The joint changes the brick count
A brick never occupies its own dimensions in a wall. It occupies its dimensions plus one joint, because every brick carries one perpend on its end and one bed joint above it. The unit that repeats across the wall is:
(brick length + joint) × (brick height + joint)
For a standard 203 × 57 mm brick with a 10 mm joint, that repeating face is 213 × 67 mm, or 0.01427 m². A square metre of wall therefore takes about 70 bricks in a single leaf.
Change the joint to 12 mm and the unit becomes 215 × 69 mm, and the same square metre takes about 67 bricks. Two millimetres of joint has removed three bricks per square metre — around 4 per cent of the order.
The standard is 10 mm, and it is not arbitrary
Ten millimetres — or 3/8 of an inch — is the near-universal nominal joint, and it exists because it is the thickness at which mortar does its job:
- Thin enough to be strong: Mortar is weaker than brick. The more of it in the wall, the weaker the wall. A thick joint is a soft layer.
- Thick enough to bed properly: Brick is not dimensionally perfect. The joint absorbs manufacturing tolerance and lets the bricklayer keep courses level.
- Thick enough to bond: The mortar must key into the surface texture of the brick on both faces.
Below about 6 mm the mortar cannot accommodate the brick's own variation. Above about 15 mm it starts to shrink noticeably as it cures, and it becomes the weak plane in the wall.
Why the tolerance matters over a wall
Joint thickness compounds vertically. A brick course is one brick plus one bed joint, so a wall 20 courses high with 10 mm joints stands at 20 × 67 mm = 1340 mm.
Let the joints drift to 12 mm and that same 20 courses reaches 1380 mm — 40 mm higher. That is enough to miss a damp-proof course level, a window opening or a wall plate, and it is why bricklayers gauge courses against a storey rod rather than trusting the eye.
The horizontal equivalent is less critical structurally but very visible: perpends that wander are the first thing anyone notices about bad brickwork.
Joint finishes and weather
The profile you tool into the joint is not only cosmetic. It decides where water goes:
- Bucket handle (concave): Compresses the mortar surface and sheds water. The default for exposed walls, and the most durable.
- Weatherstruck: Angled to throw water clear. Traditional and effective on exposed elevations.
- Flush: Simple, acceptable in sheltered positions.
- Raked: A recess for shadow and definition. It looks striking and it holds water on the ledge it creates — a poor choice anywhere exposed or subject to frost.
Tooling is done when the mortar is thumbprint-hard. Too early and it smears; too late and it will not compress.
What this means for ordering
Because the joint changes the count, the joint belongs in the estimate rather than being assumed. Ordering bricks from a face area and a brick size alone, with no joint, over-orders by roughly 15 per cent — you have effectively priced the wall as though the bricks touched.
And the mortar is the mirror image. Everything the bricks do not fill, the mortar does. A wider joint means fewer bricks and more mortar, in the same wall, at the same time.