How do I calculate how much screed I need?
Screed volume is floor area multiplied by depth, with the depth in metres. Then check the depth against the specification, allow for the slab not being level, add waste, and order by volume.
Area x depth
Volume (m3) = area (m2) x depth (m). Convert millimetres to metres first: 65 mm is 0.065 m.
Worked example. A room 5.2 m x 3.8 m is 19.76 m2. At 65 mm the volume is 19.76 x 0.065 = 1.284 m3. For a whole floor, add up the areas of each room and passage, and keep the depth for each area, as it may differ.
Where the depth comes from
The depth is set by the specification, not by a rule of thumb. It depends on the type of screed, what lies beneath it (insulation, a damp-proof layer, underfloor heating pipes) and the floor finish. Take it from the drawings and specification, and if the screed manufacturer or supplier gives a minimum for the product, that figure governs. If no depth is stated, ask before you price.
Allowing for levels that vary
A slab is rarely dead level, and the screed has to bring the floor to the finished level, or to its falls. Measure before you order:
- take slab levels on a grid across each room, against the datum the finished floor level is set from
- subtract each slab level from the finished floor level to get the depth at each point
- find the deepest point, and the thinnest, which must still meet the specified depth
- where the floor falls to a drain or wet-room outlet, measure the fall as well
Worked example. A 24 m2 floor has a finished level of 90 mm above datum, and nine slab readings of 12, 18, 25, 10, 16, 22, 8, 14 and 20 mm. The depths are 78, 72, 65, 80, 74, 68, 82, 76 and 70 mm. The average is 73.9 mm, the thinnest 65 mm and the deepest 82 mm. Volume at the average depth is 24 x 0.0739 = 1.77 m3. If the thinnest point is below the specified depth, the whole floor has to be raised and the average rises with it.
Waste and how much to order
Waste is a judgement call. It depends on access, how level the base is, whether the screed is pumped or barrowed, and how the batches divide up. Decide a percentage, then round up to the size of delivery your supplier offers.
Worked example. Allowing 5 per cent on 1.77 m3 gives 1.86 m3, so ask for 1.9 m3. Ready-mixed screed is ordered by volume, but ask the supplier about minimum loads and how long a load stays workable, so the crew and the access are ready when it arrives.
Sand and cement by volume
If you are mixing on site, the mix ratio comes from the specification. The dry sand and cement take up more volume than the laid screed, so ask your supplier what yield to assume rather than guessing.
For the sum only, say the specification is 1 part cement to 4 parts sand by volume, and the supplier says to allow 1.3 m3 of dry materials per m3 of laid screed (a made-up figure for this example). For 1.86 m3 of screed that is 1.86 x 1.3 = 2.42 m3 dry, which splits into one fifth cement (0.48 m3) and four fifths sand (1.93 m3, rounded). Convert to bags and tonnes using your supplier's bag sizes and densities.
Drying time
Drying time is set by the screed manufacturer for that product and depth. Do not use a general rule of thumb. The floor finish manufacturer may also require a moisture test before they will accept the screed, so check both sets of instructions and put the drying time in the programme.