Module 1 of 2 · 40 minutes
Mortar: the part that decides whether the wall lasts
By the end of this module you will be able to
- Choose a mortar mix appropriate to the brick and the exposure
- Mix mortar to a consistent, workable and repeatable standard
- Explain why a stronger mix is usually a worse mix
- Handle materials safely, including the real risks of cement and silica
Work through it
1 interactive for this module, built on the WAJD Teach engine. Nothing moves until you ask it to, and every one has a written version if you would rather read it.
Amara I would have guessed that more cement means a stronger wall.
Nadia Everybody does, and it is the most expensive mistake in domestic bricklaying. Mortar is supposed to be weaker than the brick.
Amara Weaker? Deliberately?
Nadia Deliberately. Mortar is a sacrificial material. Every wall moves, with temperature and with moisture. You want that movement to crack the mortar joint, because a joint can be raked out and repointed by anybody for very little money.
Amara And if the mortar is stronger than the brick?
Nadia Then the brick cracks instead. And now you are not repointing, you are cutting out and replacing bricks in a wall, which is a different job and a different bill.
Amara Is that why so many old houses look like the brick faces are falling off?
Nadia Precisely why. Somebody repointed a soft Victorian handmade brick with a hard modern cement mortar. The wall could no longer move or breathe, moisture got trapped behind that hard pointing, it froze, and the face of the brick blew off. You can drive down any street and see it.
Amara So what mix do I use?
Nadia For most external brickwork in ordinary exposure, one part cement, one lime, six sand. Softer or handmade bricks and older solid walls want one, two, nine, which is gentler still.
Amara And when do I go stronger?
Nadia Where it gets wet and freezes. Below the damp proof course, freestanding garden walls, copings, parapets. There you want one, half, four and a half. Everywhere else, resist the urge.
Amara What about sand? Sand is sand.
Nadia It really is not. Use building sand, which people also call soft sand. Sharp sand is for concrete and screed, and if you lay bricks with it you will get a harsh gritty mortar that tears off the trowel and fights you all day.
Amara How do I get consistency between batches?
Nadia Gauge by bucket, never by shovel. A shovel varies with who is holding it and how tired they are, and by four in the afternoon it is a different measure entirely. Same bucket, count it out.
Amara Why does that matter so much?
Nadia Because a wall built from batches that varied will dry with visible colour banding across it. It looks like stripes and there is nothing you can do about it afterwards.
Amara How do I know it is the right wetness?
Nadia Draw the trowel through it. It should hold a peak, cling to a vertical trowel face for a moment, then slide off cleanly. If it slumps flat it is too wet. If it tears and crumbles it is too dry.
Amara Last thing, and I want you to be blunt. What is the danger?
Nadia Two, and both are underrated. Wet cement causes chemical burns, and because it is painless at first people kneel in it or let it sit inside a glove and end up in hospital with full thickness burns. Waterproof gloves, and wash it off immediately.
Amara And the second?
Nadia Silica. Cutting bricks and blocks dry with a grinder throws out enormous quantities of respirable crystalline silica, and it causes silicosis and lung cancer. Water suppression or on tool extraction, and proper respiratory protection. It is the most serious long term risk in the entire trade and it is the one you will see ignored on almost every site you walk past.
The written material
Stronger is not better
The instinct of every beginner is to add more cement, because cement is the strong bit. It is the single most common and most expensive mistake in domestic bricklaying.
Mortar is meant to be weaker than the bricks it joins. It is a sacrificial material. When a wall moves, and every wall moves with temperature and moisture, you want the mortar to take the strain and crack in the joint, where it can be raked out and repointed for very little money. If the mortar is stronger than the brick, the brick cracks instead, and now you have a structural repair.
This is why so many repointed Victorian houses are spalling. Somebody repointed soft handmade brick with a hard cement mortar, the wall could no longer move or breathe, and the face of the brick blew off instead.
Common mixes and where they belong
Mixes are given by volume, cement to lime to sand, or cement to sand where no lime is used. A plasticiser is often used instead of lime for workability.
1:1:6 cement, lime and sand is the general purpose mix for most external brickwork in ordinary exposure. 1:2:9 is a softer mix for soft or handmade bricks and for older solid walls. 1:0.5:4.5 is a stronger mix for severe exposure, below damp proof course level, freestanding walls and parapets, where wetting and freezing are the risk.
Sand matters more than people expect. Use building sand, sometimes called soft sand, for bricklaying. Sharp sand is for concrete and screed and will give you a harsh, unworkable mortar that tears off the trowel.
- 1:1:6 general purpose external brickwork
- 1:2:9 softer bricks, older solid walls, sheltered work
- 1:0.5:4.5 severe exposure, below DPC, freestanding walls, copings and parapets
- Building or soft sand for laying, never sharp sand
- Never mix mortar in temperatures below about 3 degrees, or on a wall that may freeze overnight
Mixing to a standard you can repeat
Consistency between batches matters as much as the mix itself, because a wall built with varying batches will dry with visible colour banding and will weather unevenly.
Gauge by bucket, not by shovel. Shovels vary by who is holding them and by how tired that person is. Use the same bucket every time and count.
Mix the sand and cement dry until the colour is completely uniform with no streaks, then add water gradually. Too much water at once and you cannot recover it without changing the mix.
The finished consistency should hold a peak when you draw the trowel through it, cling to a vertical trowel face for a moment, and slide off cleanly. If it slumps flat it is too wet. If it tears and crumbles it is too dry.
The health risks that are not obvious
Wet cement causes severe chemical burns. It is strongly alkaline and the burns develop slowly, often painlessly at first, so people kneel in it or let it sit inside a glove for an hour and end up with full thickness burns. Wear waterproof gloves, keep it out of boots, and wash it off skin immediately.
Cement also causes allergic contact dermatitis, which once developed is usually permanent and can end a career.
Cutting bricks and blocks releases respirable crystalline silica, which causes silicosis and lung cancer. Dry cutting with an angle grinder or disc cutter produces enormous quantities of it. Use water suppression or on tool extraction, and wear appropriate respiratory protection. This is the most serious long term risk in the whole trade and it is routinely ignored.
Knowledge check
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