Cotswold stone slate and natural slate roofs

What the job involves

Stone slate work is either a repair or a re-lay. A repair re-hangs single slates. A re-lay strips a whole slope, sorts the slates, and lays them again on new laths.

On a re-lay, the roofer sorts the slates by length. The largest go at the eaves and the smallest at the ridge. Cotswold District Council guidance says the eaves slates are about 600 mm long and the smallest about 150 mm.

Each slate traditionally hangs from an oak peg. The roofer also re-forms ridges, verges and valleys. A swept valley uses cut slates to turn the roof in a curve, with no lead. It takes the most skill.

Local Cotswold factors

Cotswold stone slates come from the local oolitic limestone. Some were split by hand soon after quarrying, and are called presents. Others, such as Stonesfield slates from West Oxfordshire, were split by frost. The Stone Roofing Association says frost-split slates are hardly made today.

The roofs are heavy. The council guidance gives about 1,000 kg for each 10 square metres. The timbers must be sound before a re-lay.

Welsh slate is common on Victorian houses. It comes in one size and is nailed to battens. The repair method is different, so ask which material your roofer knows.

Signs the roof needs work

A stone roof needs work when slates start to slip in numbers or when water gets in. Look for these signs from the ground and the loft.

• Several slipped slates on one slope, which suggests the pegs or laths are failing.
• Slates that flake, crumble or split. Frost is the main cause of decay.
• Thick moss, which holds water against the stone.
• A dip in the roof that is new or getting worse.
• Lime mortar falling out at the ridge or the verge.

What to ask a stone slate roofer

Ask questions that show real stone slate experience. A general roofer may not have it.

• Can I see stone slate roofs you have re-laid, and speak to the owners?
• How will you sort, test and reuse my slates?
• Where will new or reclaimed slates come from? Can you prove the source?
• Will you keep the swept valleys and the stone ridge?
• Who will deal with listed building consent or building control?

The rules: consent and Building Regulations

A re-lay is building work, and a listed roof may also need consent. Check both before work starts.

GOV.UK lists replacing roof coverings as work that can need building regulations approval. Part L sets two limits: more than 50 percent of the roof, or more than 25 percent of the whole outside of the house. Above either limit, the whole roof should meet the insulation standard where this is practical. Listed buildings, conservation areas and traditional stone buildings get some flexibility. Building control should take the conservation officer's advice.

A roofer in the NFRC Competent Person Scheme can self-certify like-for-like re-roofing. The scheme says it cannot accept true historic details such as wooden pegs or riven laths. Those jobs go to building control.

On a listed building, a change of material usually needs listed building consent. Ask the council before you agree a specification.

Bats often roost in old roofs. They are protected by law. Ask for a bat survey if you think bats use your roof.

Questions

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