of felt adhesive across the top of the lower sheet, lay the upper sheet into it and press it down along the whole lap. The adhesive does the sealing; the nails only hold it while it grabs.
8. Take the slope sheets over the ridge and lap them.Each slope's top sheet runs over the ridge and at least 100mm down the far side. Then a separate capping strip, at least 300mm wide, is bedded in adhesive over the ridge covering both. Three thicknesses over the ridge line is what you are aiming at.
9. Detail the eaves and gables.Turn the felt over the edge and nail underneath, or fit a drip edge trim or a timber fascia batten over the felt so the edge cannot lift and water is thrown clear of the cladding. This is where wind gets under a felt roof.
10. Trim, then check every edge by hand.Run a knife along a straightedge to trim the overhangs, then work round the whole perimeter pressing down. Any edge you can lift with a finger will be lifted by the wind.

Tips

• Buy the felt a size up from the minimum. Offcuts are cheap; a lap 20mm short is a leak you cannot see until winter.
• Torch-on felt is not a DIY product on a timber shed roof. The self-adhesive shed felts now sold are a genuine improvement on cold adhesive if you can get the roof properly clean and dry.
• If the roof boards have rotted, look at why. Nine times out of ten it is condensation from an unventilated shed rather than the felt.
• Work the roof in the order eaves, field, ridge, edges. Doing the ridge before the slopes forces you to lap uphill somewhere.
• Keep the old felt as a template for cutting the new pieces, especially around a roof with an unequal overhang.

Common mistakes

• Felting over the old covering — It buries the nails and the ridge detail you cannot now inspect, adds weight, and the new felt only lasts as long as the surface it is stuck to. The old felt also traps a layer of damp against the boards.
• Lapping the wrong way at the ridge — If the capping strip is laid before the slope sheets, or a slope sheet stops short of the ridge, the lap faces uphill and water is driven under it by wind. Every lap must face downhill.
• Nailing through the middle of the felt — Every one of those nail heads is a hole with a raised edge. They are the first thing to leak and they are impossible to seal properly afterwards.
• Working in cold weather — Cold felt cracks when bent over a ridge or an edge, and cold adhesive never develops a bond. Both failures appear months later.

Alternatives

• EPDM rubber membrane: A single sheet with no laps at all across the roof, bonded with a contact adhesive. Considerably longer life than felt and the obvious choice on a flat or very shallow shed roof, but it needs a clean sound deck and dry, mild conditions to bond.
• Corrugated bitumen sheet: Screwed through the crowns onto battens or the deck, quick to fit and tolerant of a rough substrate. Not as neat, and the sheets need proper end and side laps and a matching ridge piece.
• Felt shingles: Nailed in courses like tiles over a felt underlay. The best-looking option and the most work, and it needs a roof pitch above the shingle manufacturer's minimum.

Keeping on top of it

• Check the ridge and all four edges for lifting (Twice a year and after any gale)
• Clear leaves and moss off the roof (Autumn)
• Check inside for damp staining on the boards (After heavy rain in winter)

Questions people ask