A brick wall is mostly brick. The mortar is the difference between the volume of the wall and the volume of the bricks in it, which sounds like a subtraction and is — but the subtraction undercounts, because bricks have frogs and perforations that fill with mortar, and because a wheelbarrow of mortar loses a good share between the board and the wall. This works out the joint volume properly, adds an allowance for the frogs, and then splits the result into cement and sand at whatever mix ratio the job is specified at, because the sand is the part you order by the tonne.
Wall, brick and mix
Mortar is what is left when the bricks have taken their share.
A standard brick is 215 x 102.5 x 65 mm, which gives a half-brick wall.
Ten millimetres is standard for both the bed joint and the perpends.
Perforations and frogs fill with mortar, and the board loses some.
Result
The wall is mostly brick
It is worth knowing what the proportions actually are, because it explains why mortar is such a small line item on a brickwork job. A half-brick wall 12 square metres in area is 1.23 cubic metres of wall, and the 712 bricks in it occupy just over one cubic metre of that. The joints — the horizontal bed and the vertical perpends — are the remaining fifth of a cubic metre.
Which means the mortar is about seventeen percent of the wall by volume, and the bricks are eighty-three. That is the honest picture before any allowances, and it is why a ten percent error in the brick count costs far more than a ten percent error in the mortar.
Why the subtraction undercounts
Bricks are not solid rectangles. Most have a frog — a depression in one face — and many are perforated, with holes through them to reduce weight and improve the key. Every one of those voids fills with mortar when the brick is laid, and the volume does not appear in the subtraction at all. On a perforated brick the frogs and holes can add a quarter again to the mortar volume.
Then there is what happens between the board and the wall. Mortar falls off the trowel, dries on the board, gets dropped and swept up, and a wheelbarrow load left in the sun for an hour is a wheelbarrow load in a skip. Ten percent is a modest allowance for all of it and fifteen is not unusual on a small job.
Sand is the bulk of the mortar by weight, and it is delivered by the tonne or in bulk bags. A cubic metre of 1:4 mortar needs well over a tonne of it.
Order in whole units and accept the leftover. Sand keeps, and the delivery charge is the expensive part of a small order.
The mix ratio sets the cement
A 1:4 mix is one part cement to four parts sand, by volume, in the bag and the barrow rather than by weight. Cement is about 1,440 kilograms per cubic metre and building sand about 1,600, so a cubic metre of 1:4 mortar works out at roughly 290 kilograms of cement — under twelve 25 kg bags — and a little over 1.2 tonnes of sand.
Weaker mixes use less cement and more sand: 1:6 is fine for an internal, non-load-bearing wall and uses about a third less cement. Stronger mixes — 1:3 — are for below ground and for anything that has to resist water, and they use more. Too strong a mix on soft brick is a real fault rather than a safety margin, because the mortar ends up harder than the brick and the brick spalls instead.
| Situation | Mix | Cement per m³ | Sand per m³ |
|---|---|---|---|
| Below ground, footings | 1:3 | 360 kg | 1.20 t |
| General purpose, external | 1:4 | 290 kg | 1.28 t |
| Above ground, sheltered | 1:5 | 240 kg | 1.33 t |
| Internal, non-load-bearing | 1:6 | 205 kg | 1.37 t |
| Repointing, soft brick | 1:5 lime | Lime-based | 1.30 t |
| Render base coat | 1:4 | 290 kg | 1.28 t |
A worked example
- BricksA 4 x 3 m wall at a 225 x 75 mm pitch is 59.3 bricks a square metre, so 712 bricks.
- Wall volumeTwelve square metres of 102.5 mm wall is 1.23 m³.
- Joint volumeThe bricks occupy 1.02 m³, so the joints are 0.21 m³.
- AllowancesAdd 30% for the frogs and 10% for waste: 0.30 m³ of mortar.
- MaterialsAt 1:4 that is 4 bags of cement and 0.38 tonnes of sand — about $55 of material.
Fifty-five dollars of mortar against seven hundred and twelve bricks is the proportion worth remembering. If a brickwork estimate has gone wrong, it has gone wrong in the brick count, in the labour, or in the scaffold — not in the sand.
Mixing and laying
Mix dry until the colour is uniform, then add water a little at a time. A mix that is too wet is weak and it slumps off the trowel; too dry and it will not fill the perpend. The test is the trowel: mortar should stand up on it and slide off cleanly when tapped. Use it within about two hours, and discard it once it has started to stiffen rather than knocking it back with more water.
Lay the bed joint full and furrow it lightly, butter the perpend end of the brick, and push the brick down into line so that mortar squeezes out on both faces. A perpend that is not full is the most common defect in amateur brickwork and the hardest to see once the wall is up — point it up as you go rather than raking it later.
Keep the perpends staggered by at least a quarter of a brick, keep the courses level off a line rather than off a level, and stop in a stepped or raked back formation rather than a straight vertical joint. Where the same job involves bedding stone or concrete rather than brick, the paver figure handles the laying course, and the adhesive figure does the same job for tiles indoors. Getting rid of what comes out is its own quantity — the haul figure sizes that.
Do
- Mix dry to a uniform colour before adding water.
- Fill the perpends as you go rather than pointing them up later.
- Stagger the perpends by at least a quarter of a brick.
- Discard mortar that has stiffened rather than knocking it back.
- Order sand in whole tonnes or bulk bags.
Avoid
- Mixing too wet, which weakens it and slumps off the trowel.
- Knocking a stiffening mix back with more water.
- Leaving perpends hollow, which is invisible once the wall is up.
- Using a mix stronger than the brick, which makes the brick spall.
- Guessing the sand and running out halfway through a course.
Where the numbers stop being true
This assumes a half-brick wall with a single thickness. A one-brick wall is two thicknesses with a collar joint between them, so roughly double the bricks and double the mortar plus the collar. A blockwork wall is a completely different arithmetic: far fewer units per square metre, and a much higher mortar share because the blocks are hollow.
The frog allowance is a percentage, and it is the least certain figure here. A solid brick wants almost none, a deep-frog brick wants twenty percent, and a heavily perforated one can want forty. If the brick type is known, weigh one brick dry and one after buttering and scale from that instead.
Nothing here covers the sand's moisture content, which is the reason the same mix by volume behaves differently on a wet day. Bulking sand — damp sand takes up more volume than dry for the same weight — means a barrow measured by volume on a wet morning carries less sand than it did yesterday, and the mix comes out stronger and harsher than intended.
How the figures are produced
The laying pitch is the brick length plus the joint, by the brick height plus the joint, and bricks per square metre is one divided by their product. Wall volume is the area times the brick width. Brick volume is the brick count times the volume of one brick, and the joint volume is the difference. The frog allowance and the waste allowance are applied in turn. At a 1:n mix the cement share is one part in n plus one, converted to weight at 1,440 kilograms per cubic metre, and the sand is the remainder at 1,600.
Questions people ask
How many bricks in a square metre?
About fifty-nine in a half-brick wall at a 225 x 75 mm pitch. Sixty is the figure usually quoted.
How much mortar for 12 square metres?
About a third of a cubic metre once the frogs and the board are allowed for, which is four bags of cement and a bit over a third of a tonne of sand.
What mix should I use?
1:4 for general external work, 1:5 or 1:6 above ground and sheltered, 1:3 below ground. Never stronger than the brick.
Can I use a stronger mix for safety?
No. Mortar harder than the brick makes the brick spall, and the wall fails at the unit rather than at the joint.
Why do I run out of mortar?
Usually the frogs and perforations, which the simple subtraction misses, plus what dries on the board.
How long does mixed mortar last?
About two hours in normal conditions, less in hot sun. Discard it once it stiffens.