How to pour self-leveling underlayment — substrate prep, primer, mixing, gauge rake, spiked roller, thickness limits, material calculation, and mistakes that ruin laminate and tile installs.

Why self-leveling compound matters on the job

A floor that looks flat to the eye is often anything but flat to a laminate click-lock, large-format tile, or vinyl sheet. High spots grind through underlay; low spots flex and crack; doors drag after the finish floor goes down. Self-leveling underlayment (SLU) — sometimes called self-smoothing screed — fills low areas and creates a plane within millimeters so the finish layer performs as designed.

This is not the same as structural screed (50–80 mm sand-cement layer that carries load). Self-leveler is typically 3–30 mm of gypsum- or cement-based flowable compound over a stable base. Use it when the substrate is sound but uneven, when you need to bury floor heating elements, or when manufacturer specs demand flatness (often ≤3 mm deviation over 2 m for floating floors).

Skip self-leveler when the slab is moving, cracked through, or soft — fix structure first. Skip it when you need slope to a drain — use a trowel-applied mortar bed instead; most SLU products are not for single-plane drainage slopes.

What you need before you start

Tools

Gauge rake (pin rake) — spreads compound to target thickness; pin height sets depth (e.g. 5 mm)
Spiked roller (spike roller / aeration roller) — removes air bubbles after pour; mandatory on large areas
Mixing drill and paddle — low-speed, high-torque; not a hammer drill on high RPM
Measuring buckets — for exact water per bag; mark water line inside bucket
Long straightedge (2–3 m) — check substrate before and after
Laser level or water level — find high and low points; set datum height at door threshold
Vacuum and stiff broom — dust is the #1 bond killer
Paint roller or brush — for primer
Needle roller — for tight areas and along walls where spiked roller does not reach
Floor squeegee — optional for pushing material on very large pours
Door threshold strips / damming tape — contain flow at doorways and drains

Materials

Self-leveling compound — gypsum-based (interior, dry areas) or cement-based (some wet-area and exterior-rated lines)
Primer — matched to compound and substrate (porous concrete vs non-absorbent old tile)
Damming foam or silicone — at door sills and pipe penetrations
Fiberglass mesh tape — at crack repairs in substrate before primer (if manufacturer allows)
Repair mortar — patch holes deeper than the pour depth before SLU

Safety and PPE

Dust mask (P2/P3) when grinding high spots or sanding repairs
Gloves and eye protection — alkaline wet mix irritates skin
Ventilation — gypsum SLU generates heat in bucket; do not mix large batches in closed rooms
Slip hazard — wet compound is extremely slick; rope off the room

When to call a pro: slab deflection, unknown asbestos tile, heated slabs with failed elements, or pours >30 mm total without engineered build-up system.

Time and conditions

Substrate moisture: concrete often ≤4% CM or per primer datasheet; damp slabs cause blistering
Ambient temp: commonly +5 °C to +30 °C during pour and 48 h cure; some products +10 °C minimum
Working time: mixed material may set in 15–30 minutes — plan batch size
Walkable: often 4–6 h; finish floor may need 24 h–7 days — read bag
No direct airflow on surface during first hours (drafts skin the top and trap voids)

How to choose self-leveling compound

Gypsum vs cement-based

| Type | Best for | Avoid when |

|------|----------|------------|

| Gypsum SLU | Dry interior rooms, over cement board or concrete, under LVT/laminate | Regular wet rooms, exterior, constant moisture |

| Cement SLU | Some bathrooms, areas needing higher hardness | Very thin pours over non-porous without right primer |

| Fast-set / fiber-modified | Repairs, thin lifts, commercial turnaround | Large beginner pours — sets too fast |

Thickness limits

Every bag states minimum and maximum thickness per layer. Typical:

Single pour: 3–20 mm common; some products allow up to 30 mm in one lift
Deep fills: use extended-set or two pours with primer between — never exceed max per layer
Over radiant heat: use SLU rated for underfloor heating; max thickness often reduced

Budget / standard / premium

Budget: basic gypsum SLU, single primer coat, manual mixing — OK for small rooms <15 m²
Standard: fiber-modified compound + matched primer + gauge rake rental — most apartment jobs
Premium: pump-applied SLU, moisture-tested slab, documented flatness — commercial and large open plans

Step-by-step installation

Step 1: Assess and mark the substrate

Measure flatness with 2 m straightedge — gap at center shows low/high. Mark lows with pencil.
Check door threshold height — finished floor height minus finish layer minus SLU must leave clearance for door swing.
Identify high spots >3 mm — grind with diamond cup on angle grinder (dust control!) or use self-leveler only in lows if highs are acceptable.
Tap test for hollow tile or delaminating old vinyl — remove unstable layers.
Cracks > hairline: chase, vacuum, fill with repair mortar; structural cracks need evaluation before covering.

Step 2: Clean, repair, and prime

Vacuum entire floor twice — primer fails on dusty concrete.
Remove oil, paint drips, curing compounds — degrease if needed.
Apply primer with roller; cut in edges. Primer color change shows coverage.
Drying time for primer — often 1–3 h, tacky-not-wet before pour; do not pour on standing puddle primer.
Dam openings: silicone or foam at doorways; wrap pipe bases; do not block floor drains intended to receive flow — use drain dam kits.

Step 3: Mix correctly (non-negotiable)

Use exact water per bag — more water = weaker, more shrink, more cracks. Weigh water if specs give kg not liters.
Mix one bag at a time until smooth 45–60 seconds; scrape bucket sides.
Do not re-temper with water after mix starts to stiffen — discard and mix fresh.
Batch planning: at 20 m² and 5 mm depth you may need 150+ kg material — multiple mixers or continuous mix station on large jobs.
Temperature of water — cold water slows set; hot water accelerates — use as spec directs.

Step 4: Pour, spread, and roll

Start at far corner, work toward door — you cannot walk back through wet material.
Pour ribbons, spread with gauge rake set to target depth (e.g. 5 mm).
Immediately pass spiked roller perpendicular to spread direction — two passes minimum on large floors.
Needle roller along walls and columns.
Material self-flows — do not over-work with rake after rolling or you drag cured grains.
Working time: if mix stiffens, stop — feathering stiff mix creates ridges.

Step 5: Cure and protect

No foot traffic until walkable time — typically 4 h minimum.
No heavy loads or finish installation until full cure — often 24 h for light vinyl, 48 h–7 days for tile or epoxy.
Protect from direct sun through windows — rapid dry crazes surface.
Verify flatness after cure with straightedge before finish floor — skim additional thin pass only if product allows minimum thickness.

Common mistakes and how to avoid them

Pouring on unprimed dusty concrete — delamination under vinyl within months.
Wrong water ratio — #1 strength failure; use scales.
Pour too thin in lows — below product minimum mm; layer crumbles.
Single batch for huge floor — front of pour sets while back is still mixing → cold joints and ridges.
No spiked roller — air bubbles telegraph through thin LVT and show as dimples.
Ignoring max thickness — slumps, cracks, exotherm heat build in thick lifts.
SLU for shower slope — water pools; use mortar bed slope instead.
Covering heating cables without rated compound — overheating, voids, cable damage.
Pouring with open window in winter — substrate freezes overnight; top cracks.
No dam at doorway — compound flows into hallway; lawsuit-grade mess.

Troubleshooting

Powdery surface after cure

Cause: over-watered mix, draft drying, or poured below minimum thickness
Fix: Remove weak layer, prime, repour thin pass if depth allows

Cracks map-pattern across room

Cause: substrate movement, no expansion gap at walls, poured on uncured repairs
Fix: Map cracks to substrate; if slab moves, SLU is wrong product — use flexible system or isolate

Bubbles visible in cured surface

Cause: insufficient spiking, porous substrate outgassing
Fix: Grind bubbles if shallow; fill with repair; next pour spike more, use outgassing primer

Compound does not flow to marked lows

Cause: mix too stiff (old bag, low water), or beyond flow distance rating (often 3–5 m from pour point)
Fix: Pour closer ribbons; check water; use extended-flow product for large rooms

Door sticks after pour

Cause: datum height not calculated; pour too thick at threshold
Fix: Grind threshold line before finish floor or plane high edge — prevention beats grinding

How much material you need

Use the Self-Leveling Floor Calculator on Yardox:

Enter room length × width (or area directly if already measured).
Set thickness in mm — average depth to fill, not maximum depth only (estimate from straightedge gaps).
Consumption from bag — often 1.5–1.8 kg/m²/mm; default 1.7 is a reasonable start.
Bag size — typically 20 or 25 kg.
Waste 5–10% — spills, bucket residue, uneven substrate absorption.

Example: Room 4 × 5 m = 20 m², average fill 5 mm, consumption 1.7 kg/m²/mm, waste 5%:

Mix = 20 × 5 × 1.7 × 1.05 = 178.5 kg
Bags (25 kg) = 8 bags

If lows need 8 mm but highs get 2 mm, use area-weighted average or divide room into zones with separate calculations.

Chain calculators: Screed Calculator for structural base → Self-Leveling Floor → Laminate or Tile for finish quantities.

FAQ

Can I self-level over old tile?

Only if tile is fully bonded, flat, and primer system is approved for non-absorbent substrate — often need special primer plus thin pass. Loose tile must come off.

Minimum thickness?

Most products 3 mm minimum — thinner lifts fail. Do not use SLU as a skim thinner than spec.

How long before laminate?

Usually 24–48 h dry subfloor moisture check for laminate specs — some brands require ≤2% CM on gypsum SLU.

Can I heat the room to speed cure?

Follow manufacturer — uncontrolled heat cracks gypsum. Gentle +15 to +25 °C stable is ideal.

Two layers allowed?

Yes with primer between when each layer respects min/max; second layer often next day.

Over plywood subfloor?

Only with approved primer + flexible SLU rated for wood — deflection must be within floor system limits; screw plywood every 150 mm on joists.

Bathroom floor?

Use cement-based SLU rated for wet areas if used at all — many installers prefer mortar bed in wet zones; never assume gypsum SLU is tanking.

Pump vs hand mix?

Pump for >100 m² or continuous depth; hand mix fine under 30–40 m² with two people.

Why did it set in the bucket?

Hot room, old material, waited too long, or mixed partial bag — always mix full units.

Do I need mesh in the SLU?

Usually no in the pour itself; mesh belongs in substrate repairs or separate uncoupling mats under tile — follow finish floor spec.

Summary checklist

Flatness survey done; highs ground; lows mapped
Door height and finish stack calculated
Substrate clean, stable, repaired, primed, dams in place
Correct product for room type (dry vs wet, wood vs concrete)
Water measured exactly; batches sized to working time
Gauge rake depth set; spiked roller ready
Pour plan from far corner out; spike within working time
Cure protected; flatness rechecked
Material quantity from Yardox calculator with waste
Finish floor install only after full cure per bag and finish manufacturer

Self-leveling compound buys you a flat datum — not structural repair. Respect thickness limits, primer, and working time, and the finish floor above will click, stick, or lay flat the way the client expects.

Underfloor heating before self-leveling

When cables or mats sit below SLU:

Heating must be off during pour unless manufacturer says otherwise; often test before, off during, gradual heat-up after cure
Embed cables at depth specified — often 3–5 mm minimum cover above element
Use flexible, fiber-reinforced SLU rated for heated floors
Do not exceed max thickness over cable — overheating kills elements

Large rooms and cold joints

For >40 m², organize pour zones with temporary dams or sequential days:

Plan joints at doorways or under future walls — not random mid-room unless covered by partition
Prime cured edge before adjacent pour next day
Feather joint with gauge rake — do not leave ridge at zone boundary

Moisture testing before sensitive finishes

Laminate and engineered wood fail from subfloor moisture, not visible dryness:

Concrete: CM meter or calcium chloride test per finish floor spec
Gypsum SLU: often must be <0.5% moisture before non-breathable vinyl — hygrometer readings on surface

Document numbers if commercial job — disputes happen months later.

When NOT to use self-leveler (use trowel screed instead)

Shower floors needing slope to drain
Exterior balconies without rated exterior compound
Structural leveling >30 mm total without engineered build-up
Active moving cracks in slab
Sound isolation mass — SLU is not acoustic mat

For structural thickness, use Screed Calculator and sand-cement or lightweight screed first, then thin SLU cosmetic pass if needed.

Self-Leveling Floor Compound: Complete Installation Guide