Liquid Screed Site Prep – The Contractor’s Guide to Avoiding Floor Failures
What is liquid screed? Also called flowing or pumpable screed, it’s a fluid flooring compound that’s designed to replace traditional, dry sand-and-cement mixes. It is pumped onto a 500-gauge polythene slip-layer, rapidly leveling itself around laser datum points to achieve a precise SR2 surface tolerance.
Capital Screeding handles these installations daily on live developments and residential self-builds. So we know from experience exactly where pours fail and how to prevent it. Don’t expect a generic overview that gives you the basics. This comes directly from our site specialists, detailing how the material behaves, what happens during the cure, and how to prep your base for a flawless floor.
Choosing Your Compound – Anhydrite vs. Cementitious Liquid Screed
Before you order your liquid screed, you’ll need to choose between the two primary liquid screed formulations. They behave differently, require separate drying schedules, and demand different surface preparation before final flooring installation.
- Anhydrite (Calcium Sulphate) Screed – This is made using synthetic or natural gypsum binders mixed with sand and water. It’s used in the majority of commercial and residential flowing pours because of its minimal shrinkage and its cost-efficiency at high volumes.
- Cementitious (Cement-Based) Screed – Made using a traditional OPC (Ordinary Portland Cement) or specialised cement binder. It mirrors the rapid drying characteristics of cement but maintains a fluid, self-smoothing consistency.
|
Attribute |
Anhydrite Liquid Screed |
Cementitious Liquid Screed |
|
Binder Base |
Calcium Sulphate (Gypsum) |
Cement (OPC or Hybrid) |
|
Typical Minimum Depth |
35mm (Floating) / 30mm (Unbonded) |
35mm (Floating) / 20mm (Bonded) |
|
Drying Time Frame |
1mm per day up to 40mm depth |
21 to 28 days total (environment dependent) |
|
Curing By-Product |
Generates a surface laitance layer |
Zero surface laitance |
|
Best Used For |
Large open-plan self-builds and UFH flooring |
Wet rooms, damp-prone basements, tight deadlines |
The Laitance Factor
There is a structural difference between these two liquid screed options that will need to be factored into your decision.
Anhydrite screeds rely on a crystallisation process to harden. As the compound settles, excess water naturally forces its way upward to escape. This upward migration carries microscopic particles of gypsum and fine aggregate to the top of the slab.
When that pooled water evaporates, those mineral deposits dry out, creating a chalky, brittle skin on top of your hardened floor. This skin is laitance – and understanding how to manage it is important if you are working with an anhydrite mix.
Step-by-Step Laitance Removal – Securing Your Final Finish Bond
Cement-based flowing screeds don’t generate a surface crust during their curing cycle, so the removal process applies only to calcium sulphate (anhydrite) installations. If you don’t remove laitance, your expensive final floor finish will shear away from the subfloor.
Step 1. Monitor the Initial Cure – Foot traffic check.
Let the liquid screed cure undisturbed until it comfortably accepts foot traffic – usually after 24 to 48 hours. Keep windows slightly open for ventilation, but wait until the elapsed site time hits day 7 before bringing grinding equipment onto the slab.
Step 2. Grind the Surface Crust – 60-grit diamond or carborundum pads.
Mount a 60-grit pad onto a single-disc floor grinder. Work across the room methodically. You will notice the floor instantly changes from a matte, chalky white appearance to a denser, darker, speckled texture as the weak laitance crust is peeled away to reveal the aggregate.
Step 3. Clear the Pores of the Slab – Heavy-duty vacuuming.
De-dust the floor completely. Any remaining dust acts as a bond-breaker, preventing your polyurethane or acrylic floor primers from sinking into the subfloor. Use a high-suction trade vacuum to ensure the pores are clear.
Step 4. Monitor Relative Humidity Levels – Hygrometer hood testing.
Leave the open floor to dry naturally. Once the required drying time has elapsed, use a sealed hygrometer hood fixed to the slab for 72 hours to verify that the moisture vapour emission rate is low enough to accept your specific tile or wood adhesive safely.
The Watertight Tanking Rule
When you’re working with liquid screed, you need to treat the room like a swimming pool base. You’ll be pouring thousands of litres of fluid onto rigid insulation boards, so the liquid naturally tries to find a path underneath them.
If your 500-gauge polythene slip-layer has un-taped seams or loose edges, the screed will slip beneath the insulation. The immense hydraulic pressure will float your heavy PIR insulation boards straight to the top of the pour, completely destroying your datum lines and snapping your underfloor heating pipe connections.
Every single lap joint in your membrane should have a minimum 150mm overlap held fast with waterproof tanking tape. Work carefully with corners, pipe penetrations, and door thresholds; there must be no gaps. If water can pass through, liquid screed will.
FAQs – Laying Liquid Flowing Screed Over UFH
Pouring liquid screed over water-fed underfloor heating pipes is the industry standard for high performance systems. The liquid material acts as a highly responsive radiator block, wrapping around the elements to transfer heat instantly up to your floor surface. Here are the precise engineering rules you need to follow to guarantee installation success.
Traditional sand-and-cement beds require a thickness of 65mm to 75mm, creating a massive, sluggish block of concrete that takes hours to heat up. Liquid screed is structurally stronger, so you can drop the depth down to just 45mm or 50mm total. A thinner slab means less material mass to warm through, shrinking your system’s heat-up response times from hours down to minutes.
This is essential. You must pressure test the complete pipe layout to a minimum of 2 bar (or your manufacturer’s spec) before the screeding team arrives on site. Keep the system pressurised throughout the entire pour and curing phase. This ensures that if a pipe gets nicked or damaged during the installation, you spot the pressure drop instantly before the material hardens around the leak.
It impacts your drying timeline. Anhydrite screeds take longer to dry naturally (typically 1mm per day up to 40mm) but can be force-dried using the UFH system after 21 days. Cement-based liquid screeds dry significantly faster and allow you to turn the underfloor heating loops on much earlier – often after just 7 days – speeding your path to final floor coverings.
You should never tile directly onto an uncommissioned floor. Once the heating system has successfully completed its step-up expansion cycle and cooled back down completely, apply a high-quality primer to the clean, sanded base. Always specify a flexible, polymer-modified adhesive (Class S1 or S2) that can handle the subtle, ongoing thermal expansion of the slab without cracking or shearing the tile grout lines.
Avoid Expensive Floor Failures – Call Capital Screeding
Before you book a delivery truck, make sure that your tanking, insulation, and datum levels pass contractor standards. Call Capital Screeding directly to talk through your project. We’re happy to offer guidance on the screed selection for your specific substrate, outline your exact curing schedule, and map out your post-pour laitance removal.
Message us, or call us today on 020 3369 3396
