Expansion Joints and Movement Joints in House Building

Concrete and masonry are not “fixed” materials: they expand with heat, contract in cold, and settle at different rates when two parts of a structure rest on separate supports. Without precautions, these natural movements always resolve the same way — through a crack, often running all the way through, sometimes structural. Expansion joints and movement joints are the devices that absorb these movements at a chosen location, rather than letting them appear anywhere at random. This article explains what each one is for, where to place them, at what spacing, and how to build them correctly in a self-build house.

Shrinkage, thermal expansion, settlement: three movements to understand

Before discussing joints, you need to understand what they absorb. Concrete and masonry undergo three families of movement on different time scales.

Shrinkage (first months)

When concrete or mortar dries, the mixing water that has not reacted with the cement evaporates. The material loses volume — this is hydraulic shrinkage. For standard concrete, this amounts to 0.3 to 0.6 mm per linear metre. Over a 10-metre slab, that represents 3 to 6 mm of shortening in the first months. If the slab cannot shorten freely (cast into walls, friction on the sub-base), the stress is released through shrinkage cracks.

Thermal expansion (annual cycle)

Concrete expands by 0.010 to 0.012 mm per metre per degree (coefficient α = 1.0 × 10⁻⁵ /°C). Between an outdoor slab at +55 °C in full summer sun and -10 °C in winter, the amplitude is 65 °C. Over 10 metres, the theoretical movement reaches 6.5 mm between summer and winter. This is considerable — and it is why external slabs (terraces, driveways) require closely spaced joints.

Differential settlement (long term)

Two parts of a structure resting on supports of different stiffnesses do not settle at the same rate: a house on deep foundations no longer moves, but an attached garage on lightweight strip footings may sink a few millimetres in the first years. Result: a crack at the junction, sometimes offset vertically. The movement joint anticipates this phenomenon.

Expansion joint vs movement joint: the difference

Both absorb movement, but not the same kind. They must be clearly distinguished.

The expansion joint

It runs through the structure from top to bottom (including foundations when they are integral to it) and divides a long run into several autonomous blocks. Each block can expand and contract independently. The joint takes the form of a vertical gap filled with a compressible material (expanded polystyrene 10-20 mm, Fibralith, dense PU foam), then sealed at the surface by an elastomeric sealant.

What it is for: absorbing thermal expansion and hydraulic shrinkage movements over long spans.

Where you find it: external slabs, boundary walls > 10 m, long buildings, joints between two sections of structure built in different phases.

The movement joint (structural separation joint)

It separates two distinct volumes that rest on different foundations (or have very different masses). It is present from the foundations to the ridge and allows differential settlement between the two parts.

What it is for: accommodating differential settlement without cracking the main structure.

Where you find it: between house and attached garage, between house and conservatory, between two adjoining building elements, during an extension built against an existing structure.

Expansion joint spacing — French building code rules

Spacing rules differ according to the type of structure and the French building code (DTU) that applies.

Masonry (DTU 20.1)

DTU 20.1 (Small-element masonry works) sets maximum spacings for walls in concrete block, brick or stone:

For a standard detached house (< 15 m long), no joint is generally needed on load-bearing walls. However, as soon as a boundary wall exceeds 10 m, a joint is required.

Indoor slabs (DTU 13.3 / NF P 11-213)

For ground-bearing indoor slabs (house slab, garage slab):

• Slab < 40 m²: no joint required
• Slab 40-100 m²: control joints (saw-cut to 1/3 of thickness) every 5-6 m
• Slab > 100 m²: full expansion joints (through the full depth) every 25-30 m

External slabs and screeds

External slabs experience maximum thermal variation. Spacing is much tighter:

Practical rule: for a concrete terrace, divide it into square or rectangular slabs of less than 16 m² with a length-to-width ratio ≤ 1.5. Elongated shapes crack diagonally.

Boundary wall

Boundary walls are exposed to full sun, thin (15-20 cm) and long. A joint every 10 to 12 metres is mandatory, taken down to the strip foundation. Forgetting this joint is the number one cause of vertical cracks in concrete block boundary walls.

Movement joint — detailed construction

This is the joint most often forgotten in self-build, and the one that causes the most costly problems. Here is how to build it correctly between a house and an attached garage (the most common case).

Step 1 — Separated foundations

The strip footings of the house and the garage are cleanly cut at the joint location. A 20 mm expanded polystyrene panel is inserted between the two footings over their full height, or they are simply left 20-40 mm apart. No reinforcement must cross the joint.

Step 2 — Substructure

The garage substructure wall is built independently of the house wall. At the junction, a compressible strip (polystyrene, dense PU foam or neoprene profile) fills the gap. The joint is vertical over the full height.

Step 3 — Slab and superstructure walls

The garage slab is cast separately from the house slab, with a separation strip. The superstructure walls of the garage are independent of the house walls. Horizontal ring beams are cut at the joint.

Step 4 — Roof and covering

The garage roof may be structurally tied or separated depending on the architecture, but the cladding must accommodate the joint movement: flexible flashing, overlap with clearance, waterproofing strip that accepts 10-20 mm movement.

Step 5 — Surface finish

On the facade, the visible joint is concealed by an elastomeric cover strip (neoprene profile, Class 25E/25HM polyurethane sealant) or by a painted polystyrene strip. Visible width: 15-25 mm.

Decision tree: which joint, where

Filling materials

A joint has two layers: the backing (compressible part that absorbs movement) and the surface (flexible waterproofing).

Backer rod and backing material

Surface waterproofing sealant

The sealant must be elastomeric Class 25 minimum (i.e. capable of absorbing ±25% movement). Three families:

• Polyurethane (PU): excellent all-round, paintable, good adhesion to concrete. The reference for facade joints. Examples: Sika Boom, Soudal Soudaflex, Mapei Mapeflex PU45.
• Neutral silicone: UV-resistant, ideal for sanitary joints and glazing, but not paintable. Not suited to exposed structural joints.
• MS Polymer (hybrid): new generation, high performance, paintable, less sensitive to moisture at application. Soudal Fix All, Sika Bond.

Special cases

Expansion joint on a timber-frame house

As timber has a low thermal expansion coefficient in the longitudinal direction (0.003-0.005 mm/m/°C), joints are rarely needed on a timber-frame house. However, the concrete slab on which the frame rests follows the rules for concrete — shrinkage joints in large slabs.

Movement joint on an extension

When building an attached extension to an existing structure, the movement joint is mandatory in the majority of cases: the ground has finished settling under the existing house; the new extension will settle in turn. If the two are tied together, cracking at the junction is guaranteed within 2-3 years. The joint must go down into the existing foundation (or into the main structure if that is not possible) and run up to the roof.

Joint at the pool / terrace junction

Between a pool surround (or the pool shell) and the adjacent terrace, a flexible joint is essential: the solid concrete pool has a different thermal inertia and moves differently from the terrace. Material: neoprene profile or Class 25 PU sealant over backer rod.

Joint in reinforced masonry

If the masonry includes horizontal ring beams (standard at each floor level), the expansion joint must cut the ring beam, leaving a starter bar in each block with no continuous steel between the two. The same rule applies to vertical stiffeners near the joint: they belong to each block independently.

Common mistakes in self-build

1. Confusing a control joint with an expansion joint — A joint saw-cut to 1/3 of the slab thickness is a control joint (directed cracking). An expansion joint runs through the full thickness and is filled with compressible material. The two are not interchangeable.

2. Forgetting the movement joint on an attached garage — A classic mistake: the garage slab is cast continuously with the house slab, and the walls are built tied together. Within 2-5 years, a through vertical crack appears at the junction.

3. Reinforcement crossing the joint — A single bar crossing it = joint neutralised. Cut all reinforcement at the joint line — non-negotiable.

4. Facade joint filled with mortar — Mortar is not elastic. It will re-crack within a year. Use an elastomeric sealant or a compressible profile.

5. Joints too widely spaced on a terrace — A 30 m² terrace without a joint will crack in a star pattern from the centre. Maximum panel area 16 m² for external concrete.

6. Joint that stops at the slab — A movement joint must go down into the foundations. If it stops at the slab, the foundation connection remains solid and the crack appears lower down.

7. Acrylic sealant outside — Acrylic sealants are cheap but crack in UV and damp. Reserved for interior decorative joints. For facades: PU or MS polymer.

Indicative costs

Key point: a joint planned from design stage costs 10 times less than repairing the crack it would have prevented.

Regulatory references

• DTU 20.1 (French building code) — Small-element masonry: partitions and walls
• DTU 13.3 (French building code) — Ground-bearing slabs: design, calculation, execution
• DTU 26.2 (French building code) — Screeds and slabs based on hydraulic binders
• NF EN 15651 — Classification of sealants (Class 25E, 25HM, 25LM)
• NF P 11-213 — Non-industrial ground-bearing slabs
• AQC Guide « Joints de dilatation et de rupture dans le batiment » (in French — Agence Qualite Construction)
• CCTG Fascicle 65 (public procurement — useful as a reference for best practice)