How level should the floor be before underfloor heating is installed?
Updated: Oct 12
The floor must be flat and sound before underfloor heating is installed, and it must be levelled beforehand rather than during. The screed Heatwell pours over the cable is about 4mm thick. It is there to position and protect the heating element, not to level the floor, and it cannot correct a dished or uneven surface.
Flat matters more than level.
Level means horizontal. A floor can run slightly downhill across a room and still take underfloor heating perfectly well. Some floors are deliberately not level — a bathroom floor falling to a drain is sloped on purpose.
Flat means free of humps and hollows. This is the one that matters. The heating cable has to sit at a consistent depth below the finished surface, because depth is what determines how quickly that patch of floor warms up. A cable that runs 15mm below the surface in one place and 25mm in another produces a floor that heats unevenly, with warm areas and cold areas that no thermostat setting will fix.
Heatwell requires the floor to be flat to within 3mm over a 1m straight edge.
Why can't the screed level the floor?
Because of what it is for and how thick it is. Heatwell's method adds approximately 4mm to the floor height — budget 5mm to be safe. That layer encases the cable and gives the flooring installer a flat level surface to work on. It is not a levelling screed and it is not deep enough to be one. Pouring more to fix a low spot would bury the cable deeper in that spot, which is the original problem made worse.
If the original floor is not flat, floor-levelling compound is likely to need to be installed twice. Once to make the floor flat and another time to bind in the underfloor heating cable. Please note that the screed to make the floor flat will need to be fully cured before the underfloor heating cable is installed. Floor primer will need to be applied for the second level of screed and is likely to need to be applied on the first.
Does the subfloor need priming?
Yes, if the subfloor is concrete, tile and slate underlay, existing tiles, or a wooden floor. The surface must be flat and clean and the floor primer applied correctly, or the self-levelling compound will not bond and can come away.
No, if there has been noise proofing, waterproofing, or a anti fracture membrane installed. Check with the noise proofing or anti fracture membrane supplier if their product is suitable for underfloor heating and if a primer is required before you install.
What condition does the floor need to be in?
Clean, free of dust, and free of paint. Free of cracks, holes and sharp protrusions. A floor being renovated needs all remains of the previous flooring removed, including every trace of glue and adhesive.
Sharp protrusions matter more than they sound. That included raised nail or screw heads. The cable is laid directly onto this surface, with any protrusion pressing into the cable for the life of the installation.
What if the subfloor is particle board or plywood?
Underfloor heating is not recommended directly over particle board or plywood. Expansion and contraction is a certainty in New Zealand conditions, and it risks cutting the element where the sheets butt together.
There are three ways to prepare it:
• Tile and slate underlay. The most common, and the default.
• 6mm insulation sheeting. This has the additional advantage of improving efficiency and reducing running cost, which makes it the option worth paying for if the room is heated daily.
• An anti-fracture rubber-based system. The most expensive, and it adds the least height.
Can underfloor heating go over existing tiles?
Yes, provided no tile is loose or broken. The glaze has to be ground off and the surface primed. The catch is height: tiles add a minimum of 12mm and the heating system adds at least 4mm on top, so that floor rises by at least 16mm. Heatwell and most installers would rather the old tiles came out.
What happens if the floor isn't prepared properly?
Heatwell will not install over an improperly prepared floor, and poor floor preparation is an explicit exclusion from the Heatwell warranty. This is not a technicality. A cable laid on a bad surface fails in ways that look like product failure when they are not. Photographs required under Best Practice will not help if the surface underneath was wrong from the start.
Who levels the floor?
Usually the builder or the tiler, before Heatwell arrives. It is worth confirming whose scope it is in, because it is a common gap between trades.
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