Common Mistakes to Avoid When Renovating a Cornish Granite Cottage

Cornish granite cottages look indestructible. Walls two feet thick, Delabole slate over your head, stone that has shrugged off Atlantic weather since before anyone thought to install a damp course. Then a gable gets repointed in cement, the walls get painted with a modern masonry paint, and two winters later the joints are cracking, salts are blowing the face off the granite and the kitchen smells faintly of cellar. Old buildings rarely fail because they are old. They fail when they are treated like new ones.

Cement Mortar Is the First Mistake

Almost every serious problem in an old granite wall starts here. Cottages like these were built with lime mortar: softer than the stone, slightly flexible, and vapour-permeable. Water that gets in through driving rain can evaporate back out through the joints. Cement mortar does the opposite. It sets hard and brittle, it is far less permeable, and in many older mixes it is actually stronger than the granite around it.

That combination causes damage in three ways. Moisture trapped behind the pointing has nowhere to go, so it moves into the stone and sideways into the wall. The rigid mortar cannot accommodate the small movements granite makes with temperature change, so cracks open at the edges of the joints. And when a joint finally fails, the cement takes chunks of granite with it rather than crumbling as it should.

What to do instead: repoint with a lime mortar matched to what is already there — a lime putty mix with a well-graded aggregate, or a natural hydraulic lime of low to moderate strength. Rake out the joints by hand, not with an angle grinder that gouges the stone. Take the joint back to roughly twice its width in depth, dampen it, and pack the mortar in thin layers. Finish it flush or very slightly recessed, never as a proud ribbon standing off the stone. Work in shade, not in full sun, and protect fresh pointing from frost and rain.

Trapping Damp Behind Modern Materials

A solid granite wall has no cavity. It manages water by absorbing a little, then drying outward when the weather turns. Anything that seals one face of that wall — plastic masonry paint, cement render, tanking slurry, injected damp-proof cream — simply relocates the problem rather than solving it.

Inside, the classic mistake is to fight a damp patch with a sealed surface. Tanking a solid wall pushes moisture further along it, and it usually reappears at a floor, in a corner, or behind a kitchen unit. Plaster skimmed over cement backing or gypsum over tanking traps water against timber lintels and floor joists.

First, find the actual source. In most cottages that is one of these:

• Overflowing or cracked gutters and leaking downpipes, which soak the wall head
• External ground levels built up above the internal floor, or a concrete path butting against the wall
• Failed flashings around chimneys, dormers and parapets
• Blocked air bricks that once ventilated a suspended timber floor
• Chimneys left open to the weather, feeding water straight down into the stack
• A leaking slate or a slipped ridge, letting in water far from where the stain appears

Then keep the walls vapour-open. Limewash or a mineral silicate paint outside, lime or clay plaster inside, and no sealant where the wall needs to dry.

Insulation That Fights the Building

Cornish cottages are cold, and it is tempting to solve that with the thickest board you can fit. The usual culprits are foil-backed plasterboard stuck straight onto the inner face of granite, and spray foam applied to the underside of slates or tiles. Both can create condensation inside the structure where you cannot see it, soaking rafters, battens and wall plates until the timber softens.

Breathable insulation suits these walls far better: wood fibre, hemp, cork, sheep's wool, or a hemp-lime mix, paired with a vapour-open finish and, where needed, a ventilated cavity behind. In a loft, keeping the roof space cold and well ventilated while insulating at ceiling level is often the least invasive route and the one conservation officers prefer. Never block the eaves vents. If you are insulating, changing heating, or altering anything structural, check whether building regulations apply and take professional advice before you start.

Roofing, Slate and Small Details

The roof is the other half of the building. Cornish roofs commonly use local slate in diminishing courses, laid to a pitch chosen for that slate. Replacing it with thin imported slate, or covering it in torch-on felt, changes both the look and the way the roof breathes.

Repair rather than replace where you can. Salvage sound slates, match sizes course by course, and fix with copper or stainless steel nails — galvanised nails rust through and cause the slow slide of slates you see on neglected roofs. Bed ridge tiles in lime mortar, not cement. Keep the eaves gap open, and avoid modern non-breathable underlays on an old, unventilated roof structure.

Planning Rules That Catch People Out

This is where an honest conversation with the council saves months. Rules vary, so check the specifics for your property rather than relying on what a neighbour did.

Listed buildings

Listing covers the whole building, inside and out, and often curtilage structures such as garden walls and outbuildings. Alterations need listed building consent, and carrying out works without it is a criminal offence. Doing it first and asking later is expensive and can require reversal.

Conservation areas and Article 4 directions

In a conservation area, demolition and some external changes need permission. Where an Article 4 direction is in place — and several Cornish towns and villages have them — permitted development rights are removed, so things that are normally automatic, such as changing windows, roof materials, cladding or porches, may need a formal application.

Windows, doors and extensions

uPVC windows are rarely acceptable on a listed building or in a sensitive conservation area. Repairing original timber sashes and casements, adding draught-proofing and secondary glazing, keeps the character and usually performs better than people expect. For extensions, matching granite and slate is not always the right answer; a well-mannered modern addition is sometimes approved more easily than a pastiche one.

A Sensible Order of Work

Do things in the right sequence and you will spend far less overall. A practical order for most cottage projects:

1. Get advice early: the council's conservation officer, a surveyor experienced with historic buildings, and a planning consultant where needed.
2. Fix water first — gutters, downpipes, drains, ground levels, flashings, chimney caps.
3. Let the walls dry out before covering anything up. This can take months.
4. Repair the roof and structure before starting internal finishes.
5. Repoint in lime, using a mortar weaker than the stone.
6. Insulate breathably, and ventilate wherever you insulate.
7. Finish with vapour-open plasters and paints, inside and out.

Choose builders who have worked on lime and historic fabric, and ask to see previous jobs nearby. If a contractor suggests sealing the walls, injecting a damp course as a first step, or spraying foam under the slates, get a second opinion. These cottages are forgiving of weather and time. They are not forgiving of the wrong materials, and with a little patience they will outlast all of us.

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