Overview (TL;DR)
Before laying flooring, you often first put down a layer of self-levelling underlayment — pour self-levelling onto the concrete substrate to make a smooth, flat base, then lay SPC, vinyl, engineered wood, tile or carpet on top. Why? Because different floors vary hugely in how flat a base they need: click floors (SPC, engineered wood) fear big dips, while thin glue-down vinyl fears tiny bumps. This article is about base prep before flooring; for the full self-levelling basics, read our complete self-levelling guide.
What is self-levelling underlayment? Why prep the base before flooring?
Underlayment self-levelling is a levelling layer laid beneath a finish — not the final walking surface itself. Mixed with water and poured onto a concrete floor, it self-levels into a smooth, solid base so the flooring on top sits flush and takes load evenly. Compared with hand-applied sand-cement, self-levelling gives easier-to-control flatness, faster work, and the option of a thin layer — which matters a lot when overlaying an existing floor (more below).
Which floors need a self-levelling underlayment first?
In short: the thinner, harder and more click-connected the floor, the more it demands a flat base. From "most demanding" to "most forgiving":
- SPC / engineered wood (click floors) — hard and click-connected, they fear big dips; large unevenness snaps the clicks, lifts edges, and causes hollow "popping" spots
- Thin glue-down vinyl / LVT — thin and soft, stuck flat, they fear tiny bumps; grit and trowel marks on the base telegraph through to the surface
- Ceramic tile — more forgiving, since the installer can vary the adhesive bed, though large-format tile still needs a flat-enough base
- Carpet / carpet tile — most forgiving, though large-area office carpet-tile jobs usually still get a base prep for evenness
"Flat" comes in two kinds: macro-flat vs micro-smooth
People think "flat" is one thing, but for flooring there are two:
- Macro-flat (no big dips or crowns) — for click floors. If the base has clear dips or slopes, a hard SPC/engineered plank sitting over them flexes at the dip, and over time the clicks take load, snap, lift, and sound hollow.
- Micro-smooth (no surface bumps) — for thin glue-down vinyl / LVT. Even if the base is macro-flat overall, any grit, trowel marks or tiny bumps get "mirrored" through the thin, stuck-down vinyl.
Self-levelling fixes both — but which one you're doing it for depends on the flooring you're laying.
When do you actually need it? When can you skip it?
You don't need to pour self-levelling for every flooring job — the practical approach is to check the base with a straightedge first:
- Base is flat, sound and dry — e.g. a new flat's handover sand-cement base is already flat, so you may not need self-levelling.
- A straightedge shows obvious gaps or slope — then self-level first before laying a click floor.
- Laying thin glue-down vinyl / LVT — even if macro-flat, if the surface isn't smooth enough, a thin self-levelling layer for a fine, smooth base is recommended.
One note: an SPC core and pad can bridge tile grout lines and minor micro-texture, but cannot bridge real macro dips — so don't believe "SPC can cover a bad floor." If it passes the straightedge check you save the step; if it fails, don't skip it.
Real Hong Kong scenarios: old buildings, public housing, offices
Old buildings / walk-ups — big level differences + rising damp
Forty-plus-year-old floors often have level differences you can see with the naked eye, plus recurring spring damp. These need a thick-fill grade applied in layers to correct the differences; for damp substrates, prime with a damp-proof primer and allow full drying before laying any impermeable floor.
Public / HOS housing — overhauls laying SPC
Public and HOS flat overhauls love ripping out the old floor and laying SPC, and a thin self-levelling base is a common step. A reminder, though: some contractors pour self-levelling straight over old tiles to skip demolition, which raises the floor so much that the front door almost scrapes the ground and won't fit a door stopper. If you want to save on demolition, work out the height and door-bottom clearance beforehand.
Offices / commercial — carpet tile, sheet vinyl, raised access floors
Offices usually base-prep before laying carpet tile, sheet vinyl or raised access floors; the priority is fast-drying, overnight turnaround that doesn't disrupt the next business day.
The thin-layer advantage: floor height and door clearance
When changing flooring, the height you can add to the floor is often limited. The thicker the base, the more likely the new floor scrapes the door — needing door trimming, or creating level differences between rooms that need transition strips. Sand-cement needs a certain thickness and raises the floor accordingly; where height is tight, self-levelling can go as a thin layer, keeping the build-up to a minimum — especially handy for old-building and public-housing SPC / vinyl overhauls.
How to choose an underlayment self-levelling (with matching primer)
Base prep mainly splits into "thin-layer" and "thick-layer" routes; follow the product TDS for actual grade, thickness and curing time:
Thin base (before thin floor coverings)
GOTECH GT 818 self-levelling — thin, fast-drying, for base prep before SPC, vinyl and other thin coverings.
General interior base levelling (replaces sand-cement screed)
Optimix SF818, Weberfloor 550, Mapei Novoplan DA — mix with water and self-level, replacing hand-troweled screed, flat and fast.
Thick fill, larger differences (old buildings)
Optimix SF806, Weberfloor 230, Sika Sikafloor 357 Level — thick single-pass levelling to correct larger differences.
Matching primer (bonding / damp-proof primer)
Optimix SF80, Weberfloor Primer, Mapei Primer SN — seal substrate pores, boost bond, reduce delamination and bubbling; especially recommended on older, absorbent or damp substrates.
To see the full range and primers, visit our self-levelling range.
FAQ
Do I always need self-levelling before laying SPC?
Not always. An SPC click floor's rigid core and pad can bridge tile grout lines and minor micro-texture; but if the floor has clear level differences (put a straightedge down and you can see the gap), the SPC planks flex over the dip, the click-joints take load, and you get 'popping' click failures and hollow footfall noise. The method is to check with a straightedge first: pass and you may not need it; fail and you should self-level first.
Which is most demanding on base flatness — SPC, vinyl, wood, tile?
Each is demanding in its own way. Click floors (SPC, engineered wood) fear 'macro dips' — big unevenness snaps the clicks, lifts edges and creates hollow spots. Thin glue-down vinyl / LVT is stuck flat, so it fears 'micro bumps' — grit or trowel marks telegraph through to the surface. Ceramic tile is more forgiving because the installer can vary the adhesive bed; carpet is the most forgiving.
Can I pour self-levelling straight over old tiles?
Technically yes (make sure the tiles are well-bonded, clean, and primed with the right bonding agent), but note two things: it raises the floor — the new flooring may then scrape the door, need door trimming, or you can't fit a door stopper; and a smooth tile face must use the right bonding primer. If you want to skip demolition to save money, work out the height first — don't finish the job only to find 'the front door almost touches the ground'.
How thin can the underlayment be? Can it feather to zero?
No. Pourable self-levelling needs a minimum thickness to flow and cure properly and can't be feathered to a knife edge; to take an edge down to zero you use a trowel patching product. For thin base prep choose a thin-layer grade and follow the product TDS thickness range.
Do I need to worry about slab moisture before laying SPC or vinyl?
Very much. SPC and vinyl are impermeable and trap the substrate's moisture. If the concrete or fresh self-levelling isn't fully cured (or the slab lacks a damp-proof layer / has rising damp), trapped moisture leads to mould, musty odours and even adhesive failure. Make sure the substrate and self-levelling are fully dry before laying; damp substrates can be primed with a damp-proof primer first.
What if an old floor has very large level differences?
Old buildings and walk-ups with big level differences need a grade that can be built up thicker, applied in layers — don't pour a thin-layer grade too thick in one pass (it cracks). For very large differences, level with a thick-fill grade first, then finish with a thin-layer grade. Damp old floors are best primed with a damp-proof primer first.
Where can I buy underlayment self-levelling?
Wah Hing stocks underlayment self-levelling and matching primers from several brands, most typically in stock. Visit our Canton Road shop in Mong Kok, or ask us on WhatsApp.
Related articles:
Ready to buy? Wah Hing stocks the self-levelling range, matching primers and installation accessories, most typically in stock — visit our Canton Road shop in Mong Kok or ask us on WhatsApp.
