5 mistakes when laying screed over underfloor heating

Screed over underfloor heating is its own discipline. Mistakes here do not show up straight away — they surface after one or two heating seasons, and the only fix is to break the floor out. Here is what goes wrong most often.

Mistake 1: too little screed above the pipes

Cost-cutting crews leave 30–35 mm above the pipes instead of the 50 mm we work to. It looks fine. In use the screed heats unevenly — hotter over each pipe, cooler between them — and a season later you get long cracks running exactly along the pipe lines.

Mistake 2: no fibre

Underfloor heating means constant expansion and contraction. Without fibre the screed does not take it. First season: microcracks. Second: visible ones. Third: the parquet starts creaking because the screed is moving.

Mesh is not a substitute for fibre. Mesh reinforces in one plane only; fibre works through the whole volume. Best of all is both together.

Mistake 3: heating it up too soon

The classic impatient-owner mistake: the screed goes down and two weeks later the heating goes on "to dry it out faster". What follows:

• The water inside turns to steam
• The pressure tears the screed apart
• Bulges, blisters and cracks appear
• In places the screed simply lets go of the substrate

Mistake 4: no edge strip

The strip goes around the full perimeter before the pour. Its job is to absorb thermal movement: underfloor heating takes the screed up to 25–30°C and it grows 2–5 mm along its length. With no strip the screed presses on the walls and cracks — mid-floor, at doorways, in corners.

A strip standing 1–2 cm proud is not a fault — the surplus is simply trimmed once the covering is down. Saving on the strip means laying the screed again in two years.

Mistake 5: the wrong pipe spacing

Not strictly a screed mistake, but it causes screed problems. If the pipes are spaced wider than 20–25 cm:

• The floor heats unevenly
• Cold patches sit between the pipes
• The screed moves less between the pipes than over them → cracks

Standard spacing: 15–20 cm across the room, 10–15 cm along external walls and under windows.

Worth knowing as well

Pressure-test before screeding

The pipes must be held at working pressure (1–2 bar) during the pour. If one leaks, finding it after screeding means breaking the floor open.

Insulation underneath

There must be 30–50 mm of XPS under the pipes. Without it 30–40% of the heat goes into the structural slab instead of the room.

Waterproofing

A waterproof membrane goes onto the substrate before the insulation, lapped 10 cm and turned up the walls. It keeps condensation out.

How to check the job was done right

• Thickness can be checked by coring at the edge — at least 50 mm above the pipe
• The edge strip should run right around — visible until it is trimmed
• The waterproofing should stand 5–10 cm up the walls
• The surface is flat, with no damp patches, cracks or delamination

In short

Screed over underfloor heating is no place to economise. The mistake shows up one or two seasons later, with the renovation finished and an expensive floor laid on top. Putting it right costs five to ten times what was saved.

Work with a crew that gives at least a one-year guarantee and lays at least 50 m² of heated-floor screed a month. Keep the "we'll save you a bit" merchants well away.