Movement Joint Width Calculator
Joint width from thermal movement plus allowances.
With the default inputs: Structural length between joints 60 · Temperature range 30 ℃ · Expansion coefficient 1.0 ×10⁻⁵/℃ etc.Minimum joint width 60 mm
Edit the inputs below to recalculate.Inputs
Result
| Thermal movement (one way) | 18.0mm |
| Two-way movement | 36.0mm |
| Allowance | 20mm |
| Calculated width | 56.0mm |
| Joint spacing reference | 60m |
ΔL = α·ΔT·L with α about 1.0×10⁻⁵ per ℃ for concrete. The width must cover two-way movement - expansion into the joint when hot, contraction when cold - plus construction tolerance, seismic displacement and sealant movement capability; twice the calculated value plus 20-30 mm is customary. Maximum spacing without joints is set in GB 50010 clause 8.1.1 (55 m for cast-in-situ frames, 45 m for shear walls, 40 m for basements, relaxable by calculation). Drawings govern.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
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 |
How to work it out
- Fix the joint spacing L from the code limits: 55 m for cast-in-situ frames indoors, 35 m exposed, 45 m for shear walls.
- Compute ΔT as the difference between the temperature at construction and the service extreme. In the north that is winter minimum against placing temperature; in the south it is the summer maximum.
- Movement ΔL = α × L × ΔT, with α = 1.0 × 10⁻⁵ per °C for concrete and 1.2 × 10⁻⁵ for steel.
- Joint width = ΔL + construction tolerance + seismic allowance. Tolerance is 10-15 mm; the seismic allowance runs 15-50 mm by intensity.
- Check the minimum: a movement joint is never less than 100 mm, and seismic joints add roughly 20 mm per 10 m of height.
Where it goes wrong
- Taking 50 mm from experience without calculating — 60 m at 30 °C moves 18 mm, and after tolerance and seismic allowance 50 mm leaves almost nothing in reserve.
- Using the annual temperature range — it must be the placing temperature against the service extreme; placing at 30 °C and a −15 °C winter gives 45 °C, not the annual swing.
- Treating a movement joint as a settlement joint — settlement joints are sized on predicted differential settlement, and using thermal movement alone lets the two halves collide.
- Filling the joint with a rigid material — movement joints need compressible fill (bitumen felt, rubber waterstop, EPS); rigid fill cancels the joint.
Material and cost
Complete joint construction with waterstop, cover and filler 150-400 CNY/m. Rubber waterstop 30-80 CNY/m, stainless cover 120-250, bitumen felt fill 15-30.
On site
- Run the joint continuously from foundation to roof; a break or offset concentrates stress exactly where it cannot be relieved.
- Roof joints leak more than any other location: use a raised omega cover plus two waterproofing layers, not filler alone.
- Keep mortar and rubble out during construction — debris fills the gap and the structure cracks when it tries to move.
Basis
- GB 50010-2010 Code for design of concrete structures, Section 8.1
- GB 50011-2010 Code for seismic design of buildings, Section 6.1
- GB 50352-2019 Unified standard for design of civil buildings, Section 6.9
FAQ
Why two-way movement?
Can a construction strip replace a movement joint?
What is wrong with a wider joint?
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Movement Joint Width Calculator. 2026-10-25. https://jianzhujisuan.com/en/expansion-joint-width