A floor finish is the smallest part of a floor. Under a tile or a timber board there is usually a screed, sometimes insulation, sometimes a membrane, and always a decision about level and flatness. The layers are measured in different units and they are ordered at different times.
Getting this wrong is expensive in a specific way. A screed that is 10 mm too thin fails, a screed that is too thick adds weight and height, and a floor that is not flat enough makes the finish fail no matter how good the finish is.
This guide takes the layers in order, from the sub floor up, and gives the quantities for each with worked examples.
Flatness is a specification, not a hope
Before any quantity, decide what flatness the finished floor requires. It is expressed as a deviation over a length, for example 3 mm under a 2 m straightedge, and it is written into the specification because different finishes need different tolerances.
A carpet tolerates more deviation than a tile
A large format tile over a small area needs a very flat base
A timber floor over a suspended slab needs a genuinely level base, not just a smooth one
Vinyl sheet shows every imperfection
The consequence for the estimate is that a floor needing a levelling compound carries a material line for the compound and a labour line for applying it. That is often 10 to 20 percent of the floor cost, and it is invisible on a plan.
Screed: the thickness that decides everything
A screed is a levelling layer, either bonded to the base, unbonded on a membrane, or floating over insulation. The minimum thickness depends on the type and on whether it is reinforced:
Screed type
Typical minimum
|---|---|
Bonded sand and cement
25 to 30 mm
Unbonded sand and cement
50 mm
Floating over insulation
65 to 75 mm
Reinforced floating
65 mm with mesh
Flowing screed, calcium sulfate
35 to 40 mm
Proprietary fast drying
40 to 50 mm
Volume is area x thickness, plus 10 percent for shrinkage and waste. A 60 m² floor at 65 mm floating screed is 60 x 0.065 = 3.9 m³, plus 10 percent = 4.3 m³.
Screed is usually delivered as a readymix or a pumped flowing screed, which is priced by cubic metre and by pump setup. That setup charge is a fixed cost that does not scale with volume, and on small floors it dominates.
Insulation under the screed
Where the floor is over a void or the ground, insulation sits below the screed. Rigid boards are measured in square metres at the specified thickness, with 5 percent waste for cutting around penetrations.
Rigid insulation boards must be laid with staggered joints and tight together, and any gaps are thermal bridges. The material quantity for 60 m² is 63 m² of board, and the taping or foil sealing is a small but real line.
Underfloor heating, where it exists, sits between the insulation and the screed, and the screed thickness must increase to cover the pipe, typically to 65 mm or more. That change in thickness is a volume change and it is regularly forgotten when a heating system is added late.
Membranes and preparation
Vapour control layer over a ground bearing slab, measured as the floor area plus laps of 100 to 150 mm, so add 10 percent
Separating membrane under an unbonded screed, same area plus laps
Acoustic membrane in flats and upper floors, by area with edge laps up the wall
Primer for bonded screeds, at the manufacturer's coverage rate
Reinforcement mesh, where specified in a floating screed, by area with laps
Preparation is the other half: sweeping, removing old adhesive, grinding high spots. On a renovation, that can be a full day for a two person gang on 60 m², and it is a labour line that no material order reveals.
Drying time, which is a programme cost
A sand and cement screed dries at roughly 1 mm per day for the first 25 mm and progressively slower after that. A 65 mm screed needs around two months before a moisture sensitive finish goes down, or a fast drying screed and a shorter wait.
That is not a material cost. It is a programme cost, and it decides when the finishes trade can start. On a tight programme, the choice of screed type is worth more than the difference in price per cubic metre, because a fast drying screed that allows flooring in two weeks can save a month of preliminaries.
Aluminium sulfate or calcium sulfate flowing screeds behave differently again and have their own drying and heating commissioning requirements.
Floor finishes: per square metre with a waste that depends on layout
Finish
Notes on quantity
|---|---|
Carpet
By m², plus 10 percent, sold in widths that set the waste
Laminate or engineered timber
By m², plus 10 percent, plus underlay
Solid timber
By m², plus 10 to 15 percent, plus battens and clips
Tile
By m², plus 10 percent, plus 15 percent for diagonal or small rooms
Vinyl sheet
By m², plus 15 percent, and joints welded
Resin
By m² at a specified thickness, and it is a specialist trade
Carpet is the finish where roll width matters most. A 4 m wide carpet in a 3.4 m wide room wastes 0.6 m across the whole length, which is 15 percent of the roll. Two smaller rooms side by side can use that same width with no waste if the layout is planned, which is why a carpet takeoff should be done room by room with the roll width in mind.
A worked example
A ground floor of 60 m², replacing a suspended timber floor in a kitchen and a concrete floor in a hall.
Vapour control layer over the concrete area of 22 m², plus 10 percent = 24 m²
Insulation over the suspended area of 38 m², 100 mm rigid board, plus 5 percent = 40 m²
Floating screed over the concrete area only, 65 mm: 22 x 0.065 = 1.43 m³, plus 10 percent = 1.57 m³
Underlay to the whole 60 m², plus 5 percent = 63 m²
Engineered timber to 38 m², plus 10 percent = 42 m²
Tile to 22 m², plus 10 percent = 24 m², plus adhesive at 4 kg/m² and grout at 0.5 kg/m²
Skirting to the perimeter of both rooms, measured separately
Threshold strips at the junction, counted as units
That is a complete order for a floor that a rate per square metre would have priced as one line.
Skirting, trims and the junctions
Skirting is measured in linear metres of wall perimeter, less door openings, plus a mitre allowance. On a rectangular room, the perimeter is straightforward. On a room with a chimney breast, columns or a bay, measure each return.
The others that are counted or measured:
Threshold strips at every change of floor finish
Stair nosings, at one per step
Door linings and architraves, where not already in the door order
Movement joints in large tiled areas, at the specified spacing
Cove or scotia where a sealed edge is required
Productivity and the trades involved
Floors involve more trades than any other single element: groundwork, screeding, heating, flooring and decoration, and each one needs the previous one to be finished and dry.
Typical outputs, for planning rather than pricing: a screeding gang lays 100 to 150 m² of sand and cement screed a day, a flowing screed crew 200 to 300 m², a tiler 10 to 15 m² of floor tile, a timber floor fitter 25 to 40 m² of engineered board, a carpet fitter 40 to 60 m².
The programme point is that each of these is separated by a drying or curing period. Two floors that take three days of work can occupy six weeks of calendar time.
Stairs are a separate assembly
A staircase is not a floor, and it is priced on its own terms: treads, risers, strings, balustrade, spindles and handrail, plus the structural opening and the trimming joists around it.
For a new stair, count the steps and measure the balustrade. For a renovation, decide early whether the stair stays, is repaired or is replaced, because a repaired staircase that has to be protected through the whole job changes the access route for every trade after it.
Stair nosings and the finish to treads and risers are measured separately again, and on an open stair the underside is either plastered or left open, which is another decision with a price.
Common errors
Pricing the finish and forgetting the screed
Screed thickness not increased for underfloor heating pipe
No flatness allowance, so levelling compound appears as a variation
Carpet ordered by area without considering roll width
Movement joints omitted in large tiled areas
Drying time not in the programme, which delays every trade after it
Skirting and thresholds missing from the order
Suspended timber floors and the repairs nobody budgets for
A suspended floor is measured by the area of deck, and priced as if the joists are sound. On a house older than fifty years, they often are not.
Two items appear once the boards come up. Rot at the joist ends where they sit in a wall, which needs splicing or a replacement section. And notching and drilling from years of plumbing and wiring, which has weakened joists that were marginal to begin with.
Neither is visible on a survey photograph and both are common. A renovation estimate for a ground floor should carry a repair allowance, stated as an allowance, so the client understands it is a provision rather than a guess hidden in the rate.
Level thresholds and the 15 mm problem
Accessibility requirements in most markets now limit the step at an external door, usually to something around 15 mm. That single requirement decides floor build up, because the finished internal floor cannot sit higher than the external paving plus the threshold limit.
The arithmetic runs backwards from the door. If the external level is fixed, the internal build up is fixed, and the insulation plus screed plus finish must fit inside it. That is why a 100 mm insulation specification sometimes becomes 60 mm of high performance board, and why the screed type changes.
Set this out before the insulation is ordered. A floor that meets the level threshold on paper and not on site is a door that will not open, and by then the screed is down.
Frequently asked questions
How thick should a screed be? 25 to 30 mm bonded, 50 mm unbonded, 65 to 75 mm floating over insulation, and deeper over underfloor heating.
How long before I can lay flooring on a screed? Roughly 1 mm per day for the first 25 mm, so about two months for a 65 mm traditional screed. Fast drying screeds cut that substantially.
How much waste on floor tiles? 10 percent for a straight layout in a square room, 15 to 20 percent for diagonal, small rooms or large format tiles.
Do I need underlay? For carpet, yes. For engineered timber, usually a combined underlay and vapour layer. For laminate, always.
How much skirting do I need? Room perimeter less door openings, plus 10 percent for mitres and waste.
Does the floor need to be level or flat? Both, and they are different properties. Level matters for furniture and doors, flat matters for the finish. Specify the tolerance before the screed is placed.
Do the layers in FRELUX
The FRELUX flooring calculator works from the sub floor up: membrane, insulation, screed volume by thickness, then finish, adhesive, underlay, skirting and trims, priced from your market's price book.