Should I remove cement render from an old Cornish wall?

Usually yes, where it is cracked and the wall behind is suffering, but it should be removed carefully and replaced with a lime system rather than stripped and left exposed through a wet winter.

Is a chemical damp proof course the answer to damp in a cottage?

Rarely. Solid walls have no cavity and often no damp proof course by design. Most cases resolve by fixing drainage, removing cement and letting the wall breathe again.

Can I extend an old cob building?

Yes, and the junction between old and new is where the thought goes. The new structure should be independent enough not to load the cob and detailed so the old wall can still dry.

Do repairs to a listed building need consent?

Like for like repair often does not, but anything that changes materials, finishes or appearance usually does. Asking first is far cheaper than unwinding it.

Traditional fabric · Cornwall

Cob and stone repairs in Cornwall, working with walls that were never meant to be sealed

A great many Cornish buildings are solid walled: rubble stone bedded in lime, cob on a stone plinth, or a mixture of both after two centuries of alteration. They stay dry not by keeping water out but by letting it move through and evaporate away. Almost every serious defect we are called to look at in an old Cornish house traces back to somebody treating that wall as though it were a modern cavity, and sealing it.

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How a solid wall stays dry

Rain wets the outer face. Lime mortar and soft stone absorb some of it, and wind and sun then drive it back out. The wall is in constant slow exchange with the weather, and it copes because the materials are permeable and the joints are weaker than the stone. Damage happens when that exchange is interrupted, because trapped moisture has to go somewhere and the somewhere is usually the inside face or the timber built into the wall.

Cement pointing is harder than the stone, so water and salts exit through the stone face instead and it spalls
Impermeable external render cracks, admits water behind it and then prevents it drying
Modern plastic paints on the inside face trap moisture and lift
Raised external ground levels and cement plinths hold water against the base of a cob wall

Cob has one enemy and it is standing water

Cob is remarkably durable while it is kept dry at the top and the bottom. A sound roof, generous eaves and a functioning gutter protect the head. A stone plinth and a free draining perimeter protect the base. Failures are nearly always the result of a leaking downpipe, a blocked gully or a concrete apron laid tight against the wall, and the cure is drainage and breathability rather than a chemical.

What a sympathetic specification contains

The specification should name the mortar mix and the aggregate, describe how joints are raked and to what depth, state how new work is protected while it cures, and set out the finish so that the wall does not end up looking like a garden feature. On listed buildings the same specification becomes part of the consent, which is why writing it properly at the start saves an awkward conversation with the conservation officer later.

Insulating a solid wall without wrecking it

It can be done, and it needs care. Vapour open insulation such as wood fibre or a lime based system allows the wall to keep drying inwards, while a plastic faced board fixed tight to damp stone creates exactly the trap the building has avoided for two hundred years. Any internal lining also has to deal with the junctions at floor and ceiling, where embedded timber joists sit inside the wall and are the first thing to rot if the moisture route changes.

Frequently asked

Straight answers.

Should I remove cement render from an old Cornish wall?: Usually yes, where it is cracked and the wall behind is suffering, but it should be removed carefully and replaced with a lime system rather than stripped and left exposed through a wet winter.
Is a chemical damp proof course the answer to damp in a cottage?: Rarely. Solid walls have no cavity and often no damp proof course by design. Most cases resolve by fixing drainage, removing cement and letting the wall breathe again.
Can I extend an old cob building?: Yes, and the junction between old and new is where the thought goes. The new structure should be independent enough not to load the cob and detailed so the old wall can still dry.
Do repairs to a listed building need consent?: Like for like repair often does not, but anything that changes materials, finishes or appearance usually does. Asking first is far cheaper than unwinding it.

Old walls behaving badly?

Send photographs and the address. We will tell you what we think is happening and what a proper repair specification would need to say.

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