Daniel Hughes
Why the Mortar Matters More Than the Brick
Repointing is one of those jobs that looks straightforward from the pavement and turns into a proper head-scratcher once you're up a ladder with a bolster chisel in hand. The brickwork you're looking at has probably stood for a century or more, and the mortar holding it together has done its job quietly, flexing with the seasons and letting the wall breathe. Get the mix wrong and you can undo decades of good service in a single summer.
The golden rule with repointing is simple enough to remember: the mortar should always be weaker and more permeable than the bricks it sits between. That sounds counterintuitive if you're used to thinking of strength as a virtue, but in old masonry it's the difference between a wall that dries out and one that quietly rots from within.
Lime Mortars and Why Old Walls Need Them
Most brickwork built before the mid-twentieth century was laid with lime mortar. Lime is softer than brick, slightly porous, and moves gently with thermal expansion and the natural settlement that any building experiences over a century or two. When rain drives into the wall, moisture finds its way into the mortar joints, and lime lets it evaporate back out again through the same route.
Lime mortar comes in a few practical guises for repointing work:
- Non-hydraulic lime putty – the traditional stuff, made from fat lime. It sets by carbonation, needs air to harden, and is wonderfully forgiving. Ideal for soft, hand-made bricks and historic fabric.
- Natural hydraulic lime (NHL) – graded from NHL 2 to NHL 5. The lower the number, the softer and more breathable the set. NHL 2 or NHL 3.5 is the usual choice for most period brickwork.
- Lime with pozzolanic additives – sometimes used where a faster set or extra early strength is needed in damp conditions, though it's rarely necessary for straightforward domestic repointing.
A simple 1:3 mix of lime putty to well-graded sharp sand is a solid starting point for many older walls, though a local supplier who knows your area's geology can often sharpen that advice considerably.
The Problem with Cement on Period Brickwork
Portland cement mortar is hard, strong and relatively impermeable. On a modern cavity wall with a damp-proof course and a hard, dense brick, that's exactly what you want. Point the same mix into a soft eighteenth-century brick and you create a wall where the mortar joint is stronger than the brick itself.
What happens next is predictable and expensive. Moisture trapped behind the cement finds the path of least resistance, which is usually through the brick face. Over a few winters, that means spalling – the front of the brick shearing off in flakes and chunks. Once the brick face is gone, there's nothing left to repoint.
There's also the movement issue. Cement mortar is rigid. Old walls move, and if the joints can't flex, the stress transfers into the bricks. You end up with cracked units, stepped cracks running diagonally up the wall, and water finding its way into places it should never reach.
Reading the Wall Before You Mix Anything
Before you commit to a mix, spend twenty minutes working out what you're dealing with. Rake out a small section of mortar with a plugging chisel and look at it closely.
- Colour and texture – lime mortar tends to be creamy, chalky and slightly gritty when broken. Cement-based mortar is grey, hard and often rings when tapped.
- Brick hardness – scratch a brick with a nail. If it scores easily, you're looking at a soft brick that wants a soft mortar.
- Exposure – a south-facing wall in Cornwall catches weather differently from a sheltered courtyard in York. Exposed elevations sometimes justify a slightly stronger NHL, while sheltered ones don't.
- Existing repairs – previous cement patching tells you a lot. If the wall has been repointed in cement before, it may need careful raking out and a return to lime, not more cement on top.
Where the wall is listed or in a conservation area, check with your local authority before starting. Many conservation officers will expect lime and will want to see the specification in writing.
Mixing and Applying Lime Mortar in Practice
Lime mortar is not a case of bunging bags together and hoping for the best. Mix it in small batches – a bucket or two at a time – because working with fresh mortar gives the best finish. Lime putty benefits from being knocked up with sharp sand the day before and left to stand, which improves workability.
When repointing, rake out the joints to roughly twice the depth of the joint width, or around 15–20mm for typical brickwork. Brush out dust and dampen the joint slightly so the new mortar doesn't have its moisture sucked straight out. Pack the mortar in firmly with a pointing iron, working from the top of the wall down to avoid staining freshly pointed courses.
Finish with a profile that suits the wall's exposure – a slightly recessed or flush joint with a brushed texture sheds water well. Avoid weatherstruck details that create ledges for rain to sit on. Keep the new work damp for a few days in hot weather by misting it, and cover it against frost, since lime needs time and moisture to carbonate properly.
Getting the Mix Right for the Long Haul
The temptation to reach for a bag of cement is understandable. It's cheap, it's quick, and it feels like it will last. On old brickwork, though, the right choice is almost always the softer, more breathable one – the mortar that flexes when the wall moves and lets moisture escape rather than trapping it.
Match the mix to the wall you actually have in front of you, not the one you'd prefer it to be. A little time spent identifying the original mortar and the brick's hardness will save you a fortune in spalled bricks, cracked panels and crumbling pointing further down the line. Get it right once and the wall will quietly thank you for another fifty years.
