What is a floor screed?
A screed is a layer of sand and cement laid over a concrete slab to give a level, smooth surface for tile, wood, vinyl or carpet. In the US it is often called a mortar bed or “floor mud”, and it is also the classic base for shower floors and underfloor heating. It is mixed with little water, to a damp, crumbly consistency, then compacted and leveled with a straightedge and a float.
How the quantities are worked out
- Wet volume = area × thickness, plus an allowance for waste and uneven floors
- Dry volume = wet volume × 1.3, because the air between dry grains disappears when the mix is wetted and compacted
- Cement volume = dry volume ÷ (1 + sand parts); sand volume is the rest
- Cement weight at about 1,440 kg/m³ (90 lb/ft³), sand at about 1,600 kg/m³ (100 lb/ft³)
- Bags = cement weight ÷ bag weight, rounded up
Example
A 200 sq ft floor with a 2-inch screed, a 1:4 mix and 10% extra:
- Volume: about 1.36 cubic yards
- Cement: about 857 lb, or 10 bags of 94 lb
- Sharp sand: about 3,800 lb, close to 2 tons
Mix ratios
| Ratio | Use |
|---|---|
| 1:3 | Thin or heavy-duty screeds, shower pans |
| 1:4 | Standard floor screed |
| 1:5 | Thick beds where strength matters less |
| 1:6 | Lean fill |
Use clean, washed sharp sand rather than fine building sand, which shrinks and cracks more.
Thickness
- Bonded to the slab with a bonding agent: from about 1 inch (25 mm)
- Unbonded, on a plastic membrane: at least 2 inches (50 mm)
- Floating, on insulation: at least 2.5 to 3 inches (65 to 75 mm)
- Over radiant heating pipes: usually at least 1.5 to 2 inches above the pipes
Drying
A screed must dry before most floor coverings go down. A common rule is about 1 day per millimeter of thickness for the first 40 mm, then slower, so a 2-inch screed needs around two months in good conditions. Ventilation and moderate heat speed it up; covering it too early traps moisture that can warp wood or lift vinyl. Test the moisture with a meter, or with the method recommended by the flooring maker, before laying.