How Do I Choose The Right Floor Screed?
What are the different types of floor screed? You can break it down into 4 main material types: calcium sulphate (anhydrite) flowing screed, cementitious flowing screed, traditional semi-dry sand and cement, and thin-bed polymer/latex smoothing compounds. Each material is then installed as either a bonded, unbonded, or floating floor depending on slab condition, damp-proofing, and underfloor heating.
The Capital Screeding team has spent over two decades working with screed and our message to anyone looking for the right screed for their job is to look at the material not the price. The type of screed you use will determine your thermal efficiency, how quickly trades can get on the floor, and whether your tiles will crack six months down the line. Matching the job to the screed is how you make the right choice.
Binders and Screed Chemistry
The binder is what holds the screed together to create your subfloor. We start with them because they are what determines how your screed cures, how it handles moisture, how quickly heat can pass through it, and which adhesive you need to use on it.
1. Calcium Sulphate (Anhydrite) Flowing Screed
This is a pumped liquid screed made from synthetic or natural anhydrite (gypsum) mixed with graded aggregates and water.
- Where it works best – Large open-plan ground floors and residential extensions with wet underfloor heating (UFH).
- The technical benefit – Anhydrite flows around heating pipes wiping out air pockets and delivering excellent thermal conductivity (up to 2.2 W/mK compared to roughly 1.0 W/mK for traditional sand and cement). There’s virtually no shrinkage as it cures, allowing pours up to 300m² over UFH without an expansion joint.
- What you need to know – It can’t be used in permanently wet areas like commercial kitchens, wet rooms, or outside. It also forms laitance as it dries; this is a weak surface skin that needs grinding before laying tiles.
2. Cementitious Flowing Screed
Cementitious liquid screeds use Portland cement (or rapid-hardening cement) combined with plasticisers to create a free-flowing, self-levelling mix.
- Where it works best – Wet rooms, bathrooms, basements, utility rooms, and jobs with tight schedules.
- The technical benefit – You get the installation speed of a pumped liquid screed and you can use it in wet areas. It cures quickly, accepts foot traffic within 24 to 48 hours, and doesn’t form laitance in the curing process.
- What you need to know – Cement shrinks as water evaporates during hydration. You’ll need to install joints across door thresholds and divide floors into bays of 40m² to 60m² (or any room with an aspect ratio greater than 6:1).
3. Sand and Cement (Semi-Dry)
Known as traditional screed, or dry screed, it’s 1 part Portland cement to 3 or 4 parts well-graded sharp sand (0:4 aggregate), mixed to a semi-dry, damp-earth consistency. It is compacted and levelled by hand using a straightedge and finished with a wood float or power float.
- Where it works best – Forming falls to drainage in wet rooms or plant rooms, small domestic areas where pump access is restricted, and on heritage renovations.
- The technical benefit – Because it is a stiff, semi-dry mix, it can be shaped to gradients (typically 1:40 to 1:60) towards drains and gullies; this is something liquid screed can’t do.
- What you need to know – It requires heavy manual compaction. If you under-compact the lower half of the bed, you end up with a weak, friable screed that crumbles under point loads. Drying is also slow; the standard rule of thumb is 1mm per day up to 50mm, and 0.5mm per day thereafter under good ambient conditions.
4. Latex and Polymer-Modified Smoothing Compounds
These are pre-blended bagged compounds that are mixed with water or liquid polymer latex, designed for thin-section application from 2mm up to roughly 15mm.
- Where it works best – Smoothing out rough structural concrete slabs, refurbishing damaged screeds, or bringing subfloors to a glass-smooth finish for glued-down Luxury Vinyl Tile (LVT).
- The technical benefit – High polymer content gives the compound flexibility, strength and an excellent chemical bond to the substrate. Most formulations are walk-on within 2 to 4 hours and ready for vinyl in 12 to 24 hours.
- What you need to know – These are surface levellers, not structural subfloors. If your base slab has structural deflection, cracks, or rising damp, a thin latex coat will show those faults in a few weeks.
Installing Screed – Bonded, Unbonded, and Floating Screeds
Screed depends for its success on the meeting point of the mix and the subfloor. How you install your screed depends on your structural load, the condition of the slab, and whether insulation or underfloor heating installation is part of the installation.
Once you’ve chosen your screed, there are 3 possible build-ups:
- Application – Mechanically keyed directly onto a prepared concrete base using a bonding agent (such as an SBR slurry or epoxy primer).
- Minimum Depth – 25mm to 40mm for traditional sand and cement; 15mm to 25mm for specialised liquid screeds.
- When to use – High-load commercial applications, warehouse floors, or tight refurbishment projects where ceiling heights don’t permit a thick insulation layer.
- Site requirement – The base must be structurally sound, free of oil, laitance, and dust, and mechanically prepared via scabbling or shot-blasting.
- Application – Separated from the concrete base by a damp-proof membrane (DPM), typically a 1200-gauge polythene sheet.
- Minimum Depth – 50mm for traditional sand and cement; 30mm for liquid screeds.
- When to use – Slabs that show signs of rising damp, ground-bearing concrete without an existing membrane, or contaminated bases where a direct bond would fail.
- Site requirement – The membrane must be lapped, taped at all joints, and turned up the perimeter walls to isolate the screed completely from the slab.
- Application – Lay over a layer of thermal or acoustic insulation boards (such as PIR or EPS), with or without underfloor heating pipes fixed to the insulation.
- Minimum Depth – 65mm (domestic) to 75mm (commercial) for traditional sand and cement; 35mm to 40mm (domestic) for liquid screeds (with at least 25mm–30mm cover over pipes).
- When to use – Almost all residential new builds, ground-floor extensions, and flat developments requiring Part E acoustic compliance.
- Site requirement – Insulation boards must be laid flat with tightly butted joints and no deflection. Perimeter expansion foam must be fitted along all walls and columns to absorb thermal expansion.
Drying Times, Depths, and Tolerances
Your screed installation needs to be integrated into your overall job schedule. To do this you’ll need to take into account the installation speed, thermal output, and surface regularity against your project schedule.
The table below sets out the above across the main screed types, referenced against British Standards (BS 8204-1):
Screed Type | Typical Floating Depth (over UFH) | Thermal Conductivity | BS 8204 Surface Regularity | Time to Foot Traffic | Time to 75% RH (Floor Ready) | Max Bay Size (Without Joints) |
Calcium Sulphate Liquid | 35–45mm (min 25mm over pipe) | 1.8 – 2.2 W/mK | SR1 (≤3mm gap under 2m straightedge) | 24 – 48 hours | Approx. 1mm/day up to 40mm (force-drying permitted after 7 days) | Up to 300m² (unheated up to 800m²) |
Cementitious Liquid | 40–50mm (min 30mm over pipe) | 1.4 – 1.6 W/mK | SR1 (≤3mm gap under 2m straightedge) | 24 – 48 hours | Fast-track formulations can accept tiles in 5–14 days | 40m² – 60m² (aspect ratio max 6:1) |
Traditional Sand & Cement | 65–75mm (min 35mm over pipe) | 0.9 – 1.1 W/mK | SR2 (≤5mm gap under 2m straightedge) | 5 – 7 days | Standard rule: 1mm/day up to 50mm, 0.5mm/day thereafter (60–90+ days) | 20m² – 40m² (requires stress-relief cuts) |
Polymer / Latex Smoothing | 3–10mm (over existing slab) | N/A (thin section) | SR1 (dependent on base preparation) | 2 – 4 hours | 12 – 24 hours | Varies by manufacturer (usually matched to base joints) |
What Can Go Wrong – And How To Fix It
When screed projects go wrong it’s usually because of rushed prep, poor moisture verification or chemical clashes. Here are some examples of the main issues to watch out for.
1. The Ettringite Reaction – Why Tiles Come Loose From Anhydrite Screeds
It doesn’t work to lay standard cement-based tile adhesive directly onto a dry calcium sulphate (anhydrite) screed:
- Moisture from the tile adhesive triggers a chemical reaction between the cement in the adhesive and the gypsum in the screed called ettringite.
- Ettringite crystals grow, expand, destroy the bond, and shear the tiles clean off the substrate.
How to prevent it – Never tile directly onto anhydrite. The screed must be dried down to 75% Relative Humidity (RH), sanded to remove laitance, vacuumed, and treated with an approved water-resistant acrylic or polyurethane barrier primer to isolate the cement adhesive from the gypsum. Alternatively, use a gypsum-based tile adhesive.
2. Laitance – Why You Always Need to Sand
During the curing phase of an anhydrite liquid screed, water comes up to the surface, bringing with it fine particles of gypsum, additive, and dust. This dries into a brittle, powdery skin called laitance.
If you lay adhesive, self-leveller, or damp-proof membranes over laitance, your floor covering is bonded to weak dust, not solid screed.
What to do with laitance: Sand the floor between 5-14 days after the pour using an orbital floor sander fitted with a 60-grit sanding disk or carborundum stone, then vacuum thoroughly. Or use a cementitious liquid screeds which produces no laitance.
3. Real Moisture Testing (BS 8203)
Trapped moisture is the top issue when laying engineered wood, laminate, or bonded LVT.
- To comply with British Standard BS 8203, you need an insulated hair hygrometer box sealed to the floor for a minimum of 72 hours, showing an equilibrium relative humidity of less than 75% RH before non-breathable flooring is installed.
- Alternatively, use a Carbide Bomb (CM) test, which takes a physical sample from the lower third of the screed bed and measures moisture by mass (must read below 0.5% CM for unheated anhydrite, or below 0.3% CM with underfloor heating).
Decision Rules for Your Build
Every job presents specific site constraints. You can use the following checklist to identify your exact specification:
1. Are you running underfloor heating throughout a large ground-floor layout?
- Calcium Sulphate (Anhydrite) Liquid Screed (Floating).
- At 40mm thick, it delivers heat fast, encases pipes without insulating voids, and allows massive open runs without breaking up your finished floor with unsightly expansion profiles.
2. Is the installation in a basement, bathroom, or wet room?
- Cementitious Liquid Screed (for flat zones) or Traditional Sand & Cement (for falls to drains).
- Total water stability. If a pipe leaks or a waste backs up, cementitious binders don’t degrade, soften, or trigger an ettringite reaction with standard cement adhesives.
3. Are you fitting glued-down Luxury Vinyl Tile (LVT) or direct-finish resin?
- Target an SR1 surface regularity (maximum 3mm deviation under a 2m straightedge).
- LVT mirrors every imperfection underneath. Achieve this using a pumped liquid screed or by topping a sound concrete base with a 3mm to 5mm polymer latex smoothing compound.
4. Is access restricted (e.g. upper-floor flats, tight mews properties)?
- Traditional Sand & Cement or Pre-bagged Fast-Dry Screeds.
- If you can’t park a ready-mix truck and pump line on the street, or site access prevents pipe delivery, crane-lifted dry-mix bags or small on-site pan mixing is the only logistical route.
5. Are you up against an unmovable move-in deadline?
- Fast-Track Cementitious Screed.
- Avoid standard sand and cement (which can take 60 to 90 days to dry at depth). Fast-track cementitious formulations hydrate chemically rather than relying solely on evaporation, letting you commission underfloor heating from day 7 and lay tiles in under a fortnight.
Book Your Screed Pour Across London, Essex & the South East
Planning a pour? The Capital Screeding team supply and pump liquid screeds directly to site across London, Essex, and the South East. Our team handles everything from laser-level set-up to high-volume pumping, delivering dead-flat SR1 and SR2 finishes with no mess and minimal downtime.
Call our team on 020 3369 3396 or contact us for advice and a clear, fixed-price quote.
