Rough Screed & Self-Leveller: Steps
Golden rule (sequence)
Demolition → MEP rough-in (pipes, conduits, drains) → Pressure tests → Rough screed → Waterproofing (wet rooms) → Tiling / or Self-leveller → Final floor.
Cutting corners here = uneven floors, hollow tiles, warped wood, and moisture problems.
Part A — Rough screed (traditional/base screed)
Purpose
Create structural, level base, embed falls (wet rooms), cover services, and set finished floor heights (FFH).
Substrate prep
- Clean slab; remove dust, laitance, oils.
- Mark datums (laser), FFH, falls to drains; set perimeter isolation strip.
- Check MEP: pressure test plumbing; confirm conduit depths; protect services.
Mix & placement
4) Choose system: bonded / unbonded / floating (over insulation) per design.
5) Prime (for bonded) or install slip membrane (for unbonded/floating) as specified.
6) Place screed (semi-dry or flowing); compact and rule to guides; form required falls in wet rooms.
7) Create control joints and respect structural joints.
Early care
8) Protect from rapid drying (draft, sun); no trafficking until initial cure.
9) If UFH in screed: install pipes/cables on clips; pressure test; pour; later commission with gradual heat-up after cure.
Quality checks (hold points)
- Flatness: typically ≤ 3–5 mm under 2 m straightedge (project spec prevails).
- Thickness per system; no exposed services.
- Moisture before finishes: test with CM/RH method to product limits.
- Falls verified at drains (laser/level).
Common mistakes
- Skipping slip membrane on unbonded builds → curling/cracks.
- No isolation strip → sound bridging and edge cracking.
- Bad curing → dusting/weak surface (later debonding).
- Wrong FFH coordination → doors/threshold clashes.
Part B — Self-levelling compound (SLC / self-smoothing)
Purpose
Fine-tune flatness/planarity to SR1 for tiles, LVT, wood/engineered wood, or large-format stone/porcelain. Typical build 3–15 mm (check data sheet).
Pre-checks
- The rough screed is sound, dry, and clean; remove laitance by grind/shot-blast if required.
- Fill cracks/voids; honor movement joints.
- Verify moisture meets SLC and final floor limits (CM/RH).
Priming & edge control
4) Apply manufacturer-approved primer (absorbent vs non-absorbent substrates differ).
5) Seal edges/penetrations to prevent leaks; leave perimeter expansion strip.
Pour & finish
6) Mix exactly to ratio (clean water); honor pot life.
7) Pour wet-on-wet, gauge with spike roller to de-air; maintain continuous flow room by room.
8) Respect min/max thickness and ambient conditions (temperature, RH, airflow).
Cure & readiness
9) Light foot traffic after initial set; do not trap moisture (no plastic sheeting).
10) Test surface hardness/adhesion if specified; then proceed:
- Tiles/stone: C2/S1/S2 adhesive as required.
- LVT/wood: check residual moisture and manufacturer’s flatness tolerances.
Common mistakes
- No primer / wrong primer → debonding or pinholes.
- Drafts/sun during cure → crazing and differential gloss.
- Over-watering mix → weak surface and dusting.
- Ignoring movement joints → reflective cracks later.
Wet rooms (special notes)
- Build falls in the rough screed, not with SLC alone.
- After screed cures: apply full waterproofing system (primer + band/tape + 2 coats) before tiling.
- 24-hour flood test prior to finishes.
Underfloor heating (UFH)
- Embed in rough screed or place below; never in thin SLC unless system-rated.
- Commission only after screed cure: raise flow temperature gradually; switch off during SLC pour and until cured.
Tolerances & targets (guide)
- Rough screed flatness: typically SR2 (≤5 mm/2 m) unless specified tighter.
- SLC flatness: aim SR1 (≤3 mm/2 m) for LVT/wood/large tiles.
- Moisture: comply with adhesive/flooring limits (e.g., ≤2.0 CM% cement screed for many resilient floors—follow product data).
Coordination & sequencing wins
- Lock FFH early (include tile + adhesive + SLC + underlay thickness).
- Install door frames after screeds/SLC where possible to avoid trimming.
- Keep other trades off fresh layers; protect with boards once traffic resumes.
Cost & programme (percentages only; baseline = rough screed only = 0%)
- Add SLC (to achieve SR1): +20–50% materials, −10–25% labour on tiling/wood (less fiddling/rework).
- Rework risk if SLC skipped on marginal substrates: +20–40% (lippage, hollow spots, call-backs).
- Programme impact: SLC adds +5–15% time but can cut overall schedule risk −10–20% by smoothing downstream trades.
Quick checklist (print-ready)
- ☐ MEP pressure tests passed before screeds
- ☐ Datums/FFH marked; isolation strips installed
- ☐ Bonded/unbonded/floating method confirmed
- ☐ Moisture tests passed before waterproofing/SLC/finish
- ☐ Primer type matches substrate & SLC
- ☐ Movement joints carried through layers
- ☐ Wet rooms flood-tested 24 h
- ☐ UFH commissioned after cure (gradual heat-up)
