Screeding Underfloor Heating Pipes

Pouring the screed itself is quick. Most of the work is in the preparation. A careful installer usually works in this order:

1. Checking the base. The structural slab has to be level, clean and dry. On ground-bearing floors or above an unheated space, a working damp-proof membrane is needed.
2. Insulation. Floor-grade EPS goes under the pipes, XPS in damper areas, or system boards with studs. The insulation sends the heat up into the room instead of down into the ground.
3. Edge strip. A compressible strip is fixed around all walls, columns and door frames. The screed can then expand freely as it warms up and does not crack.
4. Laying the pipes. The pipes, usually PE-RT or PEX with an oxygen barrier, are laid in a spiral or a serpentine pattern at the spacing given in the design. They are held with clips or pressed into the studs of the boards so they do not float up during pouring.
5. Pressure test. Before pouring, the circuits are pressurised and checked for leaks. The system stays under pressure during screeding, so any damage from a shovel or a hose shows up straight away.
6. Pouring the screed. Sand and cement screed is spread, compacted and levelled with a straightedge. Liquid anhydrite screed is poured and levels itself. Screed is not vibrated like structural concrete; spreading and smoothing it thoroughly is enough.
7. Movement joints. Larger areas, irregular rooms and doorways are divided by joints. A joint should separate heating circuits and should cross pipes no more often than necessary.

Why so many layers? Each one has a job: the insulation keeps the heat in, the edge strip allows movement and the screed stores and spreads the heat. If one is missing, the floor heats worse or gets damaged over time.

Two main types of screed are used over underfloor heating. Both work well if you follow the manufacturer's instructions.

• Sand and cement screed copes well with moisture and suits bathrooms and ground floors. Under EN 1264 the cover above the pipe is usually at least around 45 mm. Fibres and an underfloor heating additive are mixed in. It dries more slowly.
• Anhydrite (calcium sulphate) screed is liquid, wraps closely around the pipes and conducts heat well. It can be thinner; the manufacturer sets the exact depth. It is sensitive to moisture, so wet rooms need careful waterproofing, and the surface is usually sanded before the floor covering goes down.

If your floor build-up has to stay low or you need a faster schedule, anhydrite is worth considering. For damp areas, or if you want a proven, robust solution, cement screed is the usual choice. The decision should follow the heating design and the type of final floor covering.

The amount of work, and therefore the cost, depends mainly on the condition of the base and the access to the site.

• Easier: a new build with a level slab, large open areas and good access for a screed pump or mixer
• Harder: a renovation where the old floor has to come up down to the structural layer and the base needs levelling
• Many small rooms, recesses and thresholds mean more movement joints and more hand work
• Upper floors without a lift or narrow access make moving materials slower
• A deeper screed uses more material and takes longer to dry

In a flat, think about the weight of the new floor build-up and impact noise for the neighbours below. Speak to the building management or owners' association before work starts.

• Pouring without a pressure test. A leaking joint under the screed means breaking up the floor.
• Missing edge strip, or trimming it too early. The screed then pushes against the walls and cracks.
• Pipes not fixed well enough. They float towards the surface and the cover above them ends up too thin.
• Too little cover. The floor heats in stripes and the screed above the pipes breaks easily.
• Drying too fast. Turning the heating up full straight after pouring, or summer draughts, cause cracks.
• Laying the floor covering on damp screed. Wooden floors cup and tile adhesive fails to bond.

Once poured, the screed needs to be left alone. For the first few days, protect it from draughts, direct sunlight and frost. The heating goes on for the first time only after the period set by the screed manufacturer. For cement screed this is usually around three weeks; for anhydrite it is sooner, roughly after one week.

Then comes the commissioning heat-up: the water temperature is raised gradually, held for several days and then lowered slowly again. The process is recorded in a log. Full drying before the floor covering goes down takes much longer, with a thick cement screed even several weeks. The calendar does not decide when the floor is ready. A moisture measurement does, and the flooring fitter should take it before laying.

At handover, make sure you have:

• A pressure test record from before the screed was poured
• A pipe layout plan with dimensions, or photos taken before pouring
• An edge strip around the full perimeter of each room and movement joints as designed
• A level surface with no cracks, chips or hollow spots when tapped
• Details of the screed type, depth and pouring date
• A heat-up log with temperatures and dates
• A record of the moisture measurement before the floor covering is laid