Underfloor heating in joisted timber floors: spreader plates vs routed chipboard

October 22, 2026

Short answer: In a joisted timber floor you either replace the deck with 22mm routed chipboard and a 6mm foil-backed top board, or fit aluminium spreader plates over or under the joists and keep a normal deck. Routed chipboard keeps the pipe at fixed 150mm centres and adds no height beyond the 6mm top board. Spreader plates are cheaper per m² but usually need a warmer flow, around 45 to 55°C in one maker's guidance, for about 80W/m² at most.

Timber floors have no screed to store and spread heat, so the job of spreading heat falls to aluminium: either the plate the pipe sits in, or the foil on the board above it. Which method suits you depends on whether the floor is coming up, how much height you can add and whether you can reach the joists from below.

This guide compares the methods for upstairs rooms and suspended timber ground floors. For a wider overview of all floor types, see underfloor heating floor systems explained.

What are the options for underfloor heating in a timber floor?

There are three water options for a joisted floor, plus overlay boards on top of an existing deck. Each one puts the pipe in a different place.

Method	Where the pipe sits	Floor must come up?
Routed chipboard	Grooves in a 22mm deck	Yes, it replaces the deck
Spreader plates from above	Plates over the joists, under the deck	Yes
Fit-from-below plates	Plates fixed to the deck underside	No, needs access below
Overlay boards	On top of the existing deck	No, adds height

Overlay boards are covered in low-profile overlay boards. The rest of this guide covers the three methods that work inside the joist zone.

How does routed chipboard underfloor heating work?

Routed chipboard is a structural floor deck with the pipe grooves already cut. You screw it to the joists in place of ordinary chipboard, press the pipe into the grooves and screw a foil-backed top board over it.

MaxHeatChip is 22mm P5 moisture-resistant chipboard with 4 grooves per board at 150mm centres and radius returns at both ends, so the pipe turns back without extra cutting.
It comes for 12mm or 16mm pipe, in 1200 x 600mm (0.72m²) or 2400 x 600mm (1.44m²) boards.
The 6mm fibre top board has a 150 micron aluminium foil backing. Lay it foil side down, facing the pipe, and stagger its joints away from the chipboard joints.
Fix the chipboard to the joists with 5 x 80mm screws and the top board to the chipboard with 4 x 20mm screws, keeping well away from the pipe runs.

The finished deck is 22mm + 6mm = 28mm. Where an existing 22mm chipboard floor comes up, the new floor sits about 6mm higher. Check that your joist centres and spans suit a 22mm deck with your structural engineer or building control before you start.

How do spreader plates work in a joisted floor?

Spreader plates are thin aluminium sheets with a channel that grips the pipe. The aluminium spreads heat sideways across the underside of the floor deck, so the floor warms evenly instead of in stripes over each pipe.

Insulate between the joists. One spreader plate maker recommends 50mm rigid board or 150mm mineral wool, set so there is room for the pipe above it.
Make room for the pipe. Either notch the joists where the pipe crosses, within the rules below, or fix battens across the joists.
Lay the plates across the joists with a small gap between plates for expansion. The same maker leaves 15mm.
Leave space to turn the pipe. The same maker leaves 250mm between the plate ends and the wall for the loop to turn and return.
Clip in the pipe, pressure test, then lay the new deck on top.

Our aluminium heat diffuser plates come in single, double and triple channel types. The single channel plate is 1000 x 140mm. Choose the type by how many pipe runs fall between each pair of joists. Multilayer pipe such as PERT-AL-PERT suits plates because it holds its shape in the channel.

Can I fit underfloor heating from below without lifting the floor?

Yes, if you can reach the underside of the joists from a cellar, an open ceiling below or a void. Fit-from-below plates fix to the underside of the floor deck between the joists, and the floorboards stay down.

Our fit-from-below plate is 150mm wide, aluminium, with a double channel for two pipe runs.
The pipe must still pass through the joists to get from bay to bay. Drill holes within the rules below, or run the pipe below the joists at the ends.
Fit insulation under the plates, so the heat goes up through the deck and not down into the room or void below.
Seal any ceiling you open up with the fire and sound performance it had before, as your building control body requires.

Can I notch or drill joists for underfloor heating pipe?

Yes, within limits. Local authority building control guidance for solid timber joists sets the maximum size and position of notches and holes. It does not apply to engineered joists such as I-joists, where you must follow the maker's instructions.

Rule	Notch (top or bottom)	Hole
Maximum size	0.125 x joist depth, max 35mm	0.25 x joist depth, max 65mm
Position along span	0.07 to 0.25 of span from support	0.25 to 0.4 of span from support
Other limits	Top or bottom edge, not both	On the centre line, 3 diameters apart

Worked example. A 200mm deep joist spanning 4m can take a notch up to 25mm deep (200 x 0.125), cut between 280mm and 1m from each support (4000 x 0.07 and 4000 x 0.25). That is enough for 16mm pipe. If the pipe has to cross outside those zones, use battens instead of notches.

What flow temperature and output can a timber floor give?

A timber floor gives less heat than screed at the same water temperature, because the deck and the air gaps resist heat flow. One spreader plate maker quotes about 80W/m² at most, with a recommended flow of 45 to 55°C.

The MCS Heat Emitter Guide shows the same pattern for heat pumps: its example of an aluminium panel floor with tiles needs 45°C to reach a 4 star rating, where a screed floor reaches 6 stars at 35°C. For a heat pump, that means:

Check each timber room's heat loss against the output at your design flow temperature. See will underfloor heating heat my room?
Choose a thin, conductive finish. Thick carpet on a timber system cuts output further.
Upgrade insulation and draught-proofing in high heat loss rooms before you rely on UFH alone.

A timber upper floor can run from the same heat source as a screed ground floor, but design each floor for its own build-up. Our guide to underfloor heating with a heat pump explains flow temperatures in more detail.

How much insulation does a timber floor need?

That depends on what is below. Over a heated room, one heating maker's summary of BS EN 1264-4 gives a minimum thermal resistance of 0.75m²K/W under the pipe. Over unheated space or the ground it is 1.25m²K/W, and over outside air it is 2.0m²K/W.

Upstairs over a heated room: insulation mainly stops heat going down into the ceiling below and helps with sound.
Suspended timber ground floor: Part L in England sets a U-value of 0.18W/m²K for a new floor in an extension and 0.25W/m²K for a renovated floor. Keep air bricks clear so the void stays ventilated.

Our guide to insulation under underfloor heating shows how to turn these figures into board thicknesses.

Routed chipboard or spreader plates: which should I choose?

Choose routed chipboard when the deck is coming up anyway and you want fixed, accurate pipe centres. Choose spreader plates when you want to keep a standard deck or the joist spacing suits plates. Choose fit-from-below when the floor finish must stay.

Situation	Best method	Why
New upper floor or full renovation	Routed chipboard	Deck and pipe guide in one board
Floor up, keep existing deck type	Spreader plates from above	Lower material cost per m²
Floor finish must stay down	Fit-from-below plates	Work from the void or ceiling
Little room under, height to spare	Overlay boards	No work in the joists

Worked example: a 12m² bedroom in routed chipboard. 16mm pipe at the board's 150mm centres gives 12 ÷ 0.15 = 80m in the floor. Add about 10m of tails for one loop of about 90m. Boards: 12 ÷ 1.44 = 8.3, so 9 MaxHeatChip-L-16 boards. Top boards: 12 ÷ 0.72 = 16.7, so 17.

Item	Qty	Price inc VAT
MaxHeatChip 16mm, 2400 x 600mm	9	£612
6mm foil-backed top board	17	£272
PERT-AL-PERT 16mm x 100m	1	£71
Screws 5 x 80mm (100) and 4 x 20mm (200)	1 each	£7.58

Prices October 2026. Add one manifold port, an actuator and a thermostat for the room.

Products for this job

Design your timber floor free with MaxHeatUFH ProLoop. Add your rooms, choose a joisted floor, and get a loop design, board count and full kit list with prices. Start your design

Frequently asked questions

Can you put wet underfloor heating upstairs?

Yes. Routed chipboard or spreader plates fit within the joist depth, so upstairs floor levels barely change. The upstairs loops usually run from their own manifold or from spare ports on the main one.

Is routed chipboard strong enough to use as the floor deck?

MaxHeatChip is 22mm P5 chipboard made to screw to the joists as the deck. Confirm your joist centres and spans with your engineer or building control, as you would for any chipboard floor.

Do spreader plates need a top board?

Spreader plates sit under the deck, so the deck itself goes on top. With routed chipboard, the 6mm foil-backed top board goes over the pipe to spread heat and give a flat base for the finish.

Can I lay engineered wood over a timber UFH floor?

Yes, if the flooring maker approves it for UFH. Use a floor probe to limit the surface temperature, often 27°C for wood. One spreader plate maker limits floorboards plus wood finish to 25mm in total. See UFH under tiles, wood, LVT and carpet.

Which pipe is best for spreader plates?

Multilayer PERT-AL-PERT pipe holds its shape when bent, so it sits neatly in the plate channels and doesn't spring out at the returns.

Will timber UFH work with a heat pump?

It can, but expect a higher flow temperature than screed for the same output. Check each room's heat loss against the output at your heat pump's design flow temperature.