This is the single most important thing to get right, and the thing most often got wrong.
The short version
- Built before about 1920 with solid walls: it was built with lime mortar. Repoint in lime.
- Built after about 1930 with cavity walls: it was built with cement mortar. Repoint in cement.
- 1920s: could be either. Needs a look.
Why it matters
Lime mortar is softer than the brick or stone around it and it breathes. Moisture that gets into the wall moves out through the joints, and any movement in the wall is taken up by the mortar rather than the masonry.
Cement mortar is harder than old brick and doesn't breathe. Put it in a solid lime-built wall and moisture is forced out through the brick faces instead of the joints. In winter that water freezes and blows the face off the brick — that's the "spalling" you see on so many Victorian terraces that were repointed in the 70s and 80s. The wall also gets damper inside, because it can no longer dry out.
How to tell what you've got
Scrape a joint with a key. Lime is soft, sandy and pale, usually with visible white flecks. Cement is hard, grey and won't scratch. If a house has been repointed before, you'll often find a hard cement skin over soft lime underneath — that's exactly the situation where the cement should come out.
When cement is right
Any house built with cement — which is the majority of housing in Middleton, Royton, Heywood and the post-war estates — should be repointed in cement. Lime on a cavity wall isn't harmful, but it's more expensive for no benefit.
Scott checks the existing mortar on every survey and tells you which applies.
