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.
