Floor build-up heights: getting wood, stone and carpet to meet at one level

At a glance

Millimetres, as each document states them. Source letters are listed under Standards and sources. These are guides, not standards: confirm every figure with the partner workshops and contractors before it goes on a drawing.

Work down from the finished floor level

A drawing gives each room one finished floor level (FFL), the top of the surface people walk on. Everything under it is a stack of layers, and the stack is different for every finish. Add the layers and you have the distance from FFL down to the top of the structural slab (SSL). Two rooms can share one FFL and still need two slab levels, or share one slab level and need two different stacks. Nobody can see which until the stacks are added, and by then the slab is usually poured.

The method is five steps, and it fits on one sheet of the drawing set.

• 1. Fix FFL for every room on the plans and sections, and put the datum on the drawing. All heights are taken from it, never from the slab.
• 2. List the layers under each finish, from the top: finish, bed or adhesive, base layers (plywood, levelling compound, screed), heating and insulation, any acoustic layer, then the structure.
• 3. Get a thickness for each layer in writing from the maker or contractor who supplies it, with its tolerance. Where a document gives only a minimum, record it as a minimum.
• 4. Add them. The sum is the distance from FFL to the slab. Do it for every finish that meets another.
• 5. Compare. Where two sums differ, the difference has to go somewhere, and it should be decided on the drawing: in the screed, in the insulation or acoustic layer, in a step in the slab, in a make-up layer, or in a step in the finished floor.

The drawing below shows the idea with example inputs. The stone hall needs 190 mm from FFL to slab. The timber room needs 178 mm and the carpet corridor 172 mm. With the same screed and insulation in all three rooms, the slab has to step up by 12 mm and then 6 mm, or the finished floors will not be level.

Common mistake: drawing the finishes flush on the plan and leaving each contractor to find the difference on site. In this example the stone hall would then sit 12 mm proud of the timber room and 18 mm proud of the carpet corridor.

Layer by layer: reading the numbers

The table at the top gives each figure as its document gives it, and the documents are not alike. Two are government guidance (Approved Documents E and M), three are trade-body papers (the Tile Association, Stone Federation GB and the US National Wood Flooring Association), and the rest are suppliers' and makers' own guides. None is a British Standard. The standards behind them were not read: BSI lists BS 8204-1:2003+A1:2009 (screeds), BS 8201:2011 (timber floors) and BS 5385-3:2024 (stone and tile floors) as current, and BS EN 1264-4:2021 (heating installation) as current and under review. The Tile Association paper dates from 2013 and predates that last edition, whose 2009 predecessor BSI lists as withdrawn on 28 May 2021.

Read each number as a minimum, a range or a norm. ‘Not less than 65 mm’ for a lightly loaded domestic floating screed is a floor, not a design value. ‘65 to 75 mm’ describes a type of system. ‘Typically 90 to 100 mm’ of insulation is what the same paper says a Type C system usually has, according to the Building Regulations. The stone supplier's manual says the thickness of the levelling screed under its stone typically varies from 25 mm to 75 mm depending on the type of screed chosen. That is the supplier's range for its own stone and screeds, and it is a different system from the heated floating screed with the 65 mm minimum.

A screed is not free height. It is the layer that seems easiest to thicken or thin, but a heated floating screed has a minimum, and every extra millimetre costs drying time. The Tile Association gives a sample table for unheated calcium sulfate screed in ideal drying conditions. For cement and sand it says an unconditioned 65 mm screed needs at least 80 days before tiling and a 75 mm screed 100 days, and that an uncoupling membrane may overcome these times. Our own installation guide uses the same rule of thumb as the table (a day for each millimetre to 40 mm, two days for each millimetre beyond).

Timber over heating. A UK timber floor maker's guide asks for a distribution layer of at least 6 mm between overlay panels or pipes and the timber, and says tongue-and-groove planks under 18 mm, and any patterned floor, are fully bonded to a structural layer of at least 18 mm. Our underfloor heating sheet sets out the temperatures and commissioning; the heights are the point here.

Acoustic layers. Approved Document E gives example constructions for separating floors in new buildings. Its regulation extract says a ‘room for residential purposes’ includes a room in a hotel, so hotel bedrooms are in scope as well as flats. Floor type 1 (a concrete base, ceiling and soft floor covering) relies on the soft covering to reduce impact sound at source (3.4). For new buildings with rooms for residential purposes the Document calls type 1 the most suitable, and says types 2 and 3, which have floating floors, can be used provided the floating floors and ceilings are not continuous between rooms (6.8). So a hard finish such as stone or timber over a type 1 base is a question for the acoustic consultant. Type 2 has a floating floor (3.5). Option (b) is a floating layer of 65 mm sand cement screed, or a proprietary screed with a mass of at least 80 kg/m², on a resilient layer of mineral wool at least 25 mm thick (density 36 kg/m³), or another resilient layer at least 5 mm thick with a dynamic stiffness not above 15 MN/m³ (3.65). Option (a) is a timber raft on 45 x 45 mm battens over the same 25 mm of mineral wool (3.64), and option (c) a tested system giving a weighted impact reduction of at least 29 dB (3.66). The Document says its guidance is not exhaustive and that other constructions may be used. So option (b) is at least 90 mm deep under the finish and its bed, before any heating is considered. Our acoustics for wood floors sheet covers the sound side.

The base has a tolerance too. Stone Federation GB says the stone installer should not be expected to overcome variations in the background that are greater than the tolerance the stone itself must meet, and that the contractor signs off the preceding trades. Where the base is out, the stone floor is out. See subfloor requirements for the flatness tests.

A worked example, checked

Three rooms share a flat slab and a finished floor level. All three have a heated floating screed on insulation. The stone hall has 20 mm stone on a bed and uncoupling membrane of up to 10 mm (the Tile Association's limit for bedding with a membrane). The timber room has a 15 mm engineered board and 3 mm of adhesive and primer. The corridor has a 9 mm carpet tile and 3 mm of adhesive. The screed is 70 mm and the insulation 90 mm throughout. Every figure is inside a range or at a value in the table above, except the two 3 mm adhesive allowances, which are example inputs (our installation guide's trowel size for planks; the adhesive maker states the real figure). The sums are checked by script.

If the base layers are the same and the finishes differ

The three finished floors are not level: the stone hall's surface is 12 mm above the timber room's and 18 mm above the corridor's, unless the slab steps. For an accessible entrance, Approved Document M volume 2 asks for a level threshold or a raised one of not more than 15 mm in total (2.7(e)), so 18 mm would be over that figure if this were an entrance. Inside the building it gives no figure, but asks that changes of floor material do not create a trip hazard (2.7(h)).

If the slab stays flat and the screed absorbs the difference

On a flat slab 190 mm below FFL, the timber room needs an 82 mm screed and the corridor an 88 mm one. Both are above the 65 to 75 mm range the Tile Association describes, and the document gives no drying time above 75 mm. Raising the insulation in the timber room to 100 mm brings its screed back to 72 mm, but only the heating engineer can say whether that suits the thermal design, and the corridor would still need 78 mm. The honest options are a stepped slab, a deeper recess for the thin finishes, or a different heating build-up in those rooms. That is a decision for the structural and heating engineers, taken before the slab is cast.

Where the levels meet

Approved Document M. Volume 2 (England, buildings other than dwellings) says a level threshold is preferred, especially for doors in frequent use (2.6). An accessible entrance has a level threshold or, if a raised one is unavoidable, a total height of not more than 15 mm, a minimum number of upstands and slopes, and any upstand higher than 5 mm chamfered or rounded (2.7(e)). Floor surfaces next to the threshold do not impede wheelchairs, and changes in floor material do not create a potential trip hazard (2.7(h)). The ramp limits in Table 1 (for example 1:12 and a rise of 166 mm for a 2 m going) are for ramped access, not for a taper at a doorway. We found no figure for an ordinary internal threshold. Our floor transitions sheet gives the full table of paragraphs and the junction details.

Where else FFL shows.

• Stairs. A stair is set out from FFL at the bottom and at the top, so an unequal build-up at either end changes the first and last risers. See stair rise, going and headroom.
• Doors. The gap under a door leaf is taken from FFL, and a thicker finish, a rug or a carpet changes it. See door sizes and tolerances. A threshold under a fire-rated door also needs the door maker's say.
• Skirting and joinery. Fitted joinery is scribed to the finished floor, so say which floor: skirting and architrave heights.
• Survey. Take the levels of the existing slab before the stacks are fixed: site survey and dimension verification.

A stack-up sheet for the drawing set

A one-page stack-up sheet for the drawing set. Copy the table, fill one column for each finish that meets another, and have each layer's thickness confirmed in writing by the person who supplies it.

• Is the difference between any two totals shown on the drawing, with who absorbs it?
• Is every layer's thickness a stated figure with a tolerance, or only a minimum?
• Does any screed fall outside its minimum, or does the programme allow for its drying time?
• Is any acoustic layer under the finish included in the stack?
• Is every threshold either flush or within the limit that applies, and is the limit confirmed?
• Are stairs, doors, skirting and joinery set from the same FFL?

Finished floor levels (FFL) are as shown on drawing [ref]. All floor heights are measured from FFL. Where two floor finishes meet, their finished surfaces are flush at FFL [ ] unless the drawing shows a step. Each finish has a build-up drawing [ref] stating every layer and its thickness; the total from FFL to the top of the structural slab (SSL [level]) is [ ] mm for [finish A] and [ ] mm for [finish B]. [Contractor] checks the totals against the slab levels before the base is laid and reports in writing any difference over [3] mm before work starts.

[Finish]: [stone, [20] mm, schedule ref] bedded on [adhesive and uncoupling membrane, total [ ] mm, by the stone fixer], on [floating screed, [ ] mm, not less than [65] mm, by the screed contractor], on [insulation, [ ] mm to the heating engineer's design], on [structural slab]. Thicknesses are as installed, with a tolerance of [ ] mm, and each is confirmed in writing by the party that supplies it.

Where finishes differ in total height, the difference is taken up by [screed thickness / insulation thickness / a step in the slab of [ ] mm / a make-up layer of [ ] mm] as shown on drawing [ref], and by no other means. No layer under a finish is added to, thinned or omitted on site without the designer's written approval.

Where Approved Document M volume 2 applies and the door is an accessible entrance, the threshold is level or, if a raised threshold is unavoidable, not more than 15 mm in total height, with any upstand over 5 mm chamfered or rounded (paragraph 2.7(e)) [confirm current edition]. Elsewhere finished surfaces meet flush; any change of level is shown on the drawing and approved before the floor is laid. Where the floor is a separating floor under Approved Document E, its construction is [floating floor (b) of paragraph 3.65 / a tested system, report [ref]] as the acoustic consultant specifies, and its full thickness is included in the build-up on drawing [ref].

Standards and sources

• A. The Tile Association, Tiling to Heated Floors (2013, file updated 2014): read in full; screed, insulation, bedding, drying, Table 4.
• B. A UK stone supplier's technical manual, internal flooring: read; stone and screed figures.
• C. Approved Document M volume 2 (2015 ed. incl. 2020 and 2024 amendments, England): read; 2.6, 2.7, Table 1.
• D. Approved Document E (reprint Oct 2015, England): read; 3.64 to 3.66.
• E, F, G, H. A UK distributor's, a timber maker's and a parquet supplier's guides; NWFA B300 (2015); a retailer's article; our live pages: read; named in the notes file.
• Stone Federation GB, Inspection and snagging (Aug 2022): read; substrate tolerances.
• BS 8204-1:2003+A1:2009, BS 8201:2011, BS 5385-3:2024, BS EN 1264-4:2021: BSI listings read, all current; texts not read. Confirm the current editions.

Questions we are asked

What is finished floor level (FFL)?

The height of the finished walking surface, shown on the drawings so that different finishes meet at one level. Floor heights are measured down from it, and the distance from FFL to the structural slab is the sum of every layer in the build-up.

How thick is a floor build-up?

It depends on the finish and the services. In our worked example a stone floor on a heated floating screed is 190 mm from finished floor to slab, a timber floor on the same screed 178 mm, and a timber floor on plywood over existing boards only 24 mm above the boards. The Approved Document E floating floor alone is at least 90 mm under its finish. Every layer comes from a maker or contractor, in writing.

Why does a stone floor need more height than timber?

The finish and its bed are deeper. One stone supplier recommends 20 mm stone for sizes up to 600 x 600 mm, and the Tile Association says bedding over an uncoupling membrane is not generally above 10 mm, so 30 mm against 18 mm for a 15 mm board with 3 mm of adhesive in our example. The screed and insulation can be the same under both.

Can I change the screed thickness to level two floors?

Only within limits. A heated floating screed has a minimum (the Tile Association says not less than 65 mm for lightly loaded domestic work) and every extra millimetre adds drying time. Insulation and acoustic layers follow their own design. A stepped slab or a different build-up may be the better answer, and the structural and heating engineers decide.

How big a step is allowed between two floors?

Approved Document M volume 2 (England) asks for a level threshold at an accessible entrance or, if a raised threshold is unavoidable, not more than 15 mm in total, with any upstand over 5 mm chamfered or rounded. It gives no figure for the inside of a building, but asks that changes of floor material do not create a trip hazard. Aim for flush, and ask the accessibility consultant what applies.

Does an acoustic layer change the build-up?

It can, where Approved Document E applies to a separating floor, for example in a new flat or a hotel bedroom. Floor type 1 relies on a soft floor covering; type 2 has a floating floor, and its option (b) is a 65 mm screed on at least 25 mm of mineral wool, so at least 90 mm under the finish and its bed. The Document says other constructions may be used, and the acoustic consultant decides which.

Whose job is it to coordinate the heights?

The designer draws the stacks and the datum, each supplier confirms its layer, and the engineers confirm the slab, insulation and acoustic layers. Stone Federation GB says the stone installer should not be expected to overcome variations in the base beyond the tolerance of the finished stone, and that the contractor signs off the preceding work.