Moss, frost and decay on stone slate roofs

How moss and frost damage stone slates

Moss and frost work together. Moss keeps the stone wet, and frost then breaks up wet stone.

Cotswold stone slates are limestone, which takes up some water. When water in the stone freezes, it expands. Over many winters this makes the surface flake and the slate split along its layers.

Cotswold District Council guidance says that owners once scraped moss off their roofs as routine. It says that many roofs now need near-total replacement because this stopped, and that a roof scraped once a year could last twice as long.

Shaded, north-facing slopes and roofs under trees stay wet for longer. They often show moss and frost damage first.

Signs of decay to look for

Look from the ground with binoculars and from inside the loft. Note which slopes are worst.

• Thick moss cushions, especially in the lower courses.
• Slates with flaking or powdery faces.
• Slates split along their layers, or with broken corners.
• Grit, flakes or stone dust in the gutters or on the loft floor.
• Damp patches on laths or rafters after frost and rain.

How to look after a stone roof

Good care keeps the stone as dry as possible. Small, regular jobs cost less than a re-lay.

Ask a roofer to remove heavy moss by hand. Ask how they will do it without harm to the slates. Avoid methods that soak the stone or scour the surface.

Keep gutters and valleys clear, so water does not sit at the eaves. Cotswold District Council suggests regular visual checks of gutters, valleys and slates.

Cut back branches that hang over the roof, if you can do so lawfully. Trees in a conservation area can need notice to the council before you cut them.

Re-fix slipped slates soon, before water gets under the next course.

What not to do

Do not walk on a stone slate roof. The slates crack under weight.

Do not spray foam or sealant on the underside. It traps damp in the laths and pegs, and it stops the slates being reused.

Do not use a chemical moss killer before you talk to the roofer. Ask about its effect on limestone, on run-off, and on bats or birds that use the roof.

When to call a roofer

Call a roofer when moss is thick, when slates flake or split, or when you find stone dust in the loft.

Ask for a stone slate specialist. They can tell surface weathering from real decay, and they can say if a repair or a re-lay is right. Ask them to show you the worst slates.

If the slates are badly decayed, some will need replacement. Read our guide Reclaimed or new stone slates before you agree a specification.

Bats often roost in stone roofs. They are protected by law. If you think bats use your roof, ask about a survey before work starts.

What it may cost

Moss removal and small repairs cost much less than a re-lay. The price depends on access, the size of the roof and the number of slates to replace.

Our guide Cotswold stone roof cost explains what changes the price of repairs, re-lays and new stone roofs.

Questions

Sources