How to Use Plastic Sealant and Acrylic Plastic Cement on Roofs

The plastic sealant used on roofs is an acrylic paste designed to fill cracks, joints and areas susceptible to water ingress. Used before waterproofing, it bonds to the substrate, accommodates minor building movement and helps stop rainwater reaching the inside of the concrete slab.

What is acrylic plastic cement?

Acrylic plastic cement is a paste-like repair filler, usually water-based, intended to repair small openings on external surfaces. It can be applied straight from the tub with a filling knife and serves as a preparation stage before the waterproofing product is applied.

Areas commonly treated with the product include:

• Surface cracks in concrete slabs;
• Joints and seams between different materials;
• The bases of water tanks, pipework and rooflights;
• Junctions between walls, roof tiles and roof structures;
• Small areas where moisture is beginning to penetrate.

Why does the material withstand rain and temperature changes?

Once dry, the acrylic formulation retains a degree of flexibility. As a result, the coating can accommodate slight expansion and contraction caused by heat, cold and normal roof movement without immediately detaching.

Certain products also contain reinforcing fibres, which increase the paste's strength and help it bridge the crack. Its resistance to rain, however, relies on complete curing: the sealant must not be applied while it is raining or onto a waterlogged substrate.

How should plastic sealant be applied directly to a crack?

The surface must be sound, dry, and free from dust, grease, loose material or peeling waterproofing. Applying the paste over contamination can prevent proper adhesion, causing the seal to come away with the old coating.

• Open up and thoroughly clean the crack;
• Remove loose particles using a brush;
• Push the product into the opening with a filling knife;
• Smooth the paste across the edges of the crack;
• Place reinforcing mesh between two coats in more demanding areas;
• Allow the stated drying time before waterproofing.

When is the paste not enough to solve the problem on its own?

Acrylic plastic cement is intended for ordinary cracks and localised repairs. Wide openings, holes, broken components or cracks that continue to widen require repair with a suitable building material before the filler is used.

The following signs may suggest that the leak has a more serious underlying cause:

• A crack reappears shortly after it has been repaired;
• Sections of concrete are breaking away or showing rust;
• Water pools because there is insufficient fall or drainage;
• Water ingress extends across a large section of the ceiling;
• Noticeable movement occurs between slabs, walls or roof tiles.

Sealing works best before waterproofing

Although the sealant repairs specific areas, it does not replace the complete waterproofing layer on a roof. Once the repair has dried, applying a compatible system protects the entire surface and limits water entering through pores and other cracks that may still be difficult to see.

The outcome depends on cleaning, the thickness applied and adherence to the drying period, which may vary according to the product and weather conditions. Applied to a properly prepared substrate, the acrylic filler strengthens cracks and joints before rain turns minor defects into drips and damp patches.