Why old brickwork and cement mortar don't mix

Historic bricks were fired at much lower temperatures than their modern counterparts. The result is a softer, more porous unit, often with a slightly underfired core and a surface that has spent a century or more quietly absorbing and releasing moisture. That movement of water is normal and healthy. Problems start when the water gets in but cannot get back out.

Cement mortar is dense, hard and largely impermeable. Point an old brick wall with it and you effectively seal the joints while the brick faces behind continue to take in rain. Trapped moisture has nowhere to evaporate, so it sits in the brick, feeds frost action, dissolves soluble salts and pushes the face off. This is why you so often see Victorian brickwork with spalled faces and blown corners sitting above a neat, grey, cement repoint that looks perfectly sound.

Lime mortar behaves differently. It is porous, so water can move through it and evaporate from the joint faces. It is also softer than the brick, which means that when movement or frost does occur, the joint fails first and the brick survives. In conservation terms, the mortar is sacrificial — and that is exactly what you want.

The golden rule: mortar must be weaker than the brick

Everything else follows from this. The mortar should be softer, weaker and more permeable than the masonry it sits between. If you remember nothing else, remember that.

This is why the well-meant habit of adding a handful of cement to make it go off is so damaging. It raises the strength and lowers the permeability just enough to tip the balance the wrong way. There is no safe proportion of cement in a repointing mix for old brick. Leave it out.

Picking your lime: putty, NHL, or hot-mixed

There is no single right answer, but there are sensible defaults for most historic brickwork.

• Lime putty (non-hydraulic). Slaked calcium lime, matured for months. It sets by carbonation, so it needs air and time. Extremely soft and breathable, and ideal for internal work, sheltered elevations and very soft, low-fired bricks.
• NHL 2 or NHL 3.5. Natural hydraulic limes that gain some strength by reaction with water as well as by carbonation. NHL 3.5 suits a great deal of old brickwork, especially where the wall is exposed. NHL 2 is a gentler choice for delicate or very porous bricks.
• NHL 5. Best reserved for engineering brick, hard stone and heavy exposure. It is too strong for most historic brick and its lower permeability works against you.
• Hot-mixed lime. Quicklime mixed with aggregate and water on site. Traditional, fast-setting and pleasantly workable, though it demands care with safety and technique.

If you need a putty mix to set in damper or more exposed conditions, a pozzolan — brick dust, finely ground fired clay or a natural pozzolan — will give it a gentle hydraulic set without the hardness of cement.

Matching the aggregate

The sand matters as much as the lime. You want a sharp, well-graded sand with a spread of particle sizes, so the mix locks together and the lime can still breathe.

A useful starting point is one part lime to three parts aggregate by volume, with pointing mixes sometimes slightly richer. Avoid soft building sands and fine washed sands: they are usually too rounded, too uniform and often carry too much clay or silt, which weakens the mix and increases shrinkage cracking. A simple jar test — sand, water, a shake, then leave it to settle — will show you roughly how much fine material you are dealing with.

Then look at the original mortar. Take a small sample from a hidden area, crumble it and compare colour and texture. Many old brick mortars contain crushed brick or ash, giving a warm reddish or grey-brown tone. Matching that is not just cosmetic: a well-matched aggregate helps the new work sit visually alongside the old, and often mirrors the grading the wall has always had.

Mixing, packing and curing

Rake out the joints to around 15–20mm, or roughly two and a half times the joint width, and cut them square rather than feathered. Clear out loose material and dust, then dampen the brickwork so the wall does not pull water out of the fresh mortar.

Mix by volume, not by weight, and add water sparingly. A buttery, workable mix that holds its shape on the trowel is right; a wet, sloppy one will shrink, crack and lose strength. Push the mortar firmly into the joint in layers rather than smearing it across the face.

Once the mortar has firmed up, finish it with a churn brush rather than a metal tool. This packs the surface and opens the texture, which looks right and helps evaporation. Keep the work damp for several days with a light misting or damp hessian, and protect it from frost, hot sun and drying wind. Lime gains strength slowly — rushing it is counterproductive.

Trial panels, and knowing when to leave well alone

Before committing to a whole elevation, point a small trial panel in a discreet spot. Let it cure, ideally across a season, and check the colour, texture and how the joint behaves in wet and cold weather. It is far easier to adjust a mix than to undo a finished repoint.

Finally, only repoint what actually needs it. Sound, well-bedded historic mortar is doing its job and should be left in place. Cut out the crumbling joints, match what is there, and let the wall carry on breathing as it always has.