How do I tell whether my wall is pointed in lime or cement mortar?

Almost every masonry decision on an older house runs through this question. What mortar you should mix, whether the wall should be sealed, why the bricks are spalling, whether a render is appropriate — all of them depend on it, and none of them can be answered from the outside of the building without looking closely at a joint.

Step by step

1. Pick a test spot that does not matter.Round the back, low down, behind a downpipe or inside a cupboard where the wall is exposed. You are going to scratch it, and you want that scratch invisible.
2. Scrape the joint with a key or a screwdriver.Lime mortar gives way under moderate pressure and comes off as a soft, pale, chalky dust. Cement mortar resists — you feel the tool skid, and what does come off is gritty and dark. This one test settles most cases.
3. Look at the colour and texture.Lime mortar is usually off-white, buff or a warm sandy colour, with visible sand grains and often lumps of unslaked lime showing as small white specks. Cement mortar is grey, dense, and uniform.
4. Read the relative erosion of joints and bricks.This is the most reliable whole-wall test. Where the joints have weathered back and the brick arrises stand proud, the mortar is softer than the brick — a correctly built wall, almost certainly lime. Where the joints stand out as hard raised ribs and the brick faces have eroded or spalled behind them, the mortar is harder than the brick, and someone has cement-pointed a wall that should not have been.
5. Check the age and the construction.Solid walls of one brick or more, no cavity, no visible damp proof course, thick reveals at the windows, and a build date before roughly 1919 all point to lime. Measure the wall thickness at a window or door reveal — a cavity wall is noticeably thicker than a single-brick solid wall but with a hollow between.
6. Expect to find both on the same house.Very common. An original lime-mortared wall with cement repointing on the weather elevation, or cement patches round replaced windows and after a downpipe was moved. Test several places rather than one.
7. Act on the answer.Lime wall: repoint in lime, keep the wall breathable, do not seal it and do not cement render it. Modern cement-mortared cavity wall: a conventional cement-and-lime mortar is right. Old wall with cement pointing already on it: raking that out and repointing in lime is a genuine repair rather than cosmetics, and it is a big job worth doing elevation by elevation.

Tips

• A magnet is no help and neither is a moisture meter. This is a mechanical and visual assessment.
• Take a photograph of the joint close up with something for scale before you scrape it, so you can compare against a repointed sample later.
• If the answer genuinely matters — a listed building, an insurance claim, a serious repointing budget — a laboratory mortar analysis from a small sample is inexpensive and gives you the binder, the sand and the proportions.

If it doesn't work