Pick the wrong screed for a job and you’ll know about it. Tiles cracking six months later, a floor that won’t stop moving, or a pumpable screed that’s still wet when the client wants to lay engineered oak. I’ve seen all three. Floor screed types in the UK get specified badly more often than most tradesmen would admit, and a lot of it comes down to habit rather than logic. Same product every time regardless of the substrate, the heating system underneath, or what’s going on top.
This is a practical comparison of the three main options you’ll encounter on domestic and light commercial work: bonded, unbonded and liquid (self-levelling) screed. No filler, just the decisions you need to make on site.
Bonded screed: when it works and when it doesn’t
Bonded screed is mechanically keyed directly to the substrate using a primer or bonding agent, typically SBR slurry brushed onto a prepared concrete base. The bond is the whole point. Because the screed can’t move independently, you can get away with thinner layers, generally 25mm to 40mm, which makes it useful where floor build-up is tight.
It earns its place in commercial kitchens, utility areas and garages where you’ve got a solid, well-prepared concrete slab and you need to keep the floor level high. The trade-off is preparation time. The substrate has to be clean, free from laitance and primed properly. Fail on any of those steps and the bond will fail, and you’ll have a hollow, drumming floor that sounds like a problem long before you can see one.
I’d avoid bonded screed over underfloor heating. The differential thermal movement between the screed and the slab tends to stress the bond over time, especially with the repeated heat cycles a UFH system puts it through. It also doesn’t suit damp-prone slabs for the same reason: any moisture in the substrate has nowhere to go.
Unbonded screed: the go-to for most domestic jobs
Unbonded screed sits on a separating layer, typically a polythene DPM, and floats independently of the slab beneath. That separation means you need more thickness to get structural integrity, usually a minimum of 50mm for foot traffic and 65mm to 75mm if there’s any kind of loading. Get the depth wrong and the screed will curl at the edges and crack in the middle.
On a typical domestic new-build or extension, unbonded traditional sand-and-cement screed is still the most common choice, and for most straightforward jobs it’s the right one. It’s forgiving, relatively cheap, and most groundworkers and builders are comfortable laying it. The issue is drying time. Sand-and-cement screed dries at roughly 1mm per day for the first 40mm, then 0.5mm per day beyond that. A 75mm pour could sit for three months before it’s ready to tile. That’s a long time to hold a programme.
If you’re laying over a concrete ground floor slab with a DPM already built in, unbonded is almost always the right call. For guidance on getting your sub-base right before the screed goes down, the article on choosing the correct sub-base material for UK groundworks is worth a read before you start.
Liquid screed: faster, flatter and better over UFH
Liquid screed, or self-levelling screed (also called flowing screed or anhydrite screed in most merchants’ catalogues), has changed how a lot of tradesmen approach floor builds. It’s pump-applied, self-levels to a very tight tolerance, typically ±3mm over a 2m straightedge, and can go down at 50mm to 65mm as an unbonded layer. The headline benefit is drying speed: specialist calcium sulphate-based liquid screeds can be walked on within 24 to 48 hours and reach tiling readiness in a fraction of the time compared to sand-and-cement.
Over underfloor heating, liquid screed is consistently the better specification. The flow characteristics mean the screed encapsulates the pipework fully without voids, which improves thermal efficiency and reduces hot-spot risk. Most liquid screed manufacturers recommend a cover depth of 30mm to 50mm above the pipe, and the system works best when the UFH is commissioned (brought up to temperature gradually) before any finish is applied. For anyone taking on more retrofit work, understanding floor build-ups ties closely into EPC performance, and there’s useful context in the piece on what EPC ratings mean for UK builders taking on retrofit work.
The catch with liquid screed is cost. You’re looking at pump hire or a specialist subcontractor, and the material itself costs more per square metre than traditional screed. On small areas, the economics rarely stack up. On a 100m² open-plan extension, they usually do, particularly when you factor in the labour saved on mixing and levelling.
One other thing worth knowing: calcium sulphate screeds are not compatible with cementitious adhesives until they’ve been sanded and primed. Miss that step and your tile adhesive won’t bond. It’s a common error and it costs real money to put right.
Moisture testing before you tile or finish
Screed type and screed drying are two separate conversations. Even well-specified liquid screed on a heated UFH slab can retain moisture at depth longer than readings at the surface suggest. The BSRIA guidance on flooring preparation and the British Standards around in-situ hygrometry (BS 8203 for resilient flooring and the British Ceramic Tile Council guidance for tiling) both set a target of 75% relative humidity or below before any impermeable finish goes down. Most tiling failures I’ve seen on screed trace back to this step being skipped or rushed.
Calibrated hygrometers embedded in the screed for a minimum of 72 hours give a reliable reading. Surface carbide bomb tests are faster but don’t tell you what’s happening 30mm down. Tech used to be expensive and unreliable in this space, but that’s changed. Contractors now use wireless sensor arrays that log readings continuously and flag when thresholds are met. theiceage.co.uk is one example of a source covering how those kinds of monitoring tools are being adopted across the trades. The data logging aspect is genuinely useful on commercial jobs where you need a paper trail to protect yourself if a finish fails later.
The shift toward digital monitoring in building trades mirrors a broader trend of data-backed decision making, a bit like how AI has started influencing adjacent industries, including retail styling (there’s an interesting take on that crossover in this piece on how algorithm-driven recommendations are landing with British consumers). The point is the same wherever you look: better data leads to fewer surprises.
Quick reference: which screed for which job
Bonded screed suits thin build-ups over solid, well-prepared concrete where there’s no UFH and no moisture risk. Minimum 25mm, maximum around 40mm. Prep is everything.
Unbonded sand-and-cement screed covers most domestic ground floor jobs over DPM, without UFH, where drying time isn’t a constraint. Minimum 50mm, budget three months for a 75mm pour to dry properly before impermeable finishes.
Liquid screed earns its premium on UFH installations, large open areas, and jobs where programme matters. Minimum 30mm cover over pipes, sand and prime before tiling, and commission the heating before the floor finish goes down.
Getting the wet room or bathroom floor right often ties into screed specification too. If you’re specifying a wet room build, the decisions around drainage falls and waterproofing interact directly with screed type and thickness, which is covered in detail in the guide on wet room waterproofing, tanking systems and common installer mistakes.
Floor screed types aren’t complicated once you strip it back to the actual site conditions. Substrate, build-up depth, heating system, drying programme, finish material. Answer those five things honestly and the right screed picks itself.