Summary

The screed is the wear-and-load-distribution layer that sits between the structural floor and the floor finish. Get it right and a tiled, resin or LVT finish lasts decades; get it wrong — wrong mix, too thin, dried too fast, finish laid too early — and the finish either bonds poorly, lifts, or cracks. Screed defects are one of the largest single sources of remedial work on new-build flooring, and almost all of them are avoidable.

The big shift over the last 20 years has been the move from traditional sand-cement screeds to flowing/self-levelling screeds, especially calcium sulfate (anhydrite-based) products. These are pumped on, find their own level, and reach a usable thickness from 25mm — half the depth of traditional sand-cement. They have transformed underfloor heating installation by encasing pipes in a thinner, more thermally responsive layer. But they also have rules of their own: they cannot be used in permanently wet areas (e.g. swimming pools), they require a different primer system, and the laitance on top must be removed before tile adhesive will bond reliably.

For most tradespeople the question on a given job is: which screed, how thick, and when can I tile/lay timber/seal on top of it? The answers depend on the substrate, the floor finish, whether UFH is in the screed, and how the building is heated and ventilated while the screed dries. Every one of these decisions has a published British Standard behind it; the most common mistake is following a "rule of thumb" that no longer matches current guidance.

Key Facts

Quick Reference Table

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Screed Type Standard Typical Thickness Min Thickness Bonded Min Thickness on UFH Compressive Strength
Traditional sand-cement BS 8204-1 65–75mm 25mm Pipe + 65mm C25–C30
Modified cementitious flowing BS 8204-1 40–60mm 25mm Pipe + 30mm C25–C40
Calcium sulfate (anhydrite) BS 8204-7 40–50mm 25mm Pipe + 30mm C20–C35
Polymer-modified BS 8204-1 10–40mm 5mm n/a (rare) C25–C40
Resin / synthetic BS 8204-6 4–12mm 2mm n/a per product
Finish RH Target (BS 8203/5325) Notes
Vinyl, LVT, linoleum ≤75% RH Hygrometer (insulated box) reading
Carpet (foam back) ≤75% RH Hygrometer
Ceramic tile (adhesive bonded) ≤75% RH Confirm adhesive datasheet
Engineered timber (no UFH) ≤75% RH + check timber moisture
Engineered timber over UFH ≤65% RH Critical — most common failure point
Resin floor ≤75% RH or per system Some systems tolerate higher with primer
Screed Thickness Sand-Cement Drying Estimate Calcium Sulfate Drying Estimate
25mm 25 days 25–30 days
40mm 40 days 40–50 days
50mm 50 days 50–60 days
65mm 80 days (50 + 30) 80–95 days
75mm 100 days (50 + 50) 95–115 days

Estimates assume 20°C, 50% RH, gentle air movement, no force-drying. Always confirm with a hygrometer.

Detailed Guidance

Sand-Cement (Traditional)

A 1:3 cement:sharp-sand mix at the lowest workable water content gives the strongest, most stable screed. Add too much water — typical site response to a stiff mix — and you get shrinkage cracking, dusting and a lower compressive strength. Use sharp sand to BS EN 13139, not building sand, and a clean concrete sand free of silt.

The screed is laid as a "semi-dry" mix that is rammed and floated rather than poured. A power float finishes it; a steel-trowel-only finish leaves a tighter surface that may polish to a sealed laitance the floor finish cannot bond to.

Cure under polythene for 7 days minimum. Curing under polythene matters: it stops the surface drying faster than the body of the screed, which is what causes surface crazing and dusting. After 7 days, remove the polythene and allow the screed to dry in ventilated conditions. Air movement helps; high humidity slows drying linearly.

Drying takes 1 day per mm up to 50mm, then 2 days per mm above. A 65mm screed therefore dries in roughly 50 + 30 = 80 days at 20°C and 50% RH. A 75mm screed needs roughly 100 days. These numbers are not optimistic — they are the basis of BS 8203 and confirmed by published research.

Calcium Sulfate (Anhydrite, Flowing)

Pumped on, finds its own level, and reaches structurally usable thickness from 25mm bonded, 30mm unbonded over insulation. The big advantages: thinner, faster to lay (300–500m² per day with a two-person team plus pumper), and much better encapsulation of UFH pipes — important for thermal response.

The big constraints: cannot be used in permanently wet areas (calcium sulfate is soluble), requires gypsum-compatible primer (not PVA, which prevents normal adhesion of tile adhesive), and develops a surface laitance that must be removed by mechanical abrasion before tiling.

Surface laitance is the weak, soft layer that rises with the bleed water. It is typically 1–3mm thick. Sand the screed with a 60–80 grit screening disc on an orbital sander or buffing machine until you reach the harder body of the screed (the surface goes from pale and chalky to slightly darker and harder). Vacuum off all dust, then apply the gypsum-compatible primer specified by the tile adhesive manufacturer.

Drying is by ventilation, not curing under polythene. From day 4, a dehumidifier can be used; from day 7, the UFH (if present) can be commissioned per BS 8204-1 Annex C.

Underfloor Heating Commissioning

The single most important post-installation step for any UFH screed:

  1. Allow 7 days of natural drying after pour (sand-cement: 7 days under polythene then 7 days air; calcium sulfate: 7 days air)
  2. Start UFH at 25°C flow temperature, hold for 24 hours
  3. Raise flow temperature by 5°C per day until reaching the maximum design temperature (typically 45–55°C)
  4. Hold at maximum for 3 days
  5. Cool at 10°C per day back to ambient
  6. Repeat the cycle once more
  7. Verify residual moisture content by hygrometer before laying the floor finish

Skipping the commissioning cycle is the most common cause of floor finish failure over UFH — the screed dries asymmetrically once the heating is finally turned on, and a finish that was laid on what looked like a dry screed lifts within months.

Bonded, Unbonded, Floating

Mixing layouts in a single room (e.g. floating in one bay, bonded in the next) is a recipe for differential movement and cracking. Maintain a single layout and use a movement joint at any change.

Moisture Testing Before Finishing

Hygrometer reading per BS 8203 is the only acceptable test for laying a moisture-sensitive finish. Surface meters (pin or capacitance) measure only the top few millimetres and routinely give a false "dry" reading on a screed that is still wet at depth.

Procedure (BS 8203):

  1. Place an insulated hygrometer (e.g. Protimeter MR or equivalent) on the cleaned screed surface
  2. Cover the perimeter with airtight sealing tape
  3. Leave undisturbed for at least 72 hours
  4. Read the equilibrium RH inside the hood

Take at least one reading per 30m² of floor and one reading in any area suspected of slow drying (e.g. near external walls, under window reveals). All readings must be below the target before the finish is laid; a single high reading means the screed is not ready.

Tolerance and Flatness

BS 8204 specifies two surface regularity classes:

A screed laid to SR2 that needs an SR1 finish must be smoothed with a self-levelling underlayment (latex SLC) over a suitable primer. This is a common second-fix operation; the cost is small compared to remedial work on a finish laid over an uneven screed.

Frequently Asked Questions

How can I tell if a screed is dry enough for vinyl flooring?

Only by hygrometer reading per BS 8203. Surface meters read the top 3–5mm and routinely indicate a screed is dry when 30mm down it is still saturated. Place an insulated hygrometer on the screed, tape it down, leave for 72 hours, and read the equilibrium RH inside the hood. Take readings in at least three locations per room. All must be ≤75% RH for vinyl/LVT.

Can I use sand-cement screed over underfloor heating?

Yes, but the screed must cover the pipes by at least 65mm of finished depth for sand-cement (compared to 30mm for calcium sulfate). This increases the response time of the heating and adds dead weight. For new UFH installations, calcium sulfate or modified-cementitious flowing screeds are almost always specified instead, both for thermal response and drying time.

Why does my new screed have hairline cracks at room thresholds?

Movement at door thresholds is normal and necessary — every doorway is effectively a control joint in the building's structural movement pattern. Either provide an intentional movement joint at the threshold (continuous through the screed and through the finish), or accept hairline cracking. The job of the floor finish is to span small thermal movements; for ceramic tile, install a movement joint in the tile that aligns with the joint in the screed.

Can I lay tiles directly onto a calcium sulfate screed?

Yes, but only after: (1) the screed is dry to ≤75% RH (or as the adhesive manufacturer specifies); (2) the laitance has been mechanically removed; (3) a gypsum-compatible primer has been applied; (4) a gypsum-compatible tile adhesive is used (some standard cement-based adhesives are not compatible with the calcium sulfate substrate). Each step is non-optional. Read the adhesive datasheet.

What's the minimum gap between pouring screed and starting other trades on the floor?

24 hours light foot traffic for calcium sulfate; 48 hours for sand-cement. No heavy traffic, no point loads (scaffold legs, tower bases), no stored materials for 7 days. Skipping this is the most common cause of pit/dent damage that has to be filled before the finish goes on.

Regulations & Standards