Cross-Reference: Thermal Expansion of Building Materials

Summary

Failing to account for thermal expansion is one of the most common causes of cracking, buckling, and joint failure in building work. A tiled terrace that looks perfect in summer can show cracked grout, lifting tiles, or broken edge joints by winter — because the temperature range from a hot summer day to a cold winter night can be 40–60°C, producing significant movement in long runs of material.

This cross-reference provides the coefficients of linear thermal expansion for common building materials, with worked examples showing the actual movement expected in typical building situations. Use this table when designing expansion joints, selecting sealants, or investigating cracking.

Key Facts

• Coefficient of linear thermal expansion (CLTE or α): Expressed in mm/m/°C (millimetres of movement per metre of material per degree Celsius of temperature change). Equivalently expressed as ×10⁻⁶ per °C (microstrain per degree)
• UK temperature range: In most UK locations, the surface temperature of external materials ranges from approximately -10°C (cold winter) to +50°C (metal in direct summer sun) — a range of up to 60°C. For internal materials: approximately +10°C to +30°C — a range of 20°C
• Movement formula: Movement (mm) = CLTE × Length (m) × Temperature change (°C)
• Expansion joint spacing: As a rule of thumb, expansion joints in tiling and paving are needed approximately every 3–5m for internal floors and every 2–3m for external surfaces
• Differential movement: When two materials with different CLTEs are bonded together, the difference in movement creates shear stress at the bond line — this is why tile adhesive, grout, and mortar must have adequate flexibility to accommodate differential movement

Quick Reference Table — Thermal Expansion Coefficients

Worked Examples

Example 1: External Tiled Terrace

A tiled patio 6m × 4m using porcelain tiles (CLTE 0.006 mm/m/°C). External temperature range: -5°C to +45°C = 50°C.

Movement in the 6m run: 0.006 × 6 × 50 = 1.8mm

A 1.8mm movement in 6m must be accommodated by expansion joints. Without joints, grout will crack (grout has negligible tensile strength) and tiles may de-bond. Joints at 3m intervals divide the movement to 0.9mm per section — manageable by a flexible silicone expansion joint.

Example 2: Long Copper Pipe Run

A 10m copper hot water pipe (CLTE 0.017 mm/m/°C). Hot water temperature 60°C, installation temperature 15°C. Temperature rise: 45°C.

Movement: 0.017 × 10 × 45 = 7.65mm

A 10m copper pipe expands by over 7mm when heated. Expansion loops, expansion joints, or flexible connections must accommodate this. In a straight run clipped every 600mm, the pipe will bow between clips — potentially damaging the clips and causing stress at fittings. Guidance: for copper, an expansion loop (U-bend with two offsets) every 5m of straight run, or use of flexible connections at appliance connections.

Example 3: UPVC Guttering

A 5m run of UPVC guttering (CLTE 0.070 mm/m/°C). External temperature range: -10°C to +40°C = 50°C.

Movement: 0.070 × 5 × 50 = 17.5mm

A 5m UPVC gutter expands by 17.5mm — this is why UPVC gutters must have slip joints at every section join. The slip socket allows approximately 20mm of movement. If sections are glued solid or installed without slack in the sockets, the gutter will buckle in summer heat.

Example 4: Aluminium Window Frame

An aluminium window frame 2.4m wide (CLTE 0.024 mm/m/°C). Temperature range: -10°C to +50°C = 60°C (dark anodised aluminium can reach 60°C+ in direct sun).

Movement: 0.024 × 2.4 × 60 = 3.46mm

The frame expands by 3.5mm. The structural opening is masonry (CLTE 0.007 mm/m/°C) — movement in masonry = 0.007 × 2.4 × 60 = 1.0mm. Differential movement = 2.5mm. The frame perimeter sealant must accommodate 2.5mm of differential movement — a silicone sealant (typically 25% movement capability) in a 10mm joint width can accommodate ±2.5mm. This is why aluminium window installations specify a minimum 10mm perimeter gap and a flexible sealant.

Example 5: Concrete Slab Driveway

A concrete driveway slab 6m long × 100mm thick. Temperature range (UK external): -5°C to +35°C = 40°C.

Thermal movement: 0.012 × 6 × 40 = 2.88mm

Plus drying shrinkage (not temperature-dependent but additive): typically 0.03% = 1.8mm over 6m.

Total potential movement: approximately 4.7mm. A sawn control joint at 3m and a movement joint at the edge of the slab are the minimum requirement. Without joints, concrete will crack randomly (following the weakest path through the slab).

Material-Specific Notes

Timber

Timber is orthotropic — its expansion properties are dramatically different in different directions:

• Parallel to grain (along the board length): CLTE 0.003–0.005 mm/m/°C — very low thermal expansion
• Across the grain (board width): CLTE 0.030–0.070 mm/m/°C — much higher, but still less significant than moisture-driven movement
• Moisture movement: Dominates over thermal movement for timber. A 1% change in moisture content produces approximately 0.1–0.2% change in board width. In a humid kitchen versus a dry heated room, a 150mm oak board can move 5–10mm across its width due to moisture change alone

For timber flooring, decking, and joinery, moisture movement is the primary design consideration — thermal expansion is secondary. Always acclimatise timber before installation.

UPVC Windows and Doors

UPVC has a high CLTE (0.060–0.080) — much higher than the masonry surrounds. The foam-filled reinforcement sections inside UPVC frames (usually galvanised steel or aluminium) have lower CLTE but are isolated from the masonry by the UPVC shell. The result:

• The UPVC outer frame expands/contracts significantly
• The frame must be screwed, not glued, to the structural surround so it can move
• The perimeter sealant must be flexible (silicone, not acrylic caulk)
• UPVC frames should not be painted dark colours — dark UPVC can reach temperatures that distort the frame

Brick vs Concrete Blockwork

Clay brickwork has a lower CLTE than concrete block (0.006–0.008 vs 0.010–0.012). However, clay brickwork also undergoes irreversible moisture expansion after firing — a clay brick straight from the kiln will expand by 0.02–0.05% over the first years as it reabsorbs moisture from the atmosphere. Concrete blocks undergo initial drying shrinkage. These opposite movements mean that clay brick/concrete block composite walls must have movement joints (typically at 6–9m centres and at junctions) to accommodate the differential.

• tile expansion joints — Expansion joint design and sealant specification for tiled surfaces
• screed types — Screed shrinkage and drying movement management
• concrete volume — Concrete slab design including joint spacing
• drill bit guide — Material properties for drilling