The page this follows is How to repoint ashlar masonry on The Engine Shed site, part of Historic Environment Scotland. Ashlar means large, regular, squared blocks. Sandstone was the stone most often used for it in Scotland. This is not a note about every rubble wall.
Survey first
A survey of the existing pointing mortar should be carried out before work begins to assess how much repointing work is required.
They say repointing of an entire front may not be necessary if only part of the original mortar has decayed. Mortar that is sound and shows no signs of deterioration should be left in place. Patch repointing in selected areas is often all that is needed. A mortar mix similar to the original helps the wall keep an even appearance.
Complete repointing of a front may sometimes be essential. Their example is where weathering of softer sandstone has led to widespread erosion of the face.
When the joints have failed
Repointing is necessary when bedding or jointing mortar washes out and the blocks risk becoming unstable. It may also be needed to stop the damage from cement mortars. If the joints are left open, rainwater gets in and the wall gets worse. Plants, especially where roots enter the wall, damage pointing too.
Signs HES lists for an ashlar wall that may need repointing:
- washed out or decayed mortar
- loose or missing ashlar blocks
- loose or missing cope stone
- plant growth in the fine joints
- stone decay caused by cement mortars
What to do with that
Send photographs of the whole wall and of the open joints, and request a stone survey. Photographs can help with the first look. The wall may still need to be seen before any recommendation or quotation. The wider note on cement and lime is on the lime or cement page.