I looked at a Victorian terrace that had been repointed in the 1980s with a standard 1:3 cement mix. The joints were solid grey, hard as rock - and the brick faces immediately around them were crumbling away in flakes. The pointing had lasted forty years and would last another forty. The bricks would not.
The repointing had prioritised the wrong thing. Strong joints are not the goal. Weathertight joints that do not damage the surrounding masonry are the goal.
Why mortar mix matters for repointing
Mortar serves two purposes: bonding masonry units, and acting as a sacrificial element that allows for thermal and moisture movement in the wall.
If the mortar is harder and stiffer than the brick, movement stresses are concentrated in the brick faces rather than the joint. The brick surface spalls (breaks off in layers), exposing more substrate to weather. The damage accelerates.
If the mortar is softer than the brick, movement is absorbed by the joint. The joint may need repointing in 30-50 years. The bricks are unharmed.
The fundamental rule: mortar should always be weaker (softer) than the brick it surrounds.
Identifying the original mortar type
Before buying materials, try to understand what is already in the wall.
| Clue | Suggests |
|---|---|
| White or cream joints | Lime mortar (original) |
| Very light grey, easily scratched | Older lime mortar or weak sand:cement |
| Dark grey, very hard | Strong Portland cement mix |
| Crumbly, sandy texture | Weathered lime or very weak mix |
| Building pre-1920 | Almost certainly lime |
| Building post-1950 | Almost certainly Portland cement |
| Pinkish tinge to joints | Hydraulic lime or natural cement |
When in doubt, use a key or screwdriver to scratch a joint. Lime mortar will scratch fairly easily; Portland cement will not.
Mortar mixes for repointing
Pre-1920 soft brick (lime mortar)
Use: Hydraulic lime mortar - NHL 2 or NHL 3.5 with sharp sand
Mix: 1 part NHL 2.5 or 3.5 to 2.5-3 parts sharp sand by volume.
NHL 2 (feebly hydraulic) - for very soft brick and sheltered positions NHL 3.5 (moderately hydraulic) - standard choice for most repointing NHL 5 (eminently hydraulic) - for very exposed positions only
Do not use Portland cement on soft pre-1920 brick. The consequences are spalling and moisture entrapment.
Post-1920 hard brick with lime original mortar
Use: Weak Portland cement mix or hydraulic lime
Mix: 1:5 to 1:6 cement:sand, or NHL 3.5 with 2.5 parts sand.
If the original mortar appears to be lime but the brick is hard (post-1920 engineering or semi-engineering brick), a weak cement mix is appropriate. Hydraulic lime is still the safer choice.
Post-1950 modern brick (Portland cement original)
Use: Portland cement mix matching original strength
Mix: 1:5 to 1:6 cement:sand for standard facing brick Mix: 1:4 for harder or more exposed work
Do not over-specify the mix strength. A 1:3 mix for standard facing brick is too hard and will create the same spalling risk as cement on old brick.
Summary table
| Brickwork type | Recommended repointing mix |
|---|---|
| Pre-1920 soft brick | NHL 2.5 or 3.5, 1:2.5-3 with sharp sand |
| Pre-1920 hard brick | NHL 3.5, 1:2.5-3 with sharp sand |
| Victorian engineering brick | NHL 5 or 1:4 cement:sand |
| Inter-war brick (1920-1950) | NHL 3.5 or 1:5 cement:sand |
| Post-1950 standard facing brick | 1:5 to 1:6 cement:sand |
| Modern hard/engineering brick | 1:4 cement:sand |
Preparing for repointing
Raking out the joints is more important than the mix. You need to remove the old mortar to a minimum depth of 15-20mm (some say 25mm for exposed work) to allow the new mortar a proper key and depth to weather correctly.
Tools: plugging chisel and club hammer, angle grinder with mortar raking disc (for large areas), oscillating multi-tool with mortar blade.
Work carefully. An angle grinder removes mortar fast but also removes brick edges if used aggressively. Hand chiselling is slower but more controlled.
After raking, brush out the joints thoroughly - loose dust and debris prevent adhesion. Dampen the joint before pointing (particularly important for lime mortar, which needs a slightly moist substrate to bond and carbonate properly).
Jointing profile
The joint profile matters for weather performance:
- Flush or slightly recessed - good for most applications
- Tooled/bucket handle (concave) - traditional choice, sheds water well
- Weathered/struck - angled profile sheds water but should not be used with brick where the top of the lower brick is exposed
Avoid recessed/raked joints on exposed elevations - they collect water and freeze-thaw cycles accelerate deterioration.
My tips on repointing
Do a small test section first. Mix a small batch, point a 0.5m² section and assess after curing. Does the colour match? Is the texture right? Adjust before committing to the whole elevation.
Protect lime mortar from rain for the first 48 hours. Unlike cement, fresh hydraulic lime mortar will wash out in heavy rain before it has hardened. Cover with hessian sacking or polythene loosely draped (not sealed - lime needs to carbonate with air).
Repoint in mild weather. Avoid hard frost (which prevents proper curing) and bright hot sun (which dries the mortar before it can carbonate). Overcast conditions at 10-15°C are ideal.
Do not add lime to Portland cement as a substitute for lime mortar. Lime putty or hydrated lime added to cement is not the same as hydraulic lime mortar. The two materials are different and the results are different.
Use the Mortar Calculator to estimate materials for your repointing project.