Floor Screed: How Much Sand, Cement or Ready-Mix Do I Need?
A 4 × 3 m floor screeded 65 mm thick needs 0.86 m³ of screed (with 10% extra). Mixed 4.5 : 1 that takes 12 bags of cement and 1.46 tonnes of sharp sand, or about 69 bags of ready-mixed dry screed. The charts cover other rooms and depths.
The materials list
| What to buy | How much | How we worked it out |
|---|---|---|
| Cement | 12 bags of 25 kg | Screed calculator: 12 m² at 65 mm, 10% extra, a 4.5 : 1 mix with the dry materials 1.3 times the finished volume |
| Sharp sand | 1.46 tonnes | About two 0.8 tonne bulk bags |
| Or: ready-mixed dry screed | 69 bags of 25 kg | At about 2 tonnes per m³. Quicker, and the mix is the same each time, but dearer |
| Damp-proof membrane and edge insulation | As needed | Over a concrete slab, a damp-proof membrane, and an edge strip round the walls for a floating screed |
| Screed rails or battens | A few | To set the level, and a straight edge to strike the surface off |
| Mixer | 1 (hire) | A forced-action mixer is best: a drum mixer will not mix a dry screed well |
Screed for common rooms
65 mm thick, 4.5 : 1 mix, 10% waste:
| Room | Screed | Cement bags | Sharp sand | Ready-mix bags |
|---|---|---|---|---|
| 3 × 2.5 m | 0.54 m³ | 8 | 0.91 tonnes | 43 |
| 4 × 3 m | 0.86 m³ | 12 | 1.46 tonnes | 69 |
| 4 × 4 m | 1.14 m³ | 16 | 1.95 tonnes | 92 |
| 5 × 4 m | 1.43 m³ | 20 | 2.43 tonnes | 115 |
| 6 × 4 m | 1.72 m³ | 24 | 2.92 tonnes | 138 |
| 8 × 5 m | 2.86 m³ | 39 | 4.87 tonnes | 229 |
How the depth changes the amount
For the same 4 × 3 m room:
| Depth | Screed | Cement bags | Sharp sand | Ready-mix bags |
|---|---|---|---|---|
| 40 mm | 0.53 m³ | 8 | 0.90 tonnes | 43 |
| 50 mm | 0.66 m³ | 9 | 1.12 tonnes | 53 |
| 65 mm | 0.86 m³ | 12 | 1.46 tonnes | 69 |
| 75 mm | 0.99 m³ | 14 | 1.68 tonnes | 80 |
Before you start
- Choose the type of screed. A bonded screed sticks to the slab (25 to 40 mm), an unbonded one sits on a membrane (50 mm or more), and a floating one sits on insulation (65 mm or more, 75 mm over underfloor heating).
- Mix it dry. A good screed holds together when squeezed and does not drip. Too much water makes it weak and prone to cracking.
- Plan the levels. Set rails to the final floor height, allowing for the tile or floor covering, and check it against the door thresholds.
- Be ready to finish. Once the mix goes down it must be compacted and finished in a continuous operation. Mix only as much as you can lay.
Drying and covering
Cover a new screed with polythene for a few days and keep off it for a day or two. Sand and cement screeds need about a day of drying for each millimetre of depth up to 50 mm and more beyond that, and heated or very thick ones longer. Do not lay tiles or wood until a moisture meter says it is dry. Waiting is cheaper than replacing a floor that has failed.
Mistakes that cost people money
- Adding too much water, which weakens the screed.
- Laying a screed that is too thin for what is under it.
- Rushing to cover it before it is dry, which traps moisture and lifts a wood or vinyl floor.
- Forgetting the movement joints in large floors and in doorways.
Frequently asked questions
How much screed do I need for a 4m x 3m room?
At 65 mm thick and with 10% extra, about 0.86 m³ (1.7 tonnes). That is 12 bags of cement and 1.5 tonnes of sharp sand at 4.5 : 1, or 69 bags of ready-mixed screed.
What is the ratio for a sand and cement screed?
A mix of 4 to 4.5 parts of sharp sand to 1 part of cement, by volume, is the usual choice. A richer mix (3 : 1) is used for thin, bonded screeds.
How thick should a floor screed be?
At least 25 to 40 mm if it is bonded to the concrete below, 50 mm if unbonded, and 65 mm or more if floating over insulation or covering underfloor heating pipes.
How long does a screed take to dry?
A traditional sand and cement screed needs about a day for each millimetre of depth up to 50 mm, and longer beyond that. Test it with a moisture meter before laying tiles or wood.
More guides
Tools & materials for this project
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.