What ashlar is
HES describes ashlar as large, regular, squared stone blocks. A range of stones was used in Scotland. Sandstone was the one used most often in most areas. The joints on the face are very fine. Rubble walls are a different build. Do not read this page as a rule for every wall you can see. If you are not sure what you have, ask for a stone survey.
Why the mortar matters
HES says the right pointing mortar, lime, gives a building its ability to breathe, and that this helps prevent decay of the masonry. The joints are a small part of the wall, but they are how water vapour gets out.
Cement
Cement mortars should not be used for repointing work as they can damage the stonework and interior finishes by trapping water inside the wall.
They add that other impermeable mortars and coatings must also be avoided.
Lime based mortars should be used instead as these will allow the building to breathe and water vapour to escape.
Where a building cannot breathe, HES lists three results:
- masonry decay
- internal damp
- rotting timbers
Signs the pointing needs a look
HES lists these signs that an ashlar wall 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
Repointing is necessary, they say, when bedding or jointing mortar washes out and the blocks are at risk of becoming unstable. It may also be needed to halt the damage from cement mortars. Left open, rainwater gets into the joints and the deterioration carries on. Plant roots in the wall do damage too.
Survey before anyone rakes the wall out
A survey of the existing pointing mortar should be carried out before work begins to assess how much repointing work is required.
A whole front may not need to be done. If only some of the original mortar has failed, mortar that is sound and shows no deterioration should be left in place. HES says patch repointing in selected areas is often all that is needed, and a mix similar to the original helps the wall look even. Full repointing can still be essential, for example where weathering of softer sandstone has led to widespread erosion of the face.
There is a shorter page on how much of a wall needs repointing.
What HES says about the mix and the tools
HES says ashlar pointing mortar was historically made using lime putty and whiting (crushed chalk) with a little linseed oil, and that this mix is still used. If sand or stone dust is added to match the original, the particle size should be no larger than one third of the minimum joint width. Coarse sand that could clog fine joints should be avoided. Specialist analysis of the existing mortar can help the replacement look right and perform as well.
On raking out, HES says care and suitable tools are needed so the stone is not damaged. Power tools should not normally be used on the fine joints. They say rotary tools such as angle grinders should never be used. Even a chisel can damage the arris if it is too wide or used badly.
They also say cement that is still sound is not always worth chiselling off, because the removal can damage the stone. That judgement is made on the wall, not from a webpage.
How this is used
Kingshill undertakes lime pointing and repointing. The mix is agreed after the joints have been seen. A wall should not be raked out from this page.
What to do next
Source: How to repoint ashlar masonry, The Engine Shed, Historic Environment Scotland.