Guide
Lime mortar or cement on Cotswold stone?
Short answer
Use lime on old Cotswold stone walls. Lime mortar is softer and more porous than the stone, so it wears first and lets the wall dry out. Hard cement traps water and makes the stone the weak part, so the stone decays faster. Cement suits modern walls built with cement, not old stone.
How an old stone wall works
An old Cotswold wall takes in rain and dries out again. Lime mortar lets this happen.
Cotswold District Council quotes the Part L guidance on old buildings. It says buildings of stone, or with lime render or mortar, absorb moisture and let it evaporate.
The mortar is meant to be the weakest part. SPAB says lime mortar wears away first, so it protects the stone.
What cement does to old walls
Hard cement changes how the wall handles water. SPAB lists these problems.
- Cement pointing makes the stone the most porous part, so the stone suffers more frost and salt damage.
- Cement render is rigid. It cracks as the wall moves, and rain gets in through the cracks.
- Water behind cement cannot dry out, so it stays in the wall.
- Cement render fails in patches and comes away. Lime wears slowly and evenly.
Removing old cement
Remove cement only with care. SPAB says removal can damage soft material underneath, so start with a trial area.
Walls under cement often hold a lot of salt. After the cement comes off, salts can come out and cause local decay at the base. The wall may need a new coat of lime later.
Do not use a grinder on the stone edges. Ask the builder how they will cut out the cement.
Choosing the lime mix
Match the old mortar where you can. SPAB suggests checking sheltered places, such as under the eaves, to see the original finish.
If you need a new mix, make it weaker than the stone and suited to how exposed the wall is. Make a trial panel first.
Lime cures slowly. Protect new work from frost, strong sun, wind and rain, and avoid lime work in winter where possible.
When cement is right
Cement mortar suits modern walls built with cement, such as a blockwork extension. Use the same type of mortar the wall was built with.
On a listed building, a change from lime to cement can need listed building consent. Ask the council conservation team.
Questions
Is lime mortar weaker than cement?
Yes, and that is the point. Lime is meant to be weaker than the stone, so it wears first and protects the stone. You can repoint lime joints again in future.
Will cement pointing cause damp?
It can. SPAB says cement restricts evaporation, so water stays in the wall. On an old solid wall, this can lead to damp inside and decay of the stone.
Does Historic England have guidance on repointing?
Yes. Historic England publishes a guide called Repointing Brick and Stone Walls. Cotswold District Council also links to SPAB advice on repointing and lime.