Getting Tolerances Right First Time
If you’re laying floating laminate, substrate flatness matters far more than what it looks like. Minor surface roughness can always be swept away, whereas structural dips and high spots will ruin a click-system floor. Self-levelling compound is the industry-standard subfloor preparation for laminate because it creates a dead-flat surface that can be relied upon to distribute foot traffic loads evenly.
With years of contract levelling experience across both domestic and commercial sites, Collie Floor Levelling operates on the rule: test first, pour second. Whether we’re dealing with power-troweled concrete, old bitumen residues, or suspended timber, we always assess moisture limits and substrate movement before selecting a compound.
This ensures that the levelling layer bonds permanently and holds tolerance for the entire lifespan of the floor.
Testing Moisture and Removing Laitance
A high-flow levelling compound needs a strong mechanical bond with the base floor. There are two subfloor issues that will destroy that bond faster than anything else; these are high moisture levels and surface laitance.
1. Clearing Laitance
When concrete or calcium sulphate screeds dry, water evaporates from the surface and leaves behind a weak, friable film called laitance. It looks like smooth concrete, but it has absolutely no structural strength. Attempting to apply levelling compound over laitance is like sticking duct tape to loose paint – it pulls right off under load.
To avoid this, you need to grind or sand the surface back to solid concrete or gypsum before priming it.
2. Controlling Moisture (75% RH Standard)
Laminate planks and underlays act as a vapour barrier, locking ground damp beneath your floor. Testing subfloor moisture with a pinless meter or hygrometer box is essential.
Below 75% RH – It’s safe to prime your subfloor and pour directly using standard latex or water-mix compounds.
Above 75% RH – Apply a surface liquid Damp Proof Membrane (DPM) to suppress moisture movement before pouring the levelling layer.
Fixing these two issues before pouring guarantees you’ll have a flat, permanently bonded subfloor that will never flex or hollow out under laminate.
Priming for Every Substrate Type
Different subfloors have different chemical and mechanical rules. It’s no good applying a generic primer across mixed substrates as this will cause delamination.
1. Concrete and Cementitious Screeds
These are absorbent bases that need acrylic priming to manage the suction. Apply a multi-coat system: a diluted penetrating coat to soak into the concrete, followed by a concentrated second coat to form a water-resistant film that holds hydration moisture inside the levelling layer while it cures.
2. Anhydrite (Calcium Sulphate) Screeds
Gypsum screeds expand and break down when they come into direct contact with wet cement. After mechanical sanding, coat the base with an anhydrite-specific acrylic or polyurethane barrier primer. This creates an impermeable film that blocks chemical interaction between the gypsum substrate and the cement-based levelling compound.
3. Legacy Bitumen Residues
Bitumen can’t absorb water-based primers. Where old floor tile adhesive remains firmly attached, scrape back heavy deposits, then apply a grit-aggregate primer formulated for non-porous surfaces. Alternatively, you can use a latex levelling compound designed to bond over firm adhesive residues without needing a separate priming coat.
Applying The Self-Levelling Compound
Pouring a self-levelling compound is a race against the clock. Once water hits the powder, most cementitious mixes give you a working window of 15 to 20 minutes before the product begins to lose flow. Application requires planning and efficiency to achieve a dead-flat substrate for laminate.
1. Water Ratios and Mechanical Mixing
Always add powder to water, never water to powder. Mixing requires a high-torque power mixer running at controlled speeds for two minutes until the compound achieves a lump-free, homogenous slurry. Don’t estimate the water volume by eye or deviate from the manufacturer’s instructions.
2. Controlling Depth with a Gauge Rake
Self-levelling compound doesn’t mean ‘self-finding’. Left alone, thick compounds puddle in depressions and drag over high spots. Use an adjustable gauge rake set precisely to the required depth. Push and pull the wet mix systematically across the floor to distribute mass evenly across the substrate.
3. Spiked Rolling and Wet Edge Management
Immediately after raking, pass a heavy-duty spiked roller through the wet compound. This releases air bubbles trapped during mixing and blends different bucket pours together. Always work sequentially to maintain a continuous wet edge.
Curing Times and Final Pre-Installation Prep
The final step before laying your laminate boards is ensuring the self-levelling compound is completely dry, clean, and flat.
Curing Timeline Guidelines
- Foot Traffic – 2 to 4 hours post-pour under standard conditions.
- Covering with Laminate – Standard compounds need 24 hours for depths up to 5mm. Specialised fast-track or rapid-setting screeds can take floor coverings in as little as 3 to 4 hours, provided moisture tests confirm the layer has dried below critical thresholds.
- Moisture Testing – Always test the cured levelling layer itself, not just the base concrete underneath.
Laying the Foundation for Laminate
- Cleanliness – Thoroughly vacuum the floor. Any gravel or hard debris left beneath the underlayment will create localized stress points under the planks.
- Vapour Barrier – Lay a minimum 200-micron DPM polythene slip-sheet with taped 200mm overlaps over moisture-sensitive bases, turned up 25mm at the perimeter walls.
- Underlayment Selection – Choose a high-density, low-compression underlayment designed for floating click floors. Avoid thick, soft foam underlays, which allow floor vertical movement and cause the joint locks to flex and fail.
Dealing with Problem Subfloors?
If you’re dealing with un-sanded anhydrite, high moisture readings, or legacy bitumen adhesives, Collie Floor Levelling provides professional diagnostics and precision floor pouring.
Get in touch with our specialists to get your substrate prep right the first time – 01206 648 158
