High-Strength, Fast-Setting Self-Levelling Compound for Interior & Select Exterior Floors

Bostik Cempolay Ultra Strong is a high-performance, cement-based self-levelling compound formulated for smoothing uneven floors prior to laying final floor coverings. With exceptional compressive strength and rapid setting properties, it is ideal for both residential and commercial environments. Suitable for application on a wide range of substrates, it is compatible with underfloor heating systems and can be used for certain exterior applications.

Key Features & Benefits

Triple-Function: Levels, smooths, and strengthens floor surfaces in a single application
High Compressive Strength: ≥20 N/mm² after 28 days (EN 13813 – CT-C20-F6)
Fast-Setting: Foot traffic in as little as 2 hours; tile in 3 hours (under 10mm)
Extended Thickness: Apply from 3mm up to 20mm in one layer
Underfloor Heating Compatible: Suitable for warm water systems (max 27°C surface)
Water & Frost Resistant: Can be used in certain external environments

Product & Technical Details

Form: Grey, free-flowing cement-based powder
Bag Size: 20kg
Coverage:
- Approx. 4m² at 3mm thickness per 20kg bag
- Approx. 0.6m² at 20mm thickness
Mix Ratio: 4 litres clean, cold water per 20kg
Working Time: 15–20 minutes at 20°C
Foot Traffic: Approx. 2 hours
Drying for Tiling: 3 hours (ceramic, under 10mm), 24 hours for impervious coverings
Application Temp: +5°C to +27°C
Shelf Life: 12 months unopened in cool, dry conditions (5°C–25°C)

Suitable Substrates

Concrete & cement/sand screeds
Stone, quarry tiles & unglazed ceramic tiles
Calcium sulphate screeds (with specialist priming)
Previously levelled or primed floors

Surface Preparation & Priming

Ensure surfaces are clean, dry, structurally sound and free from laitance, grease, oil or dust
Substrate moisture should be below 75% RH (use a hygrometer)
Prime porous surfaces with Bostik Universal Primer (1:2 dilution)
Prime non-porous surfaces with neat Bostik Universal Primer

Application Instructions

Mix 20kg of powder with 4L clean water to a smooth, lump-free consistency
Pour onto the primed subfloor and spread using a smoothing trowel
Use a spiked roller to release trapped air and finish the surface
Allow to cure per thickness and environment before applying finishes

Limitations & Precautions

Not suitable for use on timber floors, vinyl, tarmac or weak adhesive residues
Do not apply if temperatures will drop below 5°C within 24 hours
Not intended for movement-prone or flexible substrates
For thick applications >10mm, dry sharp sand can be used (not for heated floors)

Standards & Compliance

EN 13813 Classification: CT-C20-F6
Compressive Strength: ≥20 N/mm²
Flexural Strength: ≥6 N/mm²
Fire Classification: Class F

Suitable for Use With

Ceramic, porcelain and natural stone tiles
Laminate, wood, carpet and vinyl (after full cure)
Underfloor heating systems (water-fed, not electric)

Bostik Cempolay Ultra Strong offers a professional, high-performance floor smoothing solution with rapid setting and high durability. It’s ideal for preparing surfaces in time-critical installations across domestic, commercial, and light industrial spaces.

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Bostik Cempolay Ultra Strong – 20kg - Fast-Setting Self-Levelling Compound