Embedding underfloor heating (UFH) directly into a concrete slab is one of the most efficient and long-lasting ways to heat a room. By using the slab itself as a thermal mass, the system delivers slow, even warmth with low energy use. This method is ideal for new builds, extensions, and commercial slabs where the heating pipes can be integrated before pouring. In this guide, we explain how to install a wet underfloor heating system in a concrete slab — step-by-step — with all the right prep and insulation.
Begin by designing the layout and heating zones. Each room or slab section typically has its own loop, controlled via a manifold. Mark out any areas that won't receive heat (under kitchen units, large furniture, etc.). Your design should maintain a consistent pipe spacing — usually between 100 mm and 200 mm — depending on heat demand. Make sure pipe runs are balanced to avoid over- or under-heating zones. You’ll also need to plan where the manifold will go and allow for protective conduits to route the pipes cleanly into it after pouring.
Make sure the hardcore or MOT Type 1 sub-base beneath the insulation is fully compacted and level. This ensures that your insulation boards will sit flat and prevents movement or voids forming later. Use a plate compactor and laser level to achieve the correct height. Any soft spots or uneven areas should be rectified now — after the concrete goes down, fixing errors is costly and difficult. At this stage, install your damp-proof membrane (DPM) if it hasn’t already been laid. This membrane must sit under the insulation in most UFH systems to stop rising moisture.
Install rigid PIR or EPS insulation boards directly onto the prepared base. These prevent heat loss into the ground and ensure energy efficiency. Most systems use 75 mm to 150 mm of insulation depending on building regs and floor design. Fit boards snugly together and tape the joints to prevent slurry or screed from leaking through. Ensure the insulation reaches all corners and leaves no cold bridges. In some cases, a separating membrane (slip layer) is added above the insulation to allow movement or make pipe layout easier.
Install an expansion strip or edge insulation around all perimeter walls and fixed points. This soft compressible strip allows the slab to expand during heating cycles without cracking or damaging walls. It’s typically made from polyethylene foam and comes with an adhesive backing for easy fixing. The strip should be continuous and at least 10 mm thick, running to full slab depth. Corners and junctions can be taped or overlapped to ensure a thermal and expansion break around the full slab.
Start laying the heating pipework from the manifold location. Use a pre-planned loop design, typically in a serpentine (meander) or spiral layout. Secure the pipes using clips or rails fixed into the insulation board — spacing is critical and should be consistent throughout. Use a bending spring or pipe decoiler to avoid kinks. Leave enough slack where the pipe returns to the manifold, and label each loop clearly. Avoid overlapping pipes and keep tight turns smooth to prevent flow restriction. Once laid, double-check spacing and test coverage before moving on.
Before pouring concrete, pressure test the UFH pipe loops to check for leaks. Use water or compressed air to test each loop independently or as a group via the manifold. The pressure should be held at around 6 bar for at least 30 minutes without any drop. This ensures that all connections are secure and no pipes were damaged during installation. Testing before the pour is non-negotiable — a leak inside set concrete is difficult and expensive to fix. Keep the system under pressure during the concrete pour to help identify any accidental issues immediately.
If required by your design, install steel mesh reinforcement above or around the UFH pipes. This adds strength to the slab and reduces the risk of cracking. Make sure spacers are used to keep the mesh in the correct position. Check your overall floor height and pour level using laser level markers. The pipework and insulation must sit below the finished surface depth. Set up slab formwork and mark pour height clearly to help the concrete crew. Ensure the pour will fully cover the pipes with at least 50–75 mm of concrete above them for protection.
Pour the concrete directly over the pipe layout, starting from one end and working methodically across the slab. Use wheelbarrows or a concrete pump to avoid walking over the pipes. Screed the surface level as you go, taking care not to disturb the pipework. A laser screed or straightedge can help ensure a flat finish. Avoid using vibrating pokers directly near the pipes — use tamping or hand-floating instead to prevent damage. Keep the system pressurised during and after the pour, and monitor for any signs of leakage as the concrete begins to set.
Proper curing is essential to protect both the slab and the embedded pipe system. Keep the slab damp for at least 7 days using curing sheets, sprays, or light water misting. Don’t allow heavy foot or equipment traffic during this period. Never switch on the heating system until the concrete has fully cured — typically after 28 days. Starting the system too early can cause shrinkage cracks and pipe damage. Once cured, gradually commission the UFH with low flow temperatures to let the slab acclimatise slowly.
After full curing, connect the pipes to the manifold and begin the system commissioning process. Open all valves and check for flow in each zone. Gradually increase the water temperature — typically no more than 5°C per day — to avoid thermal shock. Check all controls, thermostats, and sensors are functioning correctly. Monitor the heat-up times and note any cold zones. A properly installed system should run quietly, evenly, and efficiently once commissioned. Final screeds, polished finishes, or floor coverings should only be installed after the slab is dry and stable.
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