Electric underfloor heating on a timber floor: ply, backer boards and insulation

Timber floors are the most common subfloor upstairs in UK homes, and in many older ground floors too. They move with load and with seasonal changes in moisture, they have gaps between the boards, and they burn. Each of those points changes how an electric heating mat or cable must be laid.

This guide covers why floorboards need an overlay, the plywood and backer board options, how insulation fits in, the output and temperature limits for timber, and the electrical points that matter. It is for electric systems under tile and stone. For water systems in joisted floors, see underfloor heating in joisted timber floors.

Can you lay an electric heating mat straight onto floorboards?

No. Floorboards are not flat or stable enough to tile over, and their gaps would leave the cable unsupported. Overlay them with plywood or tile backer board first.

Three problems make bare boards unsuitable:

• Movement. Boards flex as you walk on them and shrink and swell with the seasons. Tiles and grout cannot follow that, so they crack, and the cable underneath is put under strain.
• Gaps and steps. One manufacturer's UK guide asks for a subfloor that is level, secure, smooth, dry and clean. One manufacturer says mats must not be laid on irregular surfaces.
• Combustible base. BS 7671 Section 753 calls for measures such as a metal sheet, metal conduit or at least 10mm of air clearance near easily ignitable structures. Some cable makers require a metal mesh on wooden subfloors for certain build-ups. Follow the instructions for the cable you use.

Before you overlay anything, check the floor itself. Stone Specialist's summary of BS 5385-3 suggests a simple test: stand a glass of water on the floor and jump about a metre away. If water spills, the floor needs stiffening, for example with noggings between the joists.

What plywood do I need over floorboards for electric underfloor heating?

Use exterior-grade WBP plywood at least 15mm thick, screwed down at no more than 300mm centres. Seal it on all faces and edges before fixing.

Point	What the guidance says	Source
Grade and thickness	WBP exterior-grade plywood, at least 15mm	BS 5385-3, as summarised by Stone Specialist
Screw centres	No more than 300mm, into joists and existing boards	BS 5385-3 summary; tile retailer guidance
Screw centres for heating mats	15mm plywood screwed at 200mm centres	Mat manufacturer guide
Joints	0.5 to 1.0mm gaps between sheets; stagger joints	Mat manufacturer guide; tile retailer guidance
Perimeter	2 to 3mm expansion gap, filled with silicone	Tile retailer guidance
Sealing	Seal back, face and edges, for example with SBR primer	Mat manufacturer guide; Stone Specialist
Moisture content	8 to 12% in heated homes; about 8% or lower over heated floors	BS 5385-3 summary

One manufacturer's 200mm centres for heated floors are tighter than the general 300mm tiling rule. Use the closer spacing under a heating mat.

Under tongue and groove boards with no overlay, one tile retailer specifies a highly flexible S2 adhesive. Over a plywood overlay it uses an S1 flexible adhesive. Our guide to adhesives, levelling and decoupling covers the adhesive choice.

Can I use tile backer board instead of plywood on a timber floor?

Yes, on a floor that is already stiff. One tile retailer says a tile backer board can be used instead of plywood to strengthen a timber floor, and an insulated board also cuts the heat lost into the floor void.

• Minimum thickness on floorboards: one board maker says its insulated board must be at least 10mm thick when fixed to traditional floorboards.
• Fixing on sheet floors: the same maker fixes its board with a screw and washer every 300mm, with screws at least 20mm into the floor.
• Joints: stagger them and tape them with the board maker's tape.

MaxHeatUFH stocks a cement-coated XPS tile backer board in 6, 10 and 20mm, fixed with flexible tile adhesive and insulation board washers. On floorboards, use the 10mm or 20mm board. For a thin, solid tiling base with no insulation, there is also a 6mm cementitious backer board.

Do I need insulation under electric underfloor heating on a timber floor?

It helps, especially over a cold ground-floor void. One manufacturer recommends insulation under its heating to reduce downward heat loss and warm-up time, and says even a small amount of insulation is better than none.

On a suspended ground floor, heat lost downward goes into a ventilated void and is wasted. Upstairs, it warms the ceiling of the room below, which is less wasteful but slows the floor's response. An XPS backer board does two jobs at once here: it gives a firm surface to tile onto and puts a layer of insulation directly under the cable.

The heating cable must not sit inside insulation. One manufacturer says cables must be kept clear of insulation and fully bedded in adhesive, levelling compound or screed. See insulation under underfloor heating for the wider picture.

Do I need a decoupling membrane over a timber floor?

For tiles over timber, it is the safest choice. A decoupling or anti-fracture membrane separates the tiles from movement in the floor below.

Stone Specialist's BS 5385-3 summary calls an anti-fracture membrane the safest option for tiling on timber. The MaxHeatUFH uncoupling membrane is designed for floors that move, such as timber, and suits mats and loose cable. Bed it in flexible tile adhesive, then tile on top with flexible adhesive.

BS 5385-3 also calls for a perimeter movement joint in most domestic floors, and joints where tiles meet fixed structures.

What output and floor temperature are safe on a timber floor?

Keep to 100W/m² or less, and use a floor sensor to limit the floor temperature. One manufacturer gives 100W/m² as the maximum for wooden floor constructions and a 10W/m maximum for cable on a wooden subfloor.

Limit	Figure	Source
Output on wooden floor constructions	100W/m² maximum	Manufacturer application manual and mat instructions
Loose cable output on wooden subfloor	10W/m maximum	Manufacturer application manual
Floor temperature, tiles on timber	29°C maximum	Mat manufacturer guide
Floor temperature, wood finish (parquet etc.)	27°C maximum	Mat manufacturer guide and manual

That points to the 100W/m² mat, or 10W/m loose cable spaced at 100mm or more. Avoid the 200W/m² mat and 20W/m cable on timber.

The Heatmiser neoStat-e has a floor temperature limit you can set from 20 to 45°C, with 28°C as the default. On timber, set it to the lower of the floor maker's limit and the figures above. Fit the sensor in a conduit between two heating runs, 20 to 30cm into the heated area, as one manufacturer describes.

How do I build up a timber floor for electric underfloor heating?

Stiffen, overlay, prime, lay the heating, protect it and tile. Here is the usual order over floorboards.

1. Check and stiffen. Screw down loose boards. Add noggings if the floor flexes.
2. Overlay. 15mm WBP plywood screwed at 200 to 300mm centres, or a 10mm or thicker backer board fixed to the maker's instructions.
3. Prime and groove. Prime as the adhesive maker requires. Cut a groove for the sensor conduit.
4. Lay the heating. 100W/m² mat or 10W/m cable. Cut the mesh, never the cable. Keep runs at least 50mm apart.
5. Test. One manufacturer asks for resistance and insulation tests before laying, after the adhesive is applied, and after the final floor covering.
6. Membrane and tile. Decoupling membrane in flexible adhesive, then tiles in flexible adhesive and grout.
7. Connect. A qualified electrician makes the final connection with a 30mA RCD, to BS 7671.

Wait until the adhesive and grout have fully cured before switching on. One manufacturer's installation guide then brings the floor up to temperature gradually over 48 hours.

Frequently asked questions

Can electric underfloor heating go on floorboards?

Not directly. Overlay the boards with at least 15mm WBP plywood, screwed at 300mm centres or closer, or with a tile backer board at least 10mm thick. Then lay the heating and tile over a decoupling membrane.

What thickness plywood do I need over joists or floorboards?

BS 5385-3 guidance calls for WBP exterior-grade plywood at least 15mm thick. One manufacturer's mat guide uses 15mm plywood screwed at 200mm centres with small gaps between sheets.

What is the maximum floor temperature on a timber floor?

One manufacturer's installation guide gives 29°C for tiles on a timber base and 27°C for wood floor finishes. Set the thermostat's floor limit to match.

Can I use a 150W/m² mat on a timber floor?

One manufacturer sets 100W/m² as the maximum for wooden floor constructions, so use a 100W/m² mat or 10W/m loose cable on timber.

Do I need insulation boards on a wooden floor?

They are not compulsory, but they help. An insulated backer board cuts heat lost into the floor void and gives a firm tiling base. On floorboards, use at least 10mm.

Is electric underfloor heating safe on a timber floor?

Yes, when it is installed to the maker's instructions and BS 7671, kept within the output limits, controlled by a floor-sensing thermostat and protected by a 30mA RCD.

Can I lay electric underfloor heating under laminate on floorboards?

MaxHeatUFH mats and cable are designed for tile and stone floors. For laminate and wood finishes, see our guide to heating under different floor coverings.

Sources: Stone Specialist: tiles on timber (BS 5385-3), IET Wiring Matters: floor and ceiling heating (BS 7671 Section 753)