What is the minimum recommended overlap for bituminous felt sheets?

A minimum side lap of 75 mm and a minimum end lap of 100 mm should be maintained (Clause 6.3.2).

How many layers of felt are required for a flat roof?

It depends on the slope and rainfall conditions. For a nearly flat roof with a slope up to 1 in 80, a five-course treatment is recommended (Table 1).

What is the correct way to apply bonding material?

Hot bitumen at about 1.2 kg/m² should be applied uniformly just ahead of the felt roll, so the felt is laid on a hot, flowing layer of bitumen (Clause 6.3.1).

Can this waterproofing be applied to vertical surfaces like retaining walls?

Yes, Clause 7 specifies the method for vertical surfaces, typically involving a two-course treatment with a bitumen primer.

IS 9862

: 1981

Code of practice for waterproofing of roofs and walls with built-up bituminous felts

CurrentFrequently UsedCode of PracticeBIMMaterials Science · Waterproofing and Damp Proofing

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This standard provides a code of practice for the waterproofing of roofs, walls, and other building surfaces using a built-up system of bituminous felts. It covers material selection, surface preparation, application techniques for multiple layers, and finishing treatments to protect the waterproofing system. The code aims to ensure a durable and effective barrier against water ingress.

Provides recommendations for the design and installation of built-up bituminous felt systems for waterproofing roofs and walls.

Quick Reference Values

Minimum side lap for felt75 mm

Minimum end lap for felt100 mm

Bonding bitumen quantity per layer1.2 kg/m²

Primer application rate0.25 to 0.50 litres/m²

Minimum slope for flat roofs for drainage1 in 80 to 1 in 120

Bitumen heating temperatureShould not exceed 200°C

Practical Notes

Ensure the concrete or masonry surface is completely dry, clean, and free of loose material before applying the primer for proper adhesion.

Special attention must be paid to detailing at parapet junctions, drain outlets, and expansion joints, as these are critical areas prone to leakage.

The final protective layer (e.g., stone grit, tiles) is essential to shield the bitumen felt from UV degradation and mechanical damage, significantly extending the life of the waterproofing system.