Cement Screed – a Definition, Mix Ratios, and Purpose
What is cement screed? It is either a semi-dry ‘traditional screed’ or a flowing mix of sharp sand, cement binder, and water, that’s poured over concrete subfloors. Designed to British Standard 8204, the latter provides a level, structurally sound substrate that protects underfloor heating pipes and provides a durable base for tiles, resin, or timber.
Most builders know traditional screed as a stiff mix laid by hand and tamped level with a straight edge. It will get the job done in small areas, but it drags out schedules and is terrifically punishing on your knees. At Capital Screeding, we focus instead on pumpable cementitious screeds for our builds across Essex and London. These cut out the physical labour, level floors to tighter tolerances, and speed up the drying process so follow-on trades can get back to work faster.
Traditional or Flowing Cement Screed – Which Should You Use?
If you’re screeding an external ramp you’ll use a different cement screed than you would for a floor with integrated UFH. The right choice comes down to falls, site access, and floor build-up:
Use Traditional Sand & Cement When:
- You need a slope for a drain – Traditional screed holds its shape when raked. This makes it the best option for sloped surfaces in wet rooms, walk-in showers, commercial plant rooms, or external balconies.
- Access is strictly limited – If your site hasn’t room for an 8-wheel ready-mix truck or pump lines through the building, dry screed batched by hand in small areas is the right choice.
- You are tackling minor repair patches – Small, isolated floor repairs under 15 square metres rarely justify bringing in a pump truck.
Use Flowing Cementitious Screed When:
- You’re encasing underfloor heating (UFH) – Liquid screed flows around the pipes leaving no air bubbles, which makes transferring heat across the slab up to 30% faster than with traditional screed.
- Speed matters – Two experienced screeders can pump up to 1,000 square metres in a single day, which will be ready for light foot traffic 24 to 48 hours later.
- You have tight ceiling heights or heavy insulation – Flowing screed has higher compressive strength, allowing you to pour at 35mm to 40mm over insulation where traditional screed requires at least 65mm to prevent cracking.
Note for the rest of this guide – While Capital Screeding installs both systems across London and the South East, the remainder of this guide focuses specifically on flowing cementitious screed – including minimum depths, joint placement, and UFH commissioning schedules.
What Is Flowing Cementitious Screed Made Of? Mix Design and Raw Materials
Unlike traditional screed that gets mixed by hand, flowing cementitious screed is an engineered liquid product that arrives on site ready mixed. Every delivery conforms strictly to BS EN 13813 so you’re guaranteed uniform density, workability, and compressive strength.
The mix relies on four standard ingredients:
- Formulated Cement Binder – Portland cement is combined with chemical activating agents. This binder hydrates quickly and controls drying shrinkage, allowing the slab to gain structural strength quickly without curling at the perimeters.
- 0–4mm Graded Sharp Sand – Fine aggregates with a tight particle size distribution. This clean, graded aggregate ensures the mix flows through delivery lines without blocking the pump.
- Superplasticisers (PCE Technology) – Polycarboxylate ether admixtures disperse the cement particles electrostatically. This turns a dense cement mix into a self-compacting fluid without drowning the batch in excess water.
- Controlled Water-to-Cement Ratio – Plant-batched at an exact water-cement ratio (typically 0.40 to 0.45). Precise moisture control ensures full chemical hydration, stops surface bleed water, and prevents shrinkage cracks when drying.
Minimum Depth, Tolerances, and Site Prep
Flowing cement screed delivers greater flexibility than site-mixed sand and cement, so you can pour it significantly thinner. Here are the benchmarks in our quick reference guide:
System Type | Minimum Depth | Target Depth | Essential Site Preparation |
Bonded (Direct to structural slab) | 25mm | 30mm to 35mm | Scabble or shot-blast the concrete to expose the aggregate. Clear all the dust and roll on a high-grip primer to lock the bond. |
Unbonded (Separated by membrane) | 30mm | 35mm to 40mm | Lay 500-gauge polythene slip membrane flat across the slab. Overlap joints by minimum 100mm and tape them so they’re watertight. |
Floating (Domestic) (Over thermal / acoustic insulation) | 35mm | 40mm to 45mm | Stagger rigid PIR insulation boards tightly on a flat base. Tape every joint with waterproof tape to stop liquid screed bleeding underneath. |
Floating (Commercial) (Heavy foot traffic / pallet loads) | 40mm | 45mm to 50mm | Use high-density, compressive-resistant insulation boards. Fit 8mm to 10mm perimeter expansion foam to all walls and columns. |
Heated (Over UFH) (Pipes over insulation) | 25mm to 30mm (coverage above pipe) | 45mm to 50mm (total slab depth) | Secure heating pipes every 300mm to prevent pipe float. Fill and leave the UFH system pressurized to 4–6 bar throughout the pour. |
3 Site Checks Before We Pump:
- Perimeter Strip Detailing – Fit compressible expansion foam (minimum 8mm) around all perimeter walls, pillars, and pipe entry points. Pull the polythene skirt flat over the insulation and tape it down to contain the liquid mix.
- Watertight Insulation – Liquid screed finds any opening. If your insulation boards are rocky, span over voids, or you have untaped gaps, the screed will run under the boards and float them off the concrete base.
- UFH Air Displacement – Never pour over empty, unpressurised underfloor heating loops. Fill the manifolds with water and run a pressure test prior to arrival to ensure that there are no leaks and to weigh down the pipes.
Commissioning Flowing Cement Screed Without It Cracking
You can’t rush the heating cycle if you’re using cement screed. Heating an un-hydrated slab makes moisture turn to steam internally, blowing hairline cracks through the surface and debonding tiles.
Flowing cementitious screed allows you to commission heating in a fraction of the time required by traditional screeds – provided you follow these steps:
- Seal the Environment (0–48 Hours) – Don’t ventilate the room during the first two days. The screed needs to keep its initial moisture to complete chemical hydration. Trades should stay away until the 48-hour mark.
- Cross-Ventilate (Days 3–6) – Once the slab has reached initial strength, open windows and set up fans or dehumidifiers. Never use direct-fired gas space heaters; propane exhaust puts water back into the air.
- Fire the Manifold at Day 7 – Start the underfloor heating gently with a water flow temperature of 20°C.
- Run the 5°C Daily Step-Up – Raise the manifold temperature by 5°C every 24 hours. Push it up to the maximum operating temperature, sustain it for 3 days to bake the residual moisture out, then reduce the temperature by 5°C daily until the heating is off.
- Check with a Digital Hygrometer – Never guess moisture levels by eye or surface colour. A slab can look pale and dry on top while remaining soaked at the base. Perform a British Standard surface hygrometer test – only lay final floor finishes when the reading sits reliably below 75% RH.
Where to Place Movement Joints in Flowing Cement Screed
A screed slab expands as it warms and contracts as it cools. If you don’t give it room to move, thermal stress will crack the floor right across your main room.
The Capital Screeding team limits individual screed bays to 100m² for heated floors and 150m² for unheated slabs. If a room has an aspect ratio exceeding 2:1 (such as a long hallway or narrow open-plan kitchen), we divide the run with an intermediate joint to stop the slab snapping in the middle.
Install your pre-formed movement joints at these four non-negotiable points before we pump:
- Every Single Doorway – Never pour a continuous slab through a doorway. Fit an expansion profile directly below the door so the eventual floor transition strip hides the seam.
- Heating Circuit Boundaries – Separate independent underfloor heating zones with a movement joint. If the kitchen manifold is pumping at 45°C while the adjoining hallway circuit is switched off, differential thermal expansion will shear the screed along the zone boundary.
- Changes in Slab Thickness – If your floor profile steps down – transitioning, for example, from a 35mm bonded section onto a 50mm floating insulation buildup – the two depths will cure and expand at different rates. Place an expansion joint directly over the step.
- Vertical Columns, Steelwork, and Inlets – Any rigid structure protruding through the floor acts as an anchor that restrains screed movement. Wrap pillars, pipes, and staircase bases with 8mm to 10mm compressible foam stripping to allow the slab to float freely around them.
Site FAQs – Managing the Cure
To get a flawless finish, you have to manage the physical environment of the building. These four common builder queries cover the curing and drying process:
Seamless resin floors and thin luxury vinyl tiles (LVT) require an SR1 tolerance (under 3mm deviation over 2m) whereas cementitious flowing screed achieves an SR2 tolerance. For ultra-sensitive finishes, you will still need to apply a thin 3mm smoothing compound (feather-finish) to get rid of any minor aggregate texture before laying the floor.
No. The building must be completely watertight before we pump. If rainwater drips onto a wet, curing liquid screed, it will dilute the cement binder on the surface, leaving a soft, chalky, pitted crater that will crumble under foot traffic. If a sudden leak happens post-pour, you’ll have to wait for the slab to cure, mechanically grind away the damaged soft spot, and fill it with an epoxy repair mortar before tiling.
Never use direct-fired gas heaters because propane or butane gas produces massive amounts of water vapour as a byproduct of combustion. So you’re pumping moisture directly back into a damp room, which completely stalls the drying process. If you need to speed up drying, use a combination of indirect electric space heaters (to warm the air) and commercial dehumidifiers to actively extract the moisture from the room.
No. Saw-cutting risks slicing right through your underfloor heating pipes. All movement joints in a liquid thermal screed must be pre-formed joints. This means setting up compressible jointing strips, threshold profiles, or movement joint channels exactly where you need them before we start pumping the material.
Ready to Get Pouring?
Your floor is the foundation of your entire finish. By choosing a cementitious flowing screed, you are choosing compatibility, thermal efficiency, and speed. It’s simple and efficient.
As the established installer for London, Essex and the South East, Capital Screeding guarantees a precision pour that adheres to manufacturer standards. We care about perfection.
Ready to pour? Contact us today for a free estimate or call our team for advice on – 020 3369 3396
