The bed joint is the horizontal joint in masonry: the mortar layer between two successive courses of brick. The vertical joint between two bricks in the same course is called the perpend (or cross joint).
The bed joint is what makes masonry work as a structure. It spreads the compression evenly over the brick face, absorbs dimensional variation in the bricks, and keeps the masonry level.
Why the bed joint is the important one
Bricks are never exactly the same height. Without a mortar layer they would rest on each other at a few points and the load would concentrate there, causing cracks. The bed joint fills those differences and produces full contact across the whole bedding face.
Hence the rule: the bed joint must always be full and solid. A bed joint with voids is not only structurally weaker, it is also a water path. Rainwater drawn in through the joint ends up exactly there.
Perpends behave differently. They carry almost nothing, and in a cavity wall they are deliberately left open at the bottom and the top: the open perpend provides ventilation and drainage for the cavity.
Thickness and gauge
The usual bed joint thickness in traditional brickwork is around 10 to 12 mm. That figure is not arbitrary — it is built into the gauge of the brick format. With a brick 50 mm high (the Dutch waal format), a 12.5 mm joint gives a gauge of 62.5 mm; eight courses are then exactly 50 cm, which makes setting out and aligning with window dimensions predictable. British brickwork works on a 75 mm gauge from a 65 mm brick with a 10 mm joint, giving four courses to 300 mm.
In thin-bed work — usual with calcium silicate and aerated concrete blocks — the bed joint is only 2 to 5 mm thick. That only works with dimensionally calibrated blocks; there is no mortar to absorb deviations.
Finishes
How the bed joint is finished determines both the appearance and the water behaviour of the wall:
- Flush — level with the brick face. Sheds water fastest.
- Recessed (raked) — gives shadow and makes the brick stand out strongly, but holds water longer on the brick edge below.
- Weather struck — with a slope that directs water outward.
- Struck as laid — finished during laying, without separate pointing.
Outdoors the rule is: the deeper the joint sits, the longer the wall stays wet and the greater the risk of algae growth and frost damage to the joint.
Common mistakes
- Too dry a mortar on a dry brick. A strongly absorbent brick draws water out of the mortar before it can set. The result is a powdery, weak joint. In warm, dry weather, work with dampened bricks.
- Laying in frost. Water freezing in a fresh bed joint destroys the bond irreversibly. Do not lay below about 5 °C and falling without protective measures.
- Not pressed home. Laying mortar only along the edges leaves a hollow joint — a classic cause of penetrating damp.
- Pointing too thickly. When pointing separately, the joint is first raked out to sufficient depth, then filled in layers and compacted; filling it in one pass gives a joint that lets go.
Related terms
- Perpend
- Open perpend
- Mortar
- Gauge
- Pointing
- Weather struck joint
- Thin-bed mortar
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