Cement Screed For Modern Floor Levelling
Ask an old-school builder ‘what is cement screed?’ and they’ll describe a pile of sharp sand and cement that’s mixed on-site, barrowed in, and hand-trowelled to a finish. What they’re describing is the traditional semi-dry mix that’s been the industry ‘go to’ for decades. It works, but it’s slow, physically draining, and it struggles to meet the requirement of pours for modern builds.
At Capital Screeding, we’ve spent years pouring floors across London Essex and the South East. Traditional dry-pack methods still have their place, but – for the most part – we’ve shifted our focus to flowing cementitious screed. It uses the same chemistry but it’s a fluid mix that’s pumped straight from a truck. It flows effortlessly around underfloor heating pipes, gets rid of structural voids, and lays perfectly flat – all in a fraction of the time traditional screed takes.
Cementitious vs. Anhydrite – Matching Your Material to The Site Reality
Deciding between a cementitious liquid mix and an anhydrite pour comes down to considering your floor’s final environment and your finishing schedule. Anhydrite screed handles standard dry living spaces excellently, but specific structural requirements and ambient weather conditions demand the chemical and physical characteristics of a cement-based liquid slab.
Project Requirements That Are Best Served by Cementitious Screed:
- Wet and High-Moisture Zones – The only choice for domestic bathrooms, wet rooms, utility areas, and commercial kitchens where the floor will face regular exposure to water.
- Minimal Surface Preparation – When you want to avoid the need for heavy mechanical sanding to remove surface crusts. Cementitious screed needs only a light mechanical sweep to clear site dust before the flooring goes down.
- High Strength-to-Depth Ratios – Specify cement screed when you need a thinner, lighter floor profile over insulation without losing the compressive strength needed for heavy domestic or commercial foot traffic.
- Standard Tiling Workflows – Great when you want to lay ceramic or stone tiles using standard cement-based adhesives directly onto the base.
- Fast-Track Heating Cycles – If you need to get the underfloor heating running quickly, cementitious screed allows you to turn the system on just 7 days after the pour to force-dry the slab.
- Rapid Trade Turnover – Perfect for sites that need early foot traffic within 24 to 48 hours.
Environmental Conditions on Site:
- Poor Site Ventilation – You can rely on cementitious screed to cure even on sites with restricted airflow or minimal ventilation.
- Variable Atmospheric Moisture – Handles sudden drops in temperature or spikes in seasonal dampness without altering the chemical integrity of the curing slab.
- Ideal for Basements – Functions flawlessly in deep structural footprints or basement levels where damp air naturally pools.
Screed Depths on Site – Quick-Reference Table
If you pour cementitious screed too thin and the slab will crack even under normal foot traffic. Pour too thick, on the other hand, and you’re throwing money away and extending your drying times.
You can pour flowing cementitious screed much thinner because it’s structurally stronger than traditional sand and cement. When you are setting your datum lines on-site, use this table to get your depths right.
|
Installation Type |
Minimum Depth |
Recommended Depth |
Core Site Requirement |
|
Bonded (Direct to concrete slab) |
25mm |
30mm to 35mm |
Mechanically key the base slab. Apply a compatible grit primer to lock the screed down. |
|
Unbonded (Isolated from slab) |
30mm |
40mm |
Lay a plastic slip membrane flat with 100mm overlaps taped tight. |
|
Floating (Over thermal insulation) |
35mm (Domestic) 40mm (Commercial) |
45mm to 50mm |
Pack insulation boards tight with zero gaps. Tape all joints to prevent leaks. |
|
Heated (Over UFH pipes) |
30mm above the pipe |
46mm to 50mm (Total) |
Clip pipes down securely every 300mm. Fill and pressure-test the system before we pour. |
Drying and Commissioning – How to Speed Up Your Build
Traditional sand and cement screeds make you wait, and wait. It’s a minimum of 21 to 28 days before you can even think about turning on the underfloor heating. And if you turn it on early? You run the risk of thermal shock, cracking, and ruining the slab.
Flowing cementitious screed is something else entirely. Because of its dense structure, you can fast-track the entire process.
- Day 1 to 2 – Keep the building sealed. Keep direct sunlight off the pour and eliminate any harsh drafts while it’s initially setting. You can walk on it after 24 to 48 hours.
- Day 3 to 7 – Open the windows to let the moisture escape.
- Day 7 – You can safely turn on your underfloor heating. Start at a low temperature (usually around 15°C or water temperature of 20°C) and increase it by 5°C each day until you reach your operating temperature. Cool down the system by decreasing the water temperature by 5°C each day until the slab is back to room temperature
- 24-48 hrs After UFH Commissioning – Before laying tiles, wood, or vinyl, perform a hygrometer test. Once you hit a reading below 75% RH, your floor is ready for its final finish.
Where to Put Movement Joints
When we are setting up a flowing cementitious pour, our screeders look at the floor plan to identify the natural breaking points of the building. As a general rule, we aim to keep individual screed bays under 100 square metres for heated floors, and 150 square metres for unheated floors.
Before we arrive on-site, though, you need to have your movement joints installed in the following places:
- Every Single Doorway – We never pour a continuous slab through a doorway into another room. The change in wall structure restricts the screed’s movement. Put a joint directly under the door leaf so the final flooring transition covers it.
- Changes in Floor Depth – If your floor thickness changes – stepping from a 35mm bonded section onto a 50mm floating section for example – the two depths will cure and respond to heat at different rates. You need to place a joint at the transition line.
- Around Vertical Columns and Pillars – Any rigid structural element poking through the floor acts as an anchor. Put a compressible jointing strip around columns, steps, and structural steelwork to allow the screed to move 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
