Setting out
Fit units between two ends
- where
- perpend joint width (mm)
- span (mm)
- unit count
- unit length (mm)
- number of joints included
Explain this equation
This finds the uniform perpend joint needed to fit a known number of units into a fixed span, such as between two piers.
S is the span in mm. n is the number of units and l is the length of one unit, so n × l is the length taken up by the units. n_j is the number of perpend joints you are including in the span. j is the joint width each one must be.
The space the units do not fill must be taken up by the joints, so S − nl is shared equally between them. Between two free ends there is usually one joint fewer than units; building up to existing work may add end joints.
Four 215 mm bricks in an 890 mm span take 860 mm, leaving 30 mm for three joints: 10 mm each.
Watch out: a very thin or very wide answer means the layout needs changing, not forcing. The calculator stops if the units alone fill the span.
Find a uniform perpend joint from a known span, unit count and joint count.
Enter your values to see the result.
Good to know
For free ends, the internal joint count is usually units minus one. Explicitly enter any end joints included; do not force an unbuildable joint.
Sources
- Geometry / arithmetic
Estimates are not design approval or British Standards compliance checks. Structural arch, retaining-wall, wind, foundation, lintel and masonry-capacity design are not performed. Use the project engineer’s specification.