WATERSTOP Flexible Waterproof Coating

• High-performance liquid roof sealant provides long-lasting surface protection and a weatherproof seal

• Instantly waterproof - can be applied to wet and dry surfaces

• Versatile roofing sealer for flat roofs, gutters, and flashing around roof lights and chimney stacks

• No priming required, apply directly to substrates including concrete, stone, wood, metal, zinc, clay, tiles, slate, felt, polystyrene, and most plastics

• Easy to apply with a disposable brush or a short pile roller

• Solvent, bitumen, and isocyanates-free formula

• Long-lasting protection and resistance to cracking, UV, frost and temperature variations

Preparation

• Ensure the surface is sound, clean and free from dust, dirt, oil, grease, loose paint, stone chippings, rust, standing water and contaminants. Remove moss, lichen and other organic growth.
• New cement-based surfaces, mortar and brickwork must be allowed to cure for at least two months before applying the flexible waterproof coating.
• Protect windows, frames and other surfaces not requiring application with plastic sheeting.
• The waterproof roof coating can be applied to damp surfaces after removing any excess water - no primer needed.
• Store, apply and cure WATERSTOP Flexible Waterproof Coating at temperatures above +5°C. If stored below +15°C, bring to room temperature two hours before application.

Application

• Open the bucket, remove the foil pouch, carefully open with scissors and pour the liquid rubber roof coating into the empty bucket.
• Apply immediately using disposable brushes or short pile rollers as the elastic roof coating will quickly start to cure.
• Ensure complete coverage of gaps and cracks - the roofing sealer for flat roofs will bridge and seal them permanently. For roof cracks over 5mm, fill completely and cover with flexible scrim bedded into the coating.
• Apply two coats at minimum 1.5mm thickness per coat. Ensure continuous, uniform coverage, working the liquid roof sealant well into the surface.
• Allow the first coat to dry enough to walk on (approximately four hours at +20°C) before applying the second coat. Cooler temperatures require longer drying times.
• For corners or areas prone to movement or cracking, bed reinforcing scrim into the first coat. Press the reinforcing scrim into the coating and apply the second coat after four hours.
• Refer to the Technical Data Sheet for full Instructions on how to apply.

Cleaning

• Remove any splashes of the flexible waterproof coating immediately as dried product is difficult to remove. Discard the rollers and brushes after use.

What is the best flat roof sealant?

For preventative protection, we recommend our WATERSTOP Flexible Waterproof Coating. This high-performance roof sealant creates a durable barrier while remaining flexible to accommodate building movement.

What makes it the best choice for flat roofs is its immediate waterproofing action (even if it rains right after), UV and frost resistance, and more environmentally conscious formula with no solvents, bitumen, or isocyanates. It works on most surfaces without primer and is perfect for long-lasting repairs.

For emergency repairs and active leaks that need immediate attention, try our Bostik WATERSTOP Emergency Waterproof Coating. While both products effectively seal against water damage, WATERSTOP Flexible Waterproof Coating is ideal for planned maintenance where long-term durability matters most.

How much roof sealant do I need?

As a general guide, you will need 2.3-3.1kg of WATERSTOP Flexible Waterproof Coating. per square metre when properly applied in two coats. It comes in two sizes: a 6kg tub covering approximately 2-2.5m² and a 14kg tub covering approximately 4.5-6m² of roof area.

To calculate how much roof sealant you need:

1. Measure your roof area (length * width in metres)
2. Multiply this by between 2.3kg or 3.1kg
3. Round up to ensure you have enough liquid roof sealant

Two coats at 1.5mm thickness each are suggested to achieve the recommended 3mm dried film thickness. This ensures optimal performance of the waterproof roof coating. The coverage may vary depending on the substrate, application technique and performance requirements.

How often should a flat roof be coated?

WATERSTOP Flexible Waterproof Coating can last up to 10 years. However, regular inspections are recommended to ensure the seal remains waterproof. Over time, general wear can cause deterioration. If any signs of deterioration appear, reapplication or repairs may be needed to extend the lifespan of the seal.