How To Level a Concrete Floor
Whether you are preparing for new LVT (Luxury Vinyl Tile), laminate, or tiling, achieving a perfectly flat subfloor is essential. This guide adapts the techniques from This Old House for UK DIYers, using British terminology and standard materials available at local merchants.
What is Self-Levelling Compound?
Often called 'floor screed' or 'smoothing underlayment', this is a protein-free, cement-based powder mixed with water (or a latex additive). It flows across the floor to fill dips and hollows, hardening into a smooth, level surface.
UK Tip: For concrete subfloors, look for "Bag-and-Bottle" latex-based kits or water-mix "Rapid Set" compounds at stores like Screwfix, Toolstation, or Wickes.
Preparation is 90% of the job. A poorly prepared floor will lead to the compound cracking or "blown" spots (where it unbonds from the concrete).
Clean: Clear the room entirely. Use a heavy-duty vacuum to remove every speck of dust. Use a degreaser (sugar soap works well) if there are oil stains.
Repair: Fill deep cracks or holes with a rapid-set mortar or a sand-and-cement mix.
Prime: This is the most skipped but most important step. Apply a Blue Grit or a specific SBR Primer (diluted according to the bottle). This stops the dry concrete from sucking the moisture out of the compound too quickly, which causes bubbles and cracking.
2. Setting Up the "Rail System"
While many pros "free-pour," using timber rails ensures a perfectly flat finish over a large area.
Straight lengths of CLS timber (e.g., 38mm x 63mm).
Spirit level (the longer the better).
PVA or construction adhesive.
Screed bar (a straight-edged piece of timber or a metal straight-edge).
Find the High Point: Use your spirit level to find the highest spot on the floor.
Fix the First Rail: Run a length of timber from the high point across the room. Use packers underneath it until the top of the timber is perfectly level.
Scribe and Cut: If the gap under the timber is large, scribe the floor’s contour onto the wood and trim it with a circular saw so the timber sits flush on the floor but stays level on top.
Repeat: Set up parallel rails roughly 1 metre apart (or slightly less than the width of your screed bar).
The Mix: Use a large mixing bucket (a 30L "gorilla tub" is ideal). Always add the powder to the water, not the other way around.
The Drill: You must use a power stirrer/paddle mixer. Mixing by hand with a stick will result in lumps that ruin the finish.
Consistency: It should look like thick pancake batter or double cream.
Work Fast: Most UK compounds have a "wet edge" time of only 15–20 minutes.
The Pour: Start at the far corner of the room and pour the mixture between the rails. Overfill slightly.
The Screed: Rest your straight-edge across two rails and pull it towards you in a sawing motion. This "strikes off" the excess and leaves a perfectly level surface.
Spiked Roller (Optional but Recommended): Run a spiked roller over the wet pour. This releases trapped air bubbles and helps the compound knit together.
Drying Time: Most modern "Rapid Set" compounds are "foot-traffic-ready" in 3–4 hours. However, do not lay floor coverings immediately.
The 24-Hour Rule: Wait at least 24 hours before laying tiles or laminate. If laying wood flooring, you may need to wait longer or check moisture levels with a hygrometer.
Remove Rails: Once set, remove the timber rails. Fill the gaps left by the timber with more compound and smooth them over with a steel trowel.
Common UK Mistakes to Avoid
Incorrect Water Ratio: Too much water makes the finish weak and dusty; too little and it won't flow. Follow the bag instructions to the millilitre.
Drafts: Close all windows and doors while pouring. Drafts can cause the compound to dry unevenly and crack.
No Perimeter Tape: If you are pouring a thick layer, use "expansion foam tape" around the base of the walls to allow for slight movement.
Source: www.ThisOldHouse.com
