Technical guidance on expansion joint placement, spacing, pipe protection, and screed movement management for wet underfloor heating installations.
Screed expands and contracts with temperature changes. Without expansion joints, thermal movement causes cracking, buckling, and damage to floor finishes. In UFH systems, the temperature range can be 15–55°C, creating significant thermal expansion (approx. 0.01mm/m/°C for sand/cement screed).
| Criteria | Maximum | Standard |
|---|---|---|
| Area per bay | 30 m² | BS 5385 |
| Maximum dimension | 6m | BS 5385 |
| Joint-to-wall ratio | ≤2:1 | CIRIA C660 |
| Conservatory / high-glazing | 4m max | CIRIA C660 |
Pipe must NOT cross expansion joints without protection. Where a joint is unavoidable, protect the pipe with conduit (corrugated tube) over a minimum 300mm length centred on the joint. The conduit allows the pipe to move freely as the screed expands. Without conduit, the pipe can be crushed or shear the screed.
A switch line is where pipe direction changes between zones. It acts as a controlled expansion gap. All pipes must be sleeved in conduit through the switch line for a minimum 300mm length. Switch lines are typically 10–15mm wide, filled with compressible filler. The screed butts up to the compressible filler on both sides.
Retrofitting expansion joints to existing screed is difficult — it requires cutting through the screed with a diamond blade, installing compressible filler, and potentially rerouting pipe. Get the joints right before the pour. If retrofit is unavoidable, consult a structural engineer. The floor finish will also need to be cut and reinstated.
Perimeter edge insulation strip (150mm × 8–10mm compressible foam) must be installed along ALL walls and fixed columns. It serves dual purpose: thermal break (reducing edge heat loss) and expansion allowance. The floor finish should not bridge the perimeter strip.