How to Select an Expansion Joint: Movement Calculation & Sizing

Q: How do you calculate expansion joint movement?

Start with thermal movement: delta-L = alpha x L x delta-T, where alpha is the material's coefficient of thermal expansion, L the section length and delta-T the temperature range. Then add any seismic, wind-sway and settlement demands from the structural engineer to get the total design movement.

Q: How do I choose the right expansion joint size?

Pick a system whose rated movement, in mm and as a percentage of joint width, comfortably exceeds your calculated design movement, then confirm the fire, watertight and load requirements for the location.

Knowledge Bank › Design › How to Select an Expansion Joint

How to Select an Expansion Joint: Movement Calculation & Sizing

Choosing the right expansion joint comes down to six decisions: location, movement, gap size, material, performance (fire / watertight / load) and finish. This guide walks through each — including the thermal-movement calculation.

Step 1 — Location & type

Identify where the joint sits: floor, parking deck, wall/ceiling, roof, or a seismic separation. This sets the base system. See types of systems.

Step 2 — Calculate the movement

Total movement combines thermal, seismic, wind and settlement effects. Thermal movement is the usual starting point:

Thermal movement: ΔL = α × L × ΔT
where α = coefficient of thermal expansion, L = length of the structural section, ΔT = temperature range.

Material	α (per °C)
Concrete	~10–12 × 10⁻⁶
Steel	~12 × 10⁻⁶
Aluminium	~23 × 10⁻⁶

Worked example: a 30 m concrete section (α = 11 × 10⁻⁶) through a 40 °C swing:
ΔL = 11×10⁻⁶ × 30,000 mm × 40 = 13.2 mm. Add a safety margin, then any seismic/wind demand, to get the design movement.

For seismic, wind-sway and settlement, add the structural engineer’s stated demands. Seismic movement is large and multi-directional — it usually governs the joint selection where present.

Step 3 — Size the gap & movement class

Select a system whose rated movement (as a % of nominal joint width and in mm) comfortably exceeds the calculated design movement. Match the ASTM E1399 movement category to the demand — Category IV for combined thermal + wind + seismic.

Step 4 — Choose the material

EPDM for general building seals; silicone-faced foam for facades and watertightness; stainless or bronze covers for heavy-duty or feature areas. See materials.

Step 5 — Performance requirements

Fire — if the joint crosses a fire-rated wall/floor, specify a fire-rated system matching the rating (2–4 hr, UL 2079).
Watertight — roofs, decks and wet areas need a watertight seal, ideally with a test certificate.
Load — trafficked floors and decks need a rated load class (AASHTO HS-20 / IRC).

Step 6 — Finish

Mill, anodised or powder-coated aluminium, colour-matched gaskets, and flush vs. surface-mount to suit the architecture.

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Frequently asked questions

How do you calculate expansion joint movement?

Start with thermal movement: ΔL = α × L × ΔT, where α is the material’s coefficient of thermal expansion, L the section length and ΔT the temperature range. Then add any seismic, wind-sway and settlement demands from the structural engineer to get the total design movement.

How do I choose the right expansion joint size?

Pick a system whose rated movement (in mm and as a percentage of joint width) comfortably exceeds your calculated design movement, then confirm the fire, watertight and load requirements for the location.

Related: Types · Standards · Glossary