When Can You Skip the Screeding Stage?
You can only skip the screed if you are leaving the concrete completely raw as a garage floor, or if you are laying thick stone flags set in a deep, traditional mortar bed. For everything else, the answer to ‘Do I need to screed a concrete floor?’ is ‘Yes!’.
A power-floated concrete slab isn’t a finished floor base – and here’s why: concrete contains coarse aggregates like gravel and crushed stone, which makes it impossible to smooth out, whereas liquid screed uses sharp sand or liquid calcium sulphate binders to create a dense, perfectly flat crust.
Skipping the screeding stage means laying your luxury flooring directly over laitance on the surface of the concrete. Your tile adhesive will stick to this dust, lift away from the slab, and ruin your floor finish within a year.
Working across London, Essex and the South East, Capital Screeding has multiple teams of screeders who are subfloor specialists. Their expertise lies in delivering pump-applied liquid screeds engineered to hit tight SR1 and SR2 surface targets.
How to Create a Watertight Tanking Zone Before the Liquid Screed Pump Arrives
Self-levelling screed acts like water. If there is an unsealed gap, pinhole, or loose seam anywhere on your floor area, the screed will find it, drain away beneath your insulation boards, and ruin your installation.
The prep process for flow screeds involves tanking; turning your subfloor into a temporary, waterproof pool. This keeps the fluid mix exactly where it needs to be while it goes through its initial chemical cure.
Preparation Sequence
Step 1 – Clear the subfloor slab
Clear the raw concrete base completely. Use a scraper to remove any concrete nibs and sweep away any loose grit. Any debris left behind will puncture your insulation or membrane under the weight of the screed.
Step 2 – Lay insulation boards tight
Install your floor insulation boards flat across the slab. Make sure they’re tightly abutted. If the boards are uneven or rocky, use sand to blind and level the sub-slab first.
Step 3 – Use a 500-gauge slip membrane
Roll out a continuous layer of 500-gauge polythene slip membrane over the insulation. This plastic sheet acts as your primary barrier. Check the sheets overlap each other by at least 150mm, and pull the plastic up the walls to about 100mm to create a secure lip.
Step 4 – Tape the seams completely
Use waterproof packing tape to seal every single lap joint across the polythene sheet. Tape down any wrinkles so they sit completely flat, and carefully seal around pipe penetrations or drainage points. The membrane must look like a solid, unbroken plastic skin.
Step 5 – Fix the perimeter expansion strip
Install a 10mm foam perimeter expansion strip around every wall, column, and door frame. This foam absorbs the natural expansion and contraction of the floor as it cures. Check the plastic skirt attached to the foam strip is taped securely down onto your top polythene layer to seal the outer edges.
Securing Underfloor Heating Pipes
Once the floor is watertight, you can lay the underfloor heating (UFH) pipework. Be aware that when we pump the liquid screed across the floor it will lift anything lightweight, so if your heating pipes are not pinned down securely they’ll float straight to the top of the pour.
UFH Installation Sequence
Step 1 – Fix the pipe layout pattern
Lay your 16mm barrier pipe in either a serpentine (zig-zag) or snail (spiral) pattern according to your heat-loss drawings. Connect one end to the manifold before uncoiling.
Step 2 – Staple at strict 300mm intervals
Use a staple gun to drive plastic pipe staples through the polythene layer and deep into the insulation boards. Place a staple every 300mm on straight runs. On tight bends or loops, drop that spacing down to every 100mm to counteract the natural spring and tension of the plastic pipe.
Step 3 – Protect pipes at structural thresholds
Where the heating pipes pass through doorways or cross expansion boundaries, slide a length of flexible conduit sleeve over the pipe. This protects the plastic from rubbing or shearing against the screed during natural movement.
Step 4 – Carry out a mandatory water pressure test
Before the screeders arrive, fill the heating system with water and purge all air. Pressurise the system to 6 bar and leave it under pressure for 24 hours. This checks for site leaks, but more importantly, the weight of the water inside the pipes adds ballast to keep them anchored to the floor during the pour.
Step 5 – Keep the system pressurised during the pour
Leave the UFH loops pressurised to at least 2 bar while the liquid screed is being pumped and walked in. This ensures that if a trowel or tool accidentally damages a pipe during the pour, the drop in pressure will warn the screeders straight away.
Managing Liquid Screed Depth and Cure
If you pour a fluid floor too thin, it won’t have the strength to handle daily traffic and will eventually break apart. If you pour it too thick without adjusting your timeline, you’ll trap moisture in the subfloor that will rot your engineered wood or blow your tile adhesive later on. Here are some guidelines for hitting the exact spec you need.
The Technical Rules
- Bonded vs. Floating Specs – A minimum depth of 25mm applies to fully bonded applications over a clean, sound concrete base. If you are using a separation membrane over insulation, you’ll need to hit a minimum of 35mm.
- Encasing Underfloor Heating – A minimum of 30mm of solid screed sitting directly over the crown of the heating pipe is required. This thickness acts as a structural bridge to distribute weight and ensures an even thermal mass so you don’t get hot and cold patches.
- The Traffic Clock – Within 48 hours, the screed is stable enough for foot traffic. 5-14 days is the ideal window to sand off the laitance, which is a weak, chalky crust that floats to the surface during the pour. Sanding this off early opens up the pores of the material, which can speed up the remaining drying timeline.
- Calculating the Dry Rate – Budget for 1mm of drying depth per day for the first 40mm of your floor thickness. If your spec is a 50mm deep pour, the final 10mm will take an additional 20 days to cure naturally.
If you are on a tight contract window, ask Capital Screeding about rapid-drying cementitious compounds that can handle final floor tiles in as little as 24 to 48 hours.
FAQs – Liquid Screed Curing, Sanding, and Underfloor Heating
As a builder, your main concerns are timeline, site readiness, and managing other trades. Which means you’ll have questions about the on-site practicalities of installing screed. Here are the questions we’re regularly asked by builders, answered directly by the Capital Screeding team.
Yes, but you need to wait 7 days after the pour. Turning the heating on too early will shock the material, causing rapid moisture loss and structural cracking. After day 7, you can begin commissioning the system. Start with your water temperature at 20°C, and increase it by 5°C each day until you hit your maximum design temperature (usually around 45°C or 50°C). Hold it there for a few days to force out the residual moisture, then drop it down by 5°C a day before laying your flooring.
If you’re using anhydrite screed, yes. During the curing process, laitance floats to the top and hardens into a brittle crust. If you try to tile directly over it, your adhesive will stick to the crust, which will eventually delaminate and lift away from the solid screed underneath. You need to mechanically sand the floor 5 to 14 days after the pour to strip this layer away, open up the aggregate pores, and allow the floor to dry out properly.
Why Choose Capital Screeding?
Operating across London, Essex and the South East, our expert screeders bring the machinery, experience and technical know-how needed to guarantee your subfloor is delivered on time, perfectly flat, and structurally sound.
Ready to book your liquid screed pour or want to check your site prep before we arrive? Talk to a South East screeding specialist who knows the trade inside out. Call Capital Screeding directly on 020 3369 3396.
