Screed Calculator UK

How to Calculate How Much Screed You Need

1. Measure your floor area: length and width for a rectangle, or diameter for a circle. A typical UK room might be 4m by 3m.
2. Choose your depth: 50mm for bonded or unbonded screed, 65mm for the more common floating screed over insulation, or 75mm+ if you're covering underfloor heating pipes.
3. Calculate volume: area × depth in metres. Our example: 12 m² × 0.065m = 0.78 m³.
4. Add waste: add 10% for spillage, levelling and compaction losses.
5. Convert to materials: for a traditional site-mixed screed, split the volume by a 1:4 cement:sand ratio and convert to bags. For pre-mixed or flowing screed, divide by the bag's stated coverage.

Our calculator does all of this automatically based on your area, depth and screed type.

Types of Floor Screed

Flowing screeds are usually supplied ready-mixed by the m³ from a specialist supplier rather than bagged, but small DIY areas can use bagged flowing screed products.

Bonded vs Unbonded vs Floating Screed

Minimum depths for each below follow BS 8204-1, the British Standard code of practice for screeds.

• Bonded screed: laid directly onto a primed concrete subfloor and bonded to it with a slurry coat. Can be laid thinner (25-50mm) since it relies on the base for strength. Not suitable over insulation.
• Unbonded screed: laid over a separating membrane (damp-proof membrane) on top of the subfloor, not bonded to it. Needs a minimum of 50mm to avoid cracking since it isn't supported by the base.
• Floating screed: laid over a layer of insulation, common in modern floors with underfloor heating. Needs a minimum of 65-75mm depth (or per your insulation and UFH manufacturer's spec) since it's fully unsupported below.

Curing and Drying Times

Curing: screed needs to cure (harden chemically) before it can take foot traffic, usually 2-3 days for light traffic and up to 7 days before heavier loads. Keep it covered and out of direct sun or draughts during curing to avoid surface cracking.

Drying: a different process to curing. Traditional sand and cement screed dries at roughly 1mm per day for the first 40mm, then slower after that, meaning a standard 65mm screed can take 6-10 weeks to dry fully before you can lay flooring or turn on underfloor heating. Flowing screeds dry faster, often ready in around 1mm per day throughout, but always check the manufacturer's data sheet.

Underfloor heating commissioning: once dry, UFH systems need a slow, gradual warm-up over several days (following the manufacturer's commissioning schedule) before running at full temperature, to avoid thermal shock cracking the screed.

Frequently Asked Questions

How much screed do I need for a room?

A typical 4m × 3m room (12 m²) at a standard 65mm floating screed depth needs roughly 0.78 m³ of screed, plus 10% waste, which comes to around 34 bags of cement and sand at a 1:4 mix, or the equivalent bagged/flowing screed volume.

How deep should floor screed be?

Around 50mm for bonded screed laid directly onto concrete, 50mm minimum for unbonded screed over a separating membrane, 65mm minimum for floating screed over insulation, and 75mm or more if you're covering underfloor heating pipes, depending on your UFH manufacturer's minimum cover requirement. These minimums follow BS 8204-1.

How long does screed take to dry?

Traditional sand and cement screed dries at roughly 1mm per day for the first 40mm depth, so a standard 65mm screed typically takes 6-10 weeks before you can lay flooring. Flowing/liquid screed generally dries faster, but always check the specific product's data sheet.

What's the difference between screed and concrete?

Screed uses a finer sand aggregate and is laid in a thinner layer to create a smooth, level surface for flooring. Concrete uses a coarser aggregate (stone or gravel) and is used structurally for slabs, footings and bases. Screed is not designed to be load-bearing on its own.

Can I screed over underfloor heating pipes myself?

Yes, though it's a job that rewards experience. The screed needs to fully encapsulate the pipes without trapping air pockets, and the minimum cover depth above the pipes must meet your UFH manufacturer's spec. Many self-builders use a flowing/liquid screed here since it self-levels around the pipes more reliably than a hand-laid traditional mix.

Do I need a damp-proof membrane under screed?

Yes, for ground floors: a damp-proof membrane goes beneath an unbonded or floating screed to stop moisture rising from the subfloor. For bonded screed on suspended or upper floors this usually isn't required, but always check your specific build-up.