Lime or cement: knowing which wall you have
Repoint a soft old wall in hard cement and you move the damage into the brick. Here is how to tell what you are working with, and why we mix lime for anything built before the war.
Half the damp and spalling we get called to is not a building failing. It is a repair failing — a soft, breathing wall that somebody pointed in hard grey cement thirty years ago.
Old walls are meant to move
A solid brick wall built before roughly the 1920s has no cavity and no plastic damp course. It handles water by letting it in and letting it back out again, slowly, through the mortar. The mortar is lime: softer than the brick on purpose, so that when the wall moves — and every wall moves — the mortar takes the strain and cracks before the brick does. You repoint the joints every few decades and the brick lasts centuries.
Cement changes the rules
Cement mortar is harder than a soft old brick. Point that wall in cement and three things happen. Water can no longer leave through the joint, so it leaves through the face of the brick instead, bringing salts with it. The brick, now the softest thing in the wall, takes the movement and spalls. And the repair looks fine for about five winters while the damage builds up behind it.
- Soft red or yellow stocks, wide joints, a building older than the war — almost certainly lime, keep it lime.
- Hard machine-made brick, thin joints, post-1930s — usually a gauged or cement mortar, match what is there.
- Not sure? Scratch the joint. Lime gives up powder to a key; a good cement joint does not.
What we do
For anything built to breathe, we mix lime — hot-mixed or a natural hydraulic lime to suit the exposure — and we match the old joint profile so the repair disappears. It is slower to carry and slower to set, and it is the difference between a repair that helps the wall and one that quietly wrecks it.
If you have a wall that is spalling, blowing or staining, get us to look before you repoint it. The cheap fix is usually the expensive one.