Guide · Screed depth

How thick should floor screed be?

There is no single correct screed depth. The right thickness comes from the construction — bonded, unbonded or floating — together with the screed product, the substrate, any insulation below, underfloor heating pipework, the loading the floor has to carry and what the specification requires. This guide sets out how those decisions work, with current manufacturer figures used as examples of how much systems differ.

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Quick answer

Typical depths by build-up and system.

The figures below are current manufacturer and practice examples, published to show how much depth varies between systems. They are not universal design minima and they do not replace the data sheet for the product being installed or the project specification.

Current manufacturer and practice examples — not universal design minima.
System / build-up	Current product or practice example	Important caveat
Conventional sand and cement — bonded	Around 20–40 mm. Current ARDEX A29 guidance: minimum 20 mm at 1:5 or 25 mm at 1:7, design thickness up to about 40 mm.	Depends on a sound, properly prepared substrate and the bond being achieved. Mix and product change the minimum.
Conventional sand and cement — unbonded	Around 50 mm and above. Current ARDEX A29 guidance: minimum 50 mm.	The screed works independently of the slab, so thickness is doing structural work. Loading may push it higher.
Conventional sand and cement — floating, domestic	Around 65 mm and above in lightly loaded domestic locations (ARDEX A29).	Insulation type, compressive strength and loading are part of the design, not an afterthought.
Conventional sand and cement — floating, commercial	Around 75 mm and above (ARDEX A29 minimum for floating construction).	Higher loads or heavier traffic can require more than the stated minimum.
Rapid-drying cement screed (example: ARDEX A38)	Bonded 15–40 mm; unbonded 50 mm and above; floating 65 mm and above domestic, 75 mm and above commercial.	A proprietary product can change bonded minimums while unbonded and floating stay governed by the system design.
Flowing cementitious screed (example: Cemfloor)	Current Cemfloor figures: bonded 20 mm, unbonded 30 mm, floating 35 mm domestic, 40 mm commercial, 40 mm over underfloor heating.	Product-specific. Another flowing cementitious screed will have different figures.
Calcium sulphate flowing screed (example: Gyvlon)	Current Gyvlon figures for standard systems: fully bonded 25 mm, unbonded or mechanical contact 30 mm, floating 35 mm domestic, 40 mm commercial; 25 mm cover above pipes for standard ECO, 20 mm for Thermio+.	Product-specific, and calcium sulphate systems have their own preparation, drying and finish requirements.
Underfloor heating	Specified as a total depth plus a cover over the pipes. Current examples: Gyvlon 25 mm cover (20 mm Thermio+); ARDEX FLOWPLUS 25 mm cover with total depth from 50 mm domestic and 55 mm commercial; Cemfloor 40 mm floating minimum.	Total depth and pipe cover are different numbers and both have to be satisfied. Pipe size and pipe-top elevation decide the total.

Read those as examples, not as rules. Two flowing screeds from different manufacturers can differ by 10 mm or more in the same build-up, and a conventional screed and a proprietary rapid-drying screed can differ again.

The principle

Why there is no single screed depth.

Depth is not a property of screed in the abstract. It is the output of a design decision about how the floor is put together. A screed bonded to a sound slab is supported along its whole underside, so it can be relatively thin. Take the bond away and lay it on a separating membrane and it has to work on its own. Put compressible insulation underneath it and it has to span and distribute load without any support from the slab at all. Each step demands more thickness for the same material.

The material then changes the numbers again. Modern flowing screeds, both cementitious and calcium sulphate, are generally laid thinner than traditional sand and cement in the same build-up, and individual products differ from one another. Underfloor heating adds a second constraint on top of everything else: cover over the pipework.

BS 8204-1 is the code of practice for cementitious levelling screeds and gives recommendations on design, workmanship and inspection — but it does not set one universal minimum depth that applies to every screed. The build-up and the selected material or product determine the requirement. See BS 8204 screed standards for how the standard is actually used.

Build-up

Bonded screed depth.

A bonded screed is laid directly onto a prepared substrate and is intended to act with it. Because the slab supports the screed continuously, bonded construction allows the thinnest sections of any arrangement — provided the substrate is genuinely sound, properly prepared and, where the system requires it, treated with a bonding slurry or primer applied to the manufacturer's instructions.

Current ARDEX A29 guidance for conventional cement and sand screed gives a minimum of 20 mm bonded at a 1:5 mix or 25 mm at 1:7, with a design thickness up to around 40 mm. Rapid-drying cement screeds can differ: ARDEX A38 is currently stated as 15–40 mm bonded. Flowing systems differ again — Cemfloor currently states 20 mm bonded, Gyvlon 25 mm fully bonded, ARDEX FLOWPLUS 25 mm or more bonded.

The risk with bonded work is not the number, it is the bond. Where preparation is inadequate or the substrate is contaminated, a thin bonded screed has very little in reserve. Preparation is discussed in preparing screed before tiling and, for the screed itself, on the sand and cement screed page.

Build-up

Unbonded screed depth.

An unbonded screed is separated from the substrate by a membrane or slip layer, usually to deal with a damp-proof membrane, a questionable slab surface or movement. Because it is no longer acting with the slab, it needs enough thickness to work as a layer in its own right.

For conventional cement and sand screed, current ARDEX A29 guidance gives a minimum of 50 mm unbonded, and ARDEX A38 is stated as 50 mm and above. Flowing systems commonly sit lower: Cemfloor currently states 30 mm unbonded, and Gyvlon 30 mm unbonded or in mechanical contact. Those are not interchangeable numbers — a 30 mm unbonded depth is appropriate to specific flowing products, not to a traditional screed.

Build-up

Floating screed depth over insulation.

A floating screed sits on insulation, which is compressible. The screed has to span between points of support, distribute point and distributed loads, and resist the deflection that a compressible layer allows. That is why floating construction is generally the thickest arrangement for a given material, and why insulation compressive strength, insulation thickness and floor loading belong to the same design decision as the screed depth.

Current ARDEX A29 guidance for conventional cement and sand screed gives a minimum of 75 mm floating, or 65 mm in lightly loaded domestic locations, and ARDEX A38 is stated as 65 mm and above domestic, 75 mm and above commercial. Current flowing systems run thinner — Cemfloor 35 mm domestic and 40 mm commercial, Gyvlon 35 mm domestic and 40 mm commercial, ARDEX FLOWPLUS 35 mm and 40 mm respectively.

None of those figures transfers between systems, and none of them answers the structural question on its own. The build-up has to be designed as a system, with the insulation specification and the loading resolved alongside the screed.

Underfloor heating

Screed depth over underfloor heating.

This is where most depth confusion on site comes from, because two different numbers are both called “the depth”.

Total screed thickness is measured from the top of the layer below the screed to the finished screed surface. Cover is the depth of screed above the top of the pipe or conduit. They are not interchangeable, and satisfying one does not automatically satisfy the other. A 50 mm total depth with 16 mm pipe clipped to insulation gives very different cover from the same 50 mm with the pipe sitting higher on a rail.

Current manufacturer figures show the spread. Gyvlon states 25 mm cover above pipes for standard ECO calcium sulphate systems, and 20 mm for its Thermio+ product. ARDEX FLOWPLUS states 25 mm above pipes with a total depth from 50 mm domestic and 55 mm commercial. Cemfloor states a floating minimum of 40 mm over underfloor heating. The manufacturer's data for the specified system, together with the heating designer's requirements, governs.

Depth also affects performance, not just integrity. A deeper screed over underfloor heating holds more heat and responds more slowly; a thinner one responds faster but has less cover in reserve. See screed over underfloor heating for how we handle the sequence on site.

Depth and build-up

Depth follows the build-up.

Technical sectional diagram illustrating how screed depth is measured above insulation and underfloor heating pipework.

Total depth vs cover

Two numbers, both of which have to be satisfied.

Bonded, unbonded and floating construction place completely different demands on the same material. Bonded work is supported by the slab and can be thin; unbonded work has to act on its own; floating work spans over a compressible layer and carries load without help from the substrate.

Where underfloor heating is present, the total screed thickness and the cover above the top of the pipe are separate requirements. Pipe diameter, fixing method and pipe-top elevation determine what total depth is needed to achieve the cover the manufacturer specifies.

Every figure quoted on this page is a current manufacturer or practice example. The requirement for a particular floor comes from the specification and the data sheet for the product actually being installed.

Illustrative technical diagram · not a specific project

By material

Sand and cement screed depth.

Traditional sand and cement screed is a semi-dry material compacted in place, and it is the reference point most people have in mind when they ask how thick screed should be. Current ARDEX A29 guidance — 20 mm bonded at 1:5 or 25 mm at 1:7 up to around 40 mm design thickness, 50 mm unbonded, 75 mm floating or 65 mm lightly loaded domestic — broadly reflects conventional practice for this type of screed.

Mix, product and build-up all move those numbers. A rapid-drying cement screed such as ARDEX A38, currently stated as 15–40 mm bonded, shows that a proprietary product can change the bonded minimum while leaving unbonded and floating requirements governed by the same system logic. See sand and cement screed and fast-drying screed.

By material

Liquid and flowing screed depth.

Flowing screeds are pumped and self-level, achieving full compaction without being tamped, which is a large part of why they can run thinner than traditional screed in the same build-up. They divide into two families that behave differently: cementitious flowing screeds such as Cemfloor and ARDEX FLOWPLUS, and calcium sulphate (anhydrite) screeds such as Gyvlon.

Current published figures illustrate the point. Cemfloor: 20 mm bonded, 30 mm unbonded, 35 mm floating domestic, 40 mm floating commercial, 40 mm over underfloor heating. Gyvlon standard systems: 25 mm fully bonded, 30 mm unbonded or mechanical contact, 35 mm floating domestic, 40 mm floating commercial, 25 mm cover above pipes (20 mm for Thermio+). ARDEX FLOWPLUS: 25 mm or more bonded, 30 mm domestic and 35 mm commercial unbonded, 35 mm and 40 mm floating, with underfloor heating total depths from 50 mm and 55 mm.

Those are product figures, published by the manufacturers, and they differ from one another — which is the point. Choosing a flowing screed does not fix the depth; choosing a specific product and a specific build-up does. See liquid screed and screed drying times, since depth and drying are directly linked.

Limits

Can screed be too thick?

Yes, and it is a more common problem than people expect. Extra depth adds material cost and dead load to the structure. It extends drying time, often disproportionately, which can push the floor-covering date out well beyond the programme. Over underfloor heating it slows the heat-up and cool-down response of the system. And where the finished floor level is fixed, extra screed has to come from somewhere — usually from the insulation zone or from thresholds and door clearances that then do not work.

Proprietary systems also publish maximum pour thicknesses. Above those, the manufacturer may require a layered installation or a different design. Depth is a specification, not a safety margin to be added to.

Limits

Can screed be too thin?

Also yes. Below the minimum for the system, a screed loses the integrity it depends on: cracking, curling, hollowness and break-up under load all become more likely, and over underfloor heating insufficient cover leaves pipework vulnerable and can show as visible pipe lines in the finished floor.

The practical danger is rarely the nominal depth on the drawing — it is local thin spots caused by a slab that is higher than assumed. A floor specified at 50 mm over a slab with 20 mm of variation can end up at 30 mm in places unless the nominal depth carries an allowance for that variation, or the high spots are dealt with first. That is why a level survey, or at least an honest assessment of the slab, matters as much as the number in the specification.

Simple maths

How to calculate screed volume from area and depth.

The geometry is straightforward: area in square metres multiplied by depth in metres gives volume in cubic metres.

area (m²) × depth (m) = volume (m³) · 100 m² × 0.05 m = 5 m³

That is the volume of the space the screed occupies, before any project allowances. Real floors need more than the geometry: substrate variation, falls, upstands, bay layout and normal waste all mean the volume actually laid can exceed the calculation. Where the slab varies, the average depth achieved can be noticeably greater than the nominal design depth, and it is the average that drives materials.

For traditional sand and cement work within our standard parameters, an indicative estimate takes about a minute. Costs and what drives them are covered in the screed cost guide.

Specification

What depth should go on the drawing or specification?

A depth on its own is not a specification. These are the items that make a stated depth buildable and checkable.

Nominal design depth for the chosen screed system, stated as a depth rather than as a volume or a tonnage.
Minimum acceptable local depth, so that hollows and high spots in the substrate do not quietly produce thin areas.
Finished floor level and the datum it is measured from, with any intentional falls set out.
The full build-up below the screed: slab, membranes, insulation type and thickness, and any separating layer.
Bonded, unbonded or floating — stated explicitly, not inferred from the depth.
Underfloor heating pipe diameter, fixing method and pipe-top elevation, plus the required cover above the pipe.
Loading the finished floor has to carry, in service and during construction.
The screed system or product, or the performance requirements a product has to satisfy.
Surface regularity and finish requirements, which interact with depth and setting out.
Allowance for substrate variation, so the average depth actually laid is understood before the work is priced.

On commercial work these are the details we look for first when a drawing package arrives — see commercial screeding.

What we need

What Screeding.com needs to confirm depth.

Floor area in square metres, by room or zone where depths differ.
Substrate type and slab level survey, or at least an honest description of how flat and how high the slab is.
Screed system: traditional sand and cement, a rapid-drying cement screed, a flowing cementitious screed or calcium sulphate.
Bonded, unbonded or floating construction.
Insulation type and thickness where present.
Underfloor heating: pipe size, layout, fixing method and the level of the top of the pipe.
Finished floor level and the thickness of the final floor covering.
Loading and use of the finished floor.
Drawings, a specification or a bill of quantities where they exist.

If some of that is not settled yet, send what exists. Reviewing the build-up early is usually cheaper than resolving a depth clash after the insulation is down. Request a commercial quotation.

Related guides and services.

FAQs

Common questions.

What is the minimum depth for sand and cement screed?

There is no single figure — it depends on the build-up and the product. Current ARDEX A29 guidance for conventional cement and sand screed gives a minimum of 20 mm bonded at 1:5 or 25 mm bonded at 1:7, with a design thickness up to around 40 mm; a minimum of 50 mm unbonded; and a minimum of 75 mm floating, or 65 mm in lightly loaded domestic locations. Those are one manufacturer's current figures for a conventional screed and are a useful reference point, not a universal rule: the specification, the chosen product and the build-up decide.

How thick should screed be over underfloor heating?

Two separate numbers matter: the total screed thickness and the cover above the top of the pipe or conduit. They are not interchangeable. Current manufacturer figures show how much systems vary — Gyvlon states 25 mm cover above pipes for standard ECO calcium sulphate systems and 20 mm for its Thermio+ product; ARDEX FLOWPLUS states 25 mm above pipes with a total depth from 50 mm domestic and 55 mm commercial; Cemfloor states a floating minimum of 40 mm over underfloor heating. The manufacturer's data for the specified system, together with the project specification, sets the requirement.

Is 50 mm screed thick enough?

It depends entirely on the construction. 50 mm is a conventional unbonded minimum for cement and sand screed in current manufacturer guidance, comfortably above typical bonded minimums, and more than adequate for many flowing screed build-ups. It is below the conventional floating minimum for traditional sand and cement, and it may or may not give the required cover over underfloor heating pipework depending on pipe size and elevation. The build-up has to be checked, not the number on its own.

Can screed be 30 mm thick?

In some systems, yes. Current Cemfloor figures for its flowing cementitious screed give 20 mm bonded and 30 mm unbonded; current Gyvlon figures give 25 mm fully bonded and 30 mm unbonded or in mechanical contact. For conventional cement and sand screed, 30 mm sits within the bonded design range but is well below conventional unbonded and floating minimums. So 30 mm is workable in the right system and unsuitable in the wrong one.

What depth should floating screed be?

Floating screed sits on insulation and has to span and distribute load without support from the slab, so it is generally the thickest arrangement for a given material. Current ARDEX A29 guidance for conventional cement and sand screed gives a minimum of 75 mm, or 65 mm in lightly loaded domestic locations. Current flowing screed systems commonly run thinner — around 35 mm domestic and 40 mm commercial in Cemfloor and Gyvlon literature. Insulation type, compressive strength and floor loading are part of the design and should be resolved as a system.

How thick is liquid screed?

Flowing and self-smoothing screeds are usually laid thinner than traditional screed for the same build-up, but the figures are product-specific. Current Cemfloor guidance gives 20 mm bonded, 30 mm unbonded, 35 mm floating domestic and 40 mm floating commercial or over underfloor heating. Current Gyvlon guidance for standard calcium sulphate systems gives 25 mm fully bonded, 30 mm unbonded or mechanical contact, 35 mm floating domestic and 40 mm floating commercial. Always work from the data sheet for the product actually being installed.

Can screed be too thick?

Yes. Extra depth adds material and weight, extends drying time, slows the response of an underfloor heating system, and can create problems at thresholds and finished floor levels. Proprietary systems also have maximum pour thicknesses, above which the manufacturer may require a different approach or a layered installation. Deeper is not automatically safer — the right depth is the one the system and the specification call for.

How do I calculate screed volume from depth?

Area in square metres multiplied by depth in metres gives volume in cubic metres. For example, 100 m² at 50 mm is 100 × 0.05 = 5 m³. That is the geometric volume before any project allowances for substrate variation, falls, upstands or waste, and it assumes a consistent depth across the floor — where a slab varies, the average depth actually laid can exceed the nominal design depth.

Start the conversation

Not sure what depth your build-up needs?

Send drawings, a specification or the build-up as far as it is decided, and we will review the depth, the pipe cover and the practical consequences before pricing. For traditional sand-and-cement work, an indicative estimate takes a minute.

Get an indicative estimate Upload drawings

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