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Concrete & screed · full stack
What goes under a ground floor, and how much of it?
The whole floor sandwich in one go — hardcore, blinding, DPM, insulation, oversite and screed — off a single floor area. The layer depths are the ones the old sheet ran with; change them to suit the detail.
1
Measure it
Every box has its own unit next to it.
Change the unit beside any box and the number converts. The (i) beside each question says what the box wants and where the figure usually comes from. Anything marked per cent is a percentage of the quantity above it.
2
Check the rates
Rates and assumptions used
These are the figures the calculator works to. Change any of them and the order list updates — your merchant’s pack sizes and your own waste allowances beat the defaults every time.
How this one works
Each layer = floor area × its depth, then × density for the loose materials
Oversite and screed are proportioned by weight, the way the old sheet did it
DPM and insulation are worked in sheets and rolls, with laps added to the DPM
The figures the calculator works to. Every one can be changed in step 2, and your merchant’s pack sizes and your own waste allowances beat these defaults every time.
Rate
Default
Why
Hardcore / sand density
1.7 t/m³
The figure the old sheet used for both.
DPM roll coverage
100 m²
A 4 m × 25 m roll.
DPM laps & upstands
15 %
Laps at joints and the turn-up at the walls.
Insulation board
2.16 m²
2400 × 900. Change to 2.88 for 2400 × 1200.
Oversite ballast : cement
10 :1
The old sheet ran 10:1. A structural slab wants 6:1 or better — or ready-mix.
Screed sand : cement
8 :1
The old sheet ran 8:1 on ballast. Traditional screed is 4:1 on sharp sand.
Dry-to-wet factor
1.5
Cement bag
25 kg
A worked example
With the defaults the page opens on:
How long is the floor? 9 metres
How wide is it? 7 metres
How much hardcore? 150 millimetres
How much sand blinding? 25 millimetres
How thick is the insulation? 100 millimetres
How thick is the slab? 100 millimetres
How thick is the screed? 75 millimetres
Sub floor
Floor area: 63 m²
Hardcore: 16.06 t— 9.45 m³ × 1.7 t/m³
Sand blinding: 2.68 t — 1.58 m³
DPM: 1 rolls — +15% laps, 100 m² a roll
Insulation boards: 30 sheets— 100 mm, 2.16 m² a sheet
Total build-up depth: 450 mm — dig down at least this far below finished floor level
Oversite concrete
Volume: 6.3 m³ — 100 mm
Ballast: 14.6 t— mix 10:1
Cement: 59 bags — 1,460 kg
Or ready-mix: 7 m³ — +7.5% waste, rounded to ¼ m³
Screed
Volume: 4.72 m³ — 75 mm
Sand: 10.71 t— mix 8:1
Cement: 54 bags — 1,339 kg
Change any figure above and every line recalculates; the small print under each row is the sum that produced it.
Questions people ask
What are the layers under a ground floor?
Hardcore, sand blinding, a damp-proof membrane, insulation, the oversite concrete slab and the screed - the order this calculator works in. The layer depths default to the ones the old site sheet ran with; change them to the detail.
How deep is the whole build-up?
The calculator adds every layer and shows the total, so you know how far below finished floor level to dig. At the defaults that is 450 mm.
How many DPM rolls?
Floor area plus 15% for laps and the turn-up at the walls, divided by 100 m² a roll (a 4 m × 25 m roll). Both figures are in the rates panel.
Read before you order. These work out quantities, not structural design — spans, lintels, rebar and foundation depths need an engineer and Building Control sign-off. Waste allowances are defaults, not promises: awkward cuts, long runs and bad weather all eat more than 5%. Where a merchant sells whole packs, the figure is rounded up to whole packs.
