How the screed calculator works
A traditional floor screed is a semi-dry mix of sharp sand and cement that is compacted and ruled off by hand. Because it contains very little water, it's best estimated by the weight of dry materials. A compacted semi-dry screed weighs around 2,100kg per m³, which is split between sand and cement according to your mix ratio.
For example, 20m² at 65mm is 1.3m³. With 10% wastage that's about 3,003kg of dry materials. At 4:1, you'll need 601kg of cement (25 bags) and 2.4 tonnes of sharp sand (3 bulk bags).
Recommended screed depths
| Screed type | Typical depth | Notes |
|---|---|---|
| Bonded | 25–40mm | Bonded to a primed concrete slab with a slurry coat |
| Unbonded | 50mm min. | Laid on a damp-proof membrane |
| Floating | 65mm min. | Over rigid insulation boards |
| Underfloor heating | 65–75mm | At least 25mm cover over the pipes |
Drying time
Screeds dry at roughly 1mm per day for the first 40mm, then about 0.5mm per day for any depth above that, in a well-ventilated room at around 20°C. The calculator shows this as an estimate. Before laying wood, LVT or vinyl, test the moisture with a hygrometer: most manufacturers require 75% relative humidity or lower. Underfloor heating can be commissioned gradually after about 21 days to help it dry.
Tips for a strong screed
- Mix to a "damp sand" consistency: a squeezed ball should hold its shape without water running out.
- Use screeding rails or depth battens and rule off with a long straight edge.
- Compact thoroughly, especially at edges and around pipes. Poor compaction is the main cause of hollow, crumbly screeds.
- Add polypropylene fibres to reduce shrinkage cracking, and fit perimeter edge strip on floating and heated screeds.
- Cure slowly: cover with polythene for the first 7 days and keep away from direct heat.
Worked example: a 5m × 4m extension floor over insulation
A floating screed over insulation boards should be at least 65mm thick. 20m² × 65mm = 1.3m³, about 3 tonnes of dry mix with wastage: 25 bags of cement and 3 bulk bags of sharp sand at 4:1. That's a lot of mixing for one day. A forced-action (pan) mixer and two or three people make it manageable. Allow roughly 90 days before laying wood or vinyl, unless underfloor heating is used to help it dry. Choose the "75mm UFH" preset if pipes run in the screed.
Traditional screed, liquid screed or levelling compound?
- Sand and cement (semi-dry): cheap materials, easy to buy in bags, and fine for most rooms. It needs skill to get flat and is slow to dry.
- Liquid (flowing) screed: pumped in by a specialist, self-levels, and suits underfloor heating well because it flows round the pipes. Some types (anhydrite) need their surface sanded and must be dry before tiles or adhesive go on. Ask the installer for the drying advice for their product.
- Self-levelling compound: for thin layers over a sound base, usually under 10–20mm. Use the self-levelling compound calculator.
Common screeding mistakes
- Too wet a mix. Extra water makes it easier to lay but shrinks and cracks as it dries. Keep it at damp-sand consistency.
- No edge strip on floating screeds. Without room to move at the walls, the screed cracks as it shrinks.
- Laying floors too soon. Moisture trapped under wood, LVT or vinyl causes swelling, smells and failed adhesive. Test before laying, don't guess.
- Thin patches over pipes or conduits. Keep the minimum depth everywhere, not just on average.
Once the screed is dry, plan the finish with the flooring or tile calculator. For thicker structural slabs, use the concrete calculator.