What Went Wrong With My Self Levelling Screed And How Do I Fix It?
Wondering ‘why isn’t my self levelling screed level?’ If you’ve ended up with a slope, it’s because you trusted the ‘self-levelling’ descriptor. Whereas, for wide footprints, you need to set up hard indicators. Do this by mapping the low zones using a two-metre straightedge, then inserting gauge screws into the floor as depth markers. Adjust the heights of the screw heads to match your target finish line. No more guessing! This approach gives you a totally reliable visual guide to pull the screed to before it starts to set.
With years of experience supplying and installing screed across London, Essex, and the South East, the Capital Screeding team knows self levelling screeds inside out. We’ve diagnosed and managed to fix most issues on sites with just the same issues as yours. So you can rely on our professional knowledge to help you achieve the perfect finish.
Using Anhydrite? Why Cement and Gypsum Don’t Mix
If you are pouring a self levelling layer over a modern liquid screed, the chemistry clash can completely wreck your floor before your finish material even goes down. Here’s why you shouldn’t treat anhydrite like standard concrete.
Pouring cement-based levelling compound directly onto a gypsum or anhydrite subfloor doesn’t end well. The moment the wet cement hits that base, it sets off an invisible chemical reaction that forms needle-like crystals called ettringite right at the boundary line.
As the crystals grow, they expand and shatter the adhesive bond, causing your fresh floor to crack, blister, and peel away in sheets. Always check if your substrate is anhydrite; if it is, you’ll need to mechanically grind off that weak, chalky top layer, ensure the slab is completely dry, and seal it with a specialist barrier primer designed to keep the cement and sulphate isolated from each other.
Here’s a quick checklist to avoid bonding failure:
- Check the Base – Be sure you know whether your subfloor is a gypsum or anhydrite liquid screed before buying materials.
- Grind the Laitance – If you’re dealing with anhydrite, mechanically sand away the weak, powdery surface layer to expose the solid screed beneath.
- Check the Moisture – The substrate must be dry – never prime a damp anhydrite floor.
- Apply a Barrier Primer – Use a specialized, non-porous primer to completely isolate the cement from the sulphate.
Working With a Timber Subfloor? – Put Away Your Spiked Roller
Fibre-reinforced compounds are ideal for handling the natural bounce in wooden floors, but standard finishing tools will completely destroy the surface. Here’s how to blend your pour without pulling strength out of the mix.
When you are laying a fibre-reinforced mix over a wooden subfloor, forget your spiked roller. They’re great for standard compounds, but on a fibre mix they snag the internal reinforcing strands and drag them up to the surface. You’re left with a hairy, rough finish that’s virtually impossible to sand down.
What you need to use instead is a wide-blade skimming squeegee on an extension pole. Give the liquid a light skim immediately after pouring. This simple pass breaks the surface tension, closes up your seams cleanly, and keeps the reinforcing fibres buried deep in the core.
Here’s your checklist for level floors when using fibre-reinforced compound:
- Forget the Spiked Roller – Leave the plastic spiked roller off the floor to prevent snagging the internal fibres.
- Use an Alternative – Use a wide-blade skimming squeegee on an extension pole instead.
- Time it Right – Skim the surface lightly immediately after pouring to catch the wet edge before it skins over.
- Protect the Core – Keep the structural strands buried deep to maintain both floor strength and smoothness.
Troubleshooting FAQs – How to Fix Uneven Screed
A bumpy or sloped floor doesn’t have to mean disaster. Below, we look at the specific, trade-tested methods for sorting out thickness errors, powdery surfaces, and layering fresh compound over old.
Yes, but don’t treat the cured screed like normal concrete. A fresh layer needs a minimum thickness – usually 3mm – to bond without cracking. Also, cured levelling compound is very thirsty; if you don’t seal it, it will immediately suck the water out of your second pour, causing it to fail. Wait 24 hours, apply two heavy coats of acrylic primer to stop the suction, and cover the entire floor rather than just patching the low spots.
Don’t waste time with hand sanding or standard wood sanders; they’ll just glaze over the cement. What you need is a concrete angle grinder fitted with a diamond-cup wheel, hooked up to an M-class dust extractor. Mark your high spots with a pencil using a straightedge, then gently sweep the grinder across the ridges.
It’s called laitance. To remove it, you’ll need to grind away the soft top layer until you reach the hard, dark, structurally sound compound hidden beneath.
For mechanical grinding, wait at least 24 to 48 hours until the compound gets to its initial structural hardness, otherwise you could tear chunks out of the floor. If you plan to pour a second layer, check the manufacturer’s instructions. Many high-flow compounds allow a second pour within 24 hours without extensive priming, but if you miss that window, you must wait a full 3 to 7 days for total moisture cure before priming and pouring again.
Need Help with Your Screed? Capital Screeding Has You Covered
We hope this guide has given you a few answers to questions about why your self levelling screed might not have levelled perfectly and what you can do about it. While these tips can certainly help, achieving that truly flawless, glass-like finish often comes down to experience.
For guaranteed level floors on your next project in Essex, London, or the South East, trust the professionals at Capital Screeding. And if you’re currently struggling with a floor that’s gone wrong? Give us a call, and let our expert team assess the damage and find the best solution to get your screed floor back on track – 020 3369 3396
