Low-profile overlay underfloor heating: heights, tiling direct and retrofit tips

Overlay boards are the usual way to add water underfloor heating to a room without digging up the floor. The pipe sits in grooves or between bosses in a thin board, and the floor finish goes straight on top. Our guide to underfloor heating floor systems compares overlay with screed and joisted systems. This guide is the detail for when you have chosen overlay: build-up heights, tiling direct, pipe size, floor preparation, laying, and the retrofit problems to plan for.

How much height does overlay underfloor heating add?

The board adds 15 to 25mm, but plan for the full build-up: board, adhesive bed and floor finish. Here is an example with a 10mm tile on a 5mm adhesive bed. Use your own tile and adhesive figures.

If you are taking up an old floor finish first, subtract its thickness. Lifting 10mm of old tile and adhesive takes a 33mm build-up down to a 23mm rise. Measure the height at every door and at the kitchen plinths before you choose a board.

Can you tile directly onto overlay boards?

Yes, on boards made for it. MaxHeatPanel routed XPS, MaxHeatDeck with its fibre-reinforced cement face, and the MaxHeatFix panel are all made to be tiled directly, so there is no screed to pour or wait for. Use a flexible adhesive suited to heated floors, and follow the adhesive maker's instructions for the board.

For large format tiles on underfloor heating, The Tile Association sets out these rules:

• Use an adhesive of at least C1S1 classification to BS EN 12004.
• Use CG2 grout to BS EN 13888, or CG1 with a suitable admix.
• Solid bed the tiles, back-buttering each one.
• Leave joints of at least 3mm.
• Split the floor into bays of no more than 40m² for ceramic and stone, or 25m² for resin agglomerate tiles, with movement joints between them.
• Put movement joints between areas on separate heating controls, and between heated and unheated floors.

MaxHeatDeck is the stiffest choice for large or heavy tiles, because of the cement face on both sides. Don't switch the heating on until the adhesive and grout have cured to the maker's guidance.

Which overlay board suits my floor finish?

Choose the board by the finish going on top. Tiles can go straight on. Thinner, flexible finishes usually need a smooth, rigid layer over the boards first.

One overlay system maker specifies a 6mm plywood overboard under carpet and vinyl, glued or fixed so the fixings miss the pipe runs, and lists laminate and engineered wood as suitable. For LVT, a 6mm cementitious tile backer board gives a flat, solid surface. Check the flooring maker's maximum floor temperature too. Our guide to UFH floor coverings lists the tog and temperature limits by finish.

Should I use 12mm or 16mm pipe in an overlay floor?

Use 12mm where every millimetre counts, and 16mm where you can spare 20 to 25mm and want fewer loops. The trade-off is loop length.

• 12mm pipe. Keep loops to 80m including tails. At 150mm centres (6.7m per m²) with 10m of tails, one loop covers about 10m². One overlay maker gives the same 80m limit for 12mm pipe.
• 16mm pipe. Keep loops to 100m including tails. One loop covers about 12 to 14m².

Worked example: a 28m² open-plan kitchen. With 12mm pipe it needs 28 × 6.7 = 188m in the floor plus tails, so three loops of about 70m, and three manifold ports. With 16mm pipe, two loops would be about 104m each, over the 100m limit, so it still needs three. The 16mm board adds 5 to 7mm more height. Our guide to how much pipe you need covers the method in full.

Will an overlay floor heat my room?

In a well-insulated room, usually yes. One overlay maker gives a maximum output of about 100W/m² for its 12mm system at a design flow of 45 to 50°C. Overlay floors have little mass, so they warm up much faster than a screed, but they give less output than screed at the same water temperature.

Check the room's heat loss per m² before you commit. A room losing 50 to 70W/m² is comfortably within reach. A room with large glazing or solid uninsulated walls may need 100mm centres near the windows, a higher flow temperature or extra insulation. The boards hold the pipe but most are thin, so an existing uninsulated slab will draw some heat downwards. Our guides to heat loss and UFH output and insulation under UFH help you check.

How do I prepare an existing floor for overlay boards?

1. Lift old carpet, vinyl, laminate and loose or hollow tiles. Firmly bonded tiles can often stay if they are flat and sound.
2. Check the floor with a 2m straightedge in several directions. Fill dips and grind high spots, because boards bridge gaps badly and tiles crack over them.
3. On a solid floor, check there is a working damp proof membrane. A new slab or screed must be dry before you lay anything over it.
4. On timber, screw down loose boards and check the floor doesn't flex. MaxHeatPanel is made for solid floors. On a springy timber floor, consider routed chipboard instead: see UFH in joisted timber floors.
5. Clean off dust and prime the surface if your board adhesive or levelling compound needs it.
6. Fix edge insulation round the room so the floor can move and heat doesn't escape into the walls.

How do I lay overlay boards and pipe, step by step?

1. Plan the loops and board layout first, with the manifold position and where each loop enters the room.
2. Lay the boards with joints staggered, starting from the wall where the pipes come in. Where the pipe turns at the end of a run, use boards with return grooves, such as MaxHeatDeck serpentine, or cut the return into the board.
3. Bond them to a solid floor with the adhesive your board maker specifies. On timber, screw them down with a washer under each screw head, keeping fixings clear of the grooves.
4. Cut boards with a fine saw or knife to fit round pipes, door frames and the WC pan.
5. Press the pipe into the grooves from a decoiler, without kinking it. Keep the tails in conduit where they leave the room.
6. Connect the loops to the manifold and pressure test before any finish goes down.
7. Tile or lay the finish. Don't step on unprotected pipe with heavy loads.

Our water UFH installation guide covers manifold connection and pressure testing in detail.

What should I plan for in a retrofit?

• Doors. A 30 to 40mm rise usually means trimming the bottom of each door. Hinged doors that swing over the new floor need checking before tiling.
• Thresholds. Where the heated room meets an unheated floor, use a threshold strip or ramp, and a movement joint between the two.
• Kitchen units and appliances. Don't run pipe under fixed units. Plan for the plinth height and the dishwasher and fridge clearances.
• Skirting and WC. Lift skirting so the boards and edge strip go to the wall. In bathrooms, plan the WC waste and pan fixing before you lay boards.
• Heat source. Boiler water is too hot for an overlay floor, so fit a blending valve pump pack at the manifold. See mixing valves explained.

What does a small overlay floor cost?

A single 9m² kitchen or bathroom floor on one loop costs about £407 in MaxHeatUFH kit (inc VAT, October 2026), before thermostat and tile adhesive.

Example: 9m² heated area, MaxHeatPanel 18mm with 12mm pipe at 150mm centres, run off the radiators with a single room pack. Pipe is 9 × 6.7 = 60m plus 10m of tails, about 70m. Add 10%: 77m, so one 80m coil. Boards: 9 ÷ 0.72 = 12.5, so 13 plus one spare.

• 14 x MaxHeatPanel 18mm boards at £11: £154
• 3-Layer PERT-EVOH pipe, 12mm x 80m: £45
• Single room mixing pump pack with Bastion 6m pump: £201
• Overlay panel washers, pack of 100: £7

Total: £407. A larger room needs a manifold, actuators and a pump pack in place of the single room pack.

Products for this job

• MaxHeatPanel Routed XPS Board, tile direct
• MaxHeatDeck Cement-Faced Routed XPS Board
• MaxHeatFix Low Profile Panel, 12mm pipe (pack of 10)
• 3-Layer PERT-EVOH UFH Pipe (12mm and 16mm)
• Overlay Panel Washers (pack of 100)
• 6mm Cementitious Tile Backer Board
• Edge Insulation Foam Roll
• All overlay boards

Frequently asked questions

What is the thinnest water underfloor heating?

Our thinnest overlay board is the 15mm MaxHeatFix panel for 12mm pipe. Add your adhesive and finish to get the total rise.

Can you put overlay underfloor heating on top of existing tiles?

Often, yes, if the tiles are firmly bonded, flat and sound. Remove any loose or hollow tiles and level the floor first.

Do overlay boards need screed?

No. Boards such as MaxHeatPanel, MaxHeatDeck and MaxHeatFix are made to tile directly. Thin finishes like LVT and carpet need a rigid layer such as 6mm ply or tile backer board instead.

Is overlay underfloor heating as good as screed?

It warms up much faster but gives less output at the same water temperature. In a well-insulated room it heats well. In a high heat loss room, check the output first.

Can I put laminate over overlay underfloor heating?

Yes, as a floating floor on an underlay rated for underfloor heating. Check the laminate maker's maximum floor temperature.

How long can a 12mm underfloor heating loop be?

Keep 12mm loops to 80m including the tails to the manifold. At 150mm centres that covers about 10m² per loop.

Can overlay boards go on a timber floor?

Only on a sound, rigid floor screwed down well, with the boards screwed and washered. MaxHeatPanel is made for solid floors, so on springy timber floors routed chipboard is often the better choice.

Sources: The Tile Association: fixing large format tiles and panels with underfloor heating, Polypipe Overlay product guide