Movement Joint Width Calculator
Joint width from thermal movement plus allowances.
Inputs
Result
Formula
| ΔL = α × ΔT × L | α ≈ 1.0×10⁻⁵ per ℃, L in mm |
| Width = 2×ΔL + allowances | two-way movement |
| Round up to 10 mm |
60 m structure with a 30 ℃ range: 18.0 mm one way, 36.0 mm two way, plus 20 mm gives 56.0 mm, rounded to 60 mm. A settlement joint still needs checking to 50-120 mm by ground conditions.
FAQ
Why two-way movement?
Both sides move. In heat the two blocks close the gap, in cold they separate; the sealant and waterstop must both compress and stretch, so twice the movement gives travel in both directions. Sizing on one direction only lets the joint open too far in winter, tearing the waterstop - and that is where leaks begin.
Can a construction strip replace a movement joint?
A strip solves construction-period shrinkage and differential settlement, but it gets concreted and closes - after that the structure is continuous again and long-term thermal movement still needs a real joint (or prestressing, sliding layers and similar). It is not a permanent joint.
What is wrong with a wider joint?
The waterstop and sealant must accommodate more movement, raising material and workmanship demand; the floor slab is interrupted, so shear keys or dowels, finishes, waterproofing and fire separation all become harder; and in hospitals or labs a wide gap is harder to keep clean. Joint width is a balance - adequate, not generous.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.