Expansion joint
Expansion joint is a sized gap in a slab, roof sheet, masonry wall or tile field that absorbs thermal expansion. A subset of movement joints, focused on thermal cycling.
Ask Chalkline about this →An expansion joint is a deliberately sized gap or break in a continuous building surface (concrete slab, metal roof sheet run, masonry wall, tile field) that absorbs the dimensional change caused by thermal expansion and contraction over the daily and seasonal cycle. It is a specific subset of movement joints: where a control joint deals with shrinkage and minor drying movement, an expansion joint deals primarily with thermal cycling.
Why thermal expansion matters in residential:
- Steel expands by about 0.012 mm per metre per °C. A 25 m metal-roof run shifts ~9 mm between a 10°C winter morning and a 40°C summer afternoon.
- Concrete expands by about 0.01 mm per metre per °C.
- Ceramic tile expands less, but the substrate underneath (concrete or screed) expands more, putting the tile field under stress.
- Aluminium expands by 0.023 mm per metre per °C (double steel, eight times timber); long-run aluminium frames need allowance.
Where expansion joints appear in residential:
| Element | Maximum continuous run before joint | Joint detail |
|---|---|---|
| Metal roof sheet | 25 m end-to-end fixed (AS 1562.1 typical) | Step flashing or slip-fix at the joint |
| External cement render | 6 to 9 m horizontally and vertically | 10 mm sealant-filled gap |
| Internal tiled floor | 6 to 9 m grid (AS 3958.1) | Compressible filler with sealant cap |
| External brickwork | 10 to 15 m (AS 3700) | Vertical articulation joint, full height |
| Concrete driveway / path | 4.5 m (AS 3727) | Sawn or formed contraction joint |
| Long-run aluminium frame | 5 to 6 m per manufacturer | Slip-joint at frame junction |
Construction of an expansion joint:
- Compressible filler (closed-cell foam, polyethylene rod, “backer rod”) sized to the design gap, fitted to the full depth of the joint.
- Sealant cap (polyurethane or silicone) over the top of the filler, tooled flush or slightly recessed.
- Both faces of the gap kept clean and primed if the sealant manufacturer specifies.
Common defects:
- Joint missed entirely in a long run; the field cracks at a random weak point.
- Joint too narrow: filler compressed too hard, sealant cracks at the first temperature swing.
- Sealant applied without backer rod; the sealant bonds to three sides (top, plus both vertical faces) and tears when the joint moves.
- Wrong sealant: silicone where polyurethane was specified, or vice versa.
- Joint blocked by adjacent finishes (skirting, cornice, paver) that defeat the movement allowance.
Expansion joint vs control joint:
- Control joint: planned crack in concrete or render to control where the shrinkage cracking happens.
- Expansion joint: gap sized to absorb actual movement; structural break.
- Articulation joint: full-height masonry-wall break, often combining both functions.
Also known as: thermal movement joint; structural movement joint; isolation joint (in some concrete contexts).
Category: Structure.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.