Underfloor heating floor build-up heights: screed vs flowing screed vs overlay

Height is often the deciding factor when you choose an underfloor heating (UFH) system. In a new build or extension you can design the floor depth from the start. In an existing room every millimetre counts, because the new floor has to meet door thresholds, kitchen plinths, stairs and the floor in the next room.

This guide compares the build-up heights of each system, from the thinnest to the deepest, with the minimum screed depths from the standards and screed makers and the insulation that Building Regulations call for. For how each system works, see floor systems explained.

How much height does underfloor heating add?

From almost nothing to over 150mm, depending on the system. Here is each one, thinnest first. Heights are for the heating layer only, before the floor finish.

System	Heating layer	Typical use
Electric mat in tile adhesive	About 3 to 4mm	Bathrooms, kitchens, small tiled rooms
Spreader plates between joists	None above the joists	Timber floors with access from above or below
Overlay board, 12mm pipe	15 to 18mm	Retrofits on existing solid floors
Overlay board, 16mm pipe	20 to 25mm	Retrofits with a little more height
Routed chipboard on joists	22mm board (replaces the deck)	Timber floors, new or lifted
Flowing screed over insulation	Insulation plus about 35 to 50mm	New builds, extensions, dug-out floors
Sand and cement screed over insulation	Insulation plus about 65mm (domestic)	New builds, extensions, dug-out floors

The overlay heights are our own boards: MaxHeatFix at 15mm, MaxHeatPro at 16 or 20mm, MaxHeatDeck at 20mm and MaxHeatPanel at 18 or 25mm. Our electric mats are about 3 to 4mm thick. MaxHeatChip routed chipboard is 22mm and takes a 6mm top board over the pipes.

How thick does a screed over underfloor heating need to be?

About 65mm for sand and cement in a home, and about 35 to 50mm for a flowing screed. The exact minimum depends on the screed type, how it is laid and the loads it carries.

Screed	Minimum or typical depth	Source
Floating sand and cement, general	75mm minimum	MPA Mortar data sheet (BS 8204)
Floating sand and cement, domestic	65mm normally satisfactory	MPA Mortar, TTA
Flowing cement screed, floating, domestic	35mm minimum, 30mm over pipes	Screed maker data sheet
Flowing cement screed, unbonded	Nominal 50mm, 30mm over pipes	Screed maker data sheet
Panel system, 2.0kN/m² load	Cement 45mm, anhydrite 35mm	Manufacturer data sheet
Panel system, 5.0kN/m² load	Cement 75mm, anhydrite 65mm	Manufacturer data sheet

One UFH maker's data sheet asks for 35mm of calcium sulphate or liquid cementitious screed over the pipes. Always use the figure from your screed supplier, because it sets the warranty.

Depth also sets drying time. One screed products maker gives about 1mm a day for the first 40mm of a calcium sulphate screed, and 0.5mm a day after that. The TTA says an unconditioned 65mm sand and cement screed needs at least 80 days to dry, and 75mm needs 100 days. Our guide to screed depths and drying times has more.

How much insulation goes under underfloor heating?

Enough to meet Building Regulations for the floor, which is usually far more than UFH needs on its own. For a new floor in England, Approved Document L sets a limiting U-value of 0.18W/m²K. For a renovated or retained floor, it is 0.25W/m²K.

East Suffolk Council's Building Control guidance shows what that means in practice. Its ground floor examples reach 0.18W/m²K with 100mm of PIR board, based on a typical floor shape, and it notes that 150mm of PIR is often specified to offset extra glazing. The thickness you need depends on the floor's size and shape, so get the U-value calculated for your project.

BS EN 1264 also sets a minimum insulation under the pipes, so the heat goes up rather than down:

What is below the heated floor	Minimum insulation (m²K/W)
A heated room	0.75
Ground, or an unheated room	1.25
Outside air, for example over a garage	2.00

On a ground floor, the Part L figure is the one that sets the thickness. Our guide to insulation under UFH explains both.

What does a full floor build-up look like?

Add up every layer from the slab or joists to the top of the finish. These examples use the figures above. Your own insulation thickness will come from your U-value calculation.

Build-up	Layers	Total before finish
New ground floor, sand and cement	100mm PIR + 65mm screed	165mm
New ground floor, flowing screed	100mm PIR + 50mm screed	150mm
Same, with 150mm PIR	150mm PIR + 50mm screed	200mm
Overlay on an existing slab	18mm MaxHeatPanel	18mm
Electric mat on an insulated backer	10mm backer board + mat in adhesive	About 14mm

Then add the finish. As an example, a 10mm tile on a 5mm adhesive bed adds 15mm, so the 18mm overlay floor ends up about 33mm above the old slab. A 20mm engineered board adds 20mm, plus any underlay.

One manufacturer publishes full structural heights for its panel system, measured from the slab to the top of the screed. They range from 97mm (30mm of insulation, cement screed, light load, over a heated room) to 172mm (75mm of insulation, cement screed, heavier load, over outside air). On a ground floor that must meet 0.18W/m²K you will usually need more insulation than those examples include.

Should I use screed, flowing screed or overlay?

Use screed where you are building or digging out the floor, and overlay where you cannot lose the height. Height is not the only difference.

• Sand and cement screed: the deepest, with the most thermal mass. It is the slowest to dry and to warm up, and the steadiest once warm.
• Flowing screed: thinner, and poured so it flows round the pipes. It still needs careful drying and, for calcium sulphate, laitance removal before tiling.
• Castellated panels: hold the pipe in raised bosses under a screed or levelling compound, with no clips. See our castellated panels.
• Overlay boards: the thinnest water system, laid over the existing floor with no drying time. Less mass, so faster to respond, but they rely on the room being reasonably well insulated. See low-profile overlay UFH.
• Joisted systems: spreader plates add no height above the joists, and routed chipboard replaces the deck. See UFH in joisted timber floors.
• Electric mats: the thinnest of all, for tiled rooms. Our mats are made for tile and stone floors.

How do I keep the build-up as low as possible?

Choose the thinnest system that suits the room, and take out what you can before you start.

1. Measure first. Check the height you can gain at every door, the kitchen plinths, the stair nosing and the floor in the next room.
2. Lift the old finish. Taking up 10mm of old tile and adhesive gives you 10mm back.
3. Use 12mm pipe in overlay. It allows boards of 15 to 18mm, against 20 to 25mm for 16mm pipe. Loops must be shorter, so you may need more manifold ports.
4. Use flowing screed instead of sand and cement. 35 to 50mm instead of about 65mm, if the screed maker allows it for your floor.
5. Dig out if you need performance. If the room loses a lot of heat and you want a screeded floor with full insulation, lowering the slab may be the only way to get both.

Frequently asked questions

What is the thinnest underfloor heating?

An electric mat laid in the tile adhesive, at about 3 to 4mm. The thinnest water system is an overlay board, from 15mm.

How much height does overlay underfloor heating add?

The board adds 15 to 25mm, plus the adhesive and floor finish. A tiled overlay floor is often about 30 to 40mm higher than before.

What is the minimum screed depth over underfloor heating?

For sand and cement in a home, about 65mm. For flowing screed, one maker gives 35mm minimum floating, with at least 30mm over the pipes. Follow your screed maker's figure.

How deep is a typical underfloor heating floor build-up in an extension?

Often about 150 to 170mm before the floor finish, with about 100mm of insulation and 50 to 65mm of screed. More insulation means a deeper floor.

Can I put underfloor heating on an existing concrete floor without raising it much?

Yes, with overlay boards bonded to the slab, or with an electric mat under tiles. Check the existing slab is sound, flat and dry first.

Does underfloor heating need insulation under it?

Yes. BS EN 1264 sets a minimum of 1.25m²K/W on the ground, and Building Regulations usually ask for more on a new floor.

Does a thicker screed give more heat?

No. It gives more thermal mass, so the floor warms more slowly and stays warm longer. The heat output depends mainly on the water temperature, pipe centres and floor finish.

Sources: MPA Mortar Data Sheet 22 (screeds), The Tile Association: tiling to heated floors, East Suffolk Council: extensions and alterations (Part L U-values)