There's a sound that tells you a brick wall has been pointed wrong: a dull, hollow tap, followed by the crunch of a brick face flaking off under your knuckles. I've heard it on a Victorian terrace in the Midlands where a builder had "tarted up" the front elevation with a 4:1 sand-cement mix. Three winters later, the soft red bricks had spalled so badly you could read the wall like a contour map. The mortar was still perfect. That's the problem.
Repointing historic brickwork with lime mortar is the fix, and it's not difficult to learn. But it's unforgiving. Get the mix wrong, work it too dry, brush too early, and you've spent a week making a wall worse than you found it. This is what I've learned from doing it, including the parts I got wrong.
Key takeaways
- Lime mortar works because it's weaker and more vapour-permeable than the bricks around it. That's the whole point — the mortar is meant to fail first.
- Match the lime to the building: non-hydraulic putty lime for soft, sheltered brickwork; NHL 2 or 3.5 for exposed or damp walls.
- Dig out to roughly twice the width of the joint, never less than 1.5×. Shallow pointing falls out.
- Never brush fresh lime. Wait until it's crumbly-set, usually 12–24 hours depending on conditions.
- Cement mortar on old brick is the single most common and most expensive mistake. It's a slow demolition, not a repair.
Why lime mortar is the only sane choice for old brickwork
Historic bricks are soft. Fired in low-temperature clamp kilns, they're porous, and they move. Lime mortar accommodates that movement because it's flexible and because it's deliberately weaker than the brick. When the wall shifts or moisture freezes inside it, the mortar crumbles and the brick survives. Swap in cement and you reverse the hierarchy: the mortar becomes a rigid, near-impermeable shell, water gets trapped behind it, and the brick becomes the sacrificial element.
Look, the physics is simple. Water enters brickwork constantly — driven rain, rising damp, condensation. It has to leave again, and traditionally it leaves through the mortar joints. Cement pointing blocks that exit route.
The catch? Lime mortar is fiddly. It sets slowly, it's sensitive to drying, and it demands patience that most site programmes don't allow. That's why it got abandoned in the first place. Speed won. Walls lost.
What is the best mortar mix for repointing old bricks?
There's no single answer, and anyone who gives you one number is oversimplifying. The right mix depends on the brick's hardness and the wall's exposure. Here's how I break it down:
| Brick type / situation | Binder | Approximate mix (binder : sand) | Notes |
|---|---|---|---|
| Very soft, hand-made or clamp-fired brick, sheltered elevation | Non-hydraulic lime putty | 1 : 3 (by volume, putty : sharp sand) | Most breathable. Needs protection from frost while curing. |
| Soft red brick, moderate exposure | NHL 2 | 1 : 3 | Good compromise; sets in damp conditions. |
| Fletton or medium-hard brick, exposed wall | NHL 3.5 | 1 : 3 to 1 : 4 | Faster set, more frost resistance. |
| Damp or coastal situations | NHL 3.5 or 5 | 1 : 3 | Only where genuinely needed — harder isn't better. |
Two things matter more than the ratio. First, the sand: use a well-graded sharp sand with a mix of particle sizes, not a uniform builder's sand that leaves the mortar weak and crumbly. Second, never add cement "for strength." I've watched a mason stir a shovel of OPC into a lime mix because it felt sloppy. It set like iron and cracked within a year.
Getting consistency right — the test I actually use
Mix should be soft enough to push into the joint with a trowel or a pointing gun without dragging. If it's crumbly, add water gradually — a splash at a time, because lime putty mixes turn from stiff to sloppy fast.
My rule: if the mortar holds its shape on the trowel but yields under light finger pressure, it's right. If it crumbles off the trowel edge, too dry. If it slides off, too wet.
Preparing the joints — the step everyone rushes
Raking out is the unglamorous half of the job, and it's where the quality of the finished wall is decided. You need depth. A mortar that sits 3 mm proud on the brick face is cosmetic, not structural, and it will shell off within a few seasons.
Dig out to roughly twice the width of the joint, with 1.5× as an absolute floor. An 8 mm joint wants 16 mm of depth. Stop when you hit sound, original mortar — you're not trying to excavate the full bed.
Tools and technique
An angle grinder with a thin diamond blade is fast and brutal. On soft historic brick it will also gouge the arrises if your hand wobbles. I use a plugging chisel and a small lump hammer for most of the work and reserve the grinder for hard mortar and long runs, running it at low speed with a light touch.
What not to do: don't chase the joint deeper than needed "to be safe." You'll open voids behind the brick and blow out the bond. And don't leave the wall dusty — brush out every joint with a stiff brush, then flush with clean water.
How to repoint historic brickwork, step by step
Once the joints are clean, the work itself is repetitive and quiet. Sequence matters more than speed.
- Damp the joints. Historic brick will suck the water straight out of your mortar in dry weather. Spray the wall lightly and let it drink before you start. Not soaking — damp.
- Work from the top down, in small areas. Around a square metre at a time, in shaded conditions if you can manage it. Lime hates drying out too fast.
- Pack the mortar firmly. Push it in with a pointing trowel or a hawk and small jointing tool, working from the back of the joint forward. Air pockets are your enemy.
- Strike the joint. Finish flush, or slightly recessed for a traditional look, using a jointing tool or a piece of rounded dowel cut to the joint width. Do it while the mortar is still green.
- Leave it alone. This is the hard part.
On my first full elevation — a small garden wall, mercifully not someone's house — I brushed it down four hours after pointing because it looked dry. It wasn't. I dragged the surface off half the joints and had to redo roughly a third of the wall. Four hours of work gone because I couldn't wait.
When do you brush lime mortar?
Wait until it's crumbly-set. Under normal mild conditions that's roughly 12 to 24 hours, but judge with your fingertips, not the clock. When the mortar resists a light brush but still crumbles slightly under a rub, it's ready. Then brush with a soft bristle brush — not a wire one — to remove the smears from the brick face.
And protect the fresh work. Cover it with damp hessian or polythene sheeting in hot, dry or windy weather. Spray it lightly with water once or twice a day for the first few days if it's drying hard. Lime cures by carbonation, which needs moisture, and it takes weeks, not hours, to get anywhere near full strength.
Can you point over lime mortar, and other mistakes worth avoiding
Yes, you can point over existing lime mortar — but only if the old mortar is sound. Scrape back any loose or friable material, rake out the joint to some depth, dampen it, and pack fresh lime in over the top. If the original mortar crumbles to dust under a fingernail, it's failed, and pointing over it just buries a problem.
What you cannot do, sensibly, is point lime over old cement. The cement's impermeability doesn't disappear because it's buried. It keeps trapping moisture. In my experience the only honest fix is to cut it out and start again, which is slow, costly work — several times the labour of a straightforward repoint.
Two more boundaries worth knowing. Never let lime mortar dry out in direct sun or wind; it will crack and dust before it cures. And never point in frost — fresh lime has almost no frost resistance and a single hard night can ruin a day's work.
Then there's tuckpointing, which is sometimes confused with ordinary repointing. Tuckpointing uses a coloured mortar to imitate fine joints, with a contrasting filler behind it. It's a decorative treatment with a long history, and on a building where it originally existed, restoring it is legitimate. Faking it on a building that never had it is a different matter. That's a judgment call, but in my view it cheapens the wall.
The work is slow, and the results don't show up for a season or two. When the wall does hold through its first hard winter — no spalling, no cracking, joints soft when you probe them with a knife — that's the whole point. A wall that's got another fifty years in it. Hire someone who won't reach for the cement bag the moment your back is turned.
