When installing underfloor heating (UFH) in a concrete floor, setting correct slab levels is critical. An uneven slab can lead to inefficient heating, structural cracking, or pooling during the pour. More importantly, consistent levels ensure that pipe zones are fully submerged in the right thickness of concrete, allowing optimal thermal performance. This guide walks you through the process of setting and maintaining accurate slab levels before pouring around UFH pipework.
Begin by establishing the final finished floor level (FFL) throughout the site using a laser level or dumpy level. Mark this consistently across walls, door thresholds, and datum points. From the FFL, subtract the thickness of the floor finish (e.g. tile, polished concrete) and the required concrete slab depth above UFH pipes — typically 50–75 mm. This allows you to calculate the required insulation + UFH + slab thickness at each point, helping to control level tolerances before any materials are laid.
Excavate and compact the hardcore base to a consistent depth. Use a vibrating plate or roller to ensure it’s tightly compacted, checking levels frequently with a laser. The surface should follow the desired slab plane — whether level or with falls — and not vary more than ±10 mm. Uneven base levels at this stage make it far harder to keep the final slab consistent, especially when pouring over pipes and insulation. Use timber pegs or pins set to depth to verify height as you go.
Install rigid insulation boards (e.g. PIR or XPS) across the compacted base. As you lay each board, use a straightedge and laser level to check its height. Boards may vary slightly in thickness, so use packers, trimming, or levelling screed if needed to create a flat surface. Once the insulation layer is in, recheck levels across the room and mark any deviations. This becomes your working base for UFH pipe fixing and concrete cover.
Before laying UFH pipework, fix reference pins or guide rails around the slab area at the target slab height. These can be small rebar sections or screed rails set using a laser. They allow you to maintain level control during the concrete pour, especially when pouring over complex or multi-zone pipe layouts. Make sure these markers are stable and not knocked loose during pipework installation.
Once pipes are secured and pressure-tested, begin the concrete pour using your fixed reference pins or rails to guide level depth. Use a screed bar or laser screed to maintain a flat finish across pipe zones. Constantly check slab depth over the pipes — too shallow and they risk exposure, too deep and heat performance suffers. Always finish flush with your guide points, then trowel or power-float based on the specified finish.
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