What is the minimum recommended slope for a concrete roof for waterproofing?

A slope of at least 1 in 100 should be provided to ensure effective drainage towards outlets (Clause 3.1).

How far up a parapet wall should the waterproofing extend?

The waterproofing treatment should be carried up parapet walls and other vertical surfaces for a minimum height of 150 mm above the finished roof level (Clause 3.4).

What are the required overlaps for bitumen felt layers?

A minimum lap of 100 mm at longitudinal joints and 150 mm at end joints is specified for bitumen felt sheets (Clause 4.3.2).

Should a protective layer be applied over the waterproofing?

Yes, a protective surface finish like gravel spreading, lime-surkhi concrete, or tiles is recommended to protect the treatment from sun exposure and mechanical damage (Clause 4.3.4).

IS 6494

: 1988

Code of practice for waterproofing of concrete roof slabs

CurrentFrequently UsedCode of PracticeBIMArchitectural · Waterproofing and Damp Proofing

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This code of practice provides guidelines for the waterproofing of flat and sloped concrete roof slabs in buildings. It covers various waterproofing systems including bitumen felts, mastic asphalt, and plastic sheeting, detailing material requirements, surface preparation, application techniques, and protective finishes. The standard emphasizes the importance of proper drainage, detailing at junctions, and maintenance for long-term effectiveness.

Lays down the recommendations for waterproofing of various types of concrete roof slabs using different materials and techniques.

Quick Reference Values

Minimum slope for roof drainage1 in 100

Minimum upstand height at parapets/vertical surfaces150 mm

Longitudinal lap for bitumen felts100 mm

End lap for bitumen felts150 mm

Typical thickness for mastic asphalt treatment20 to 25 mm (in two layers)

Practical Notes

The single most critical factor for successful roof waterproofing is providing an adequate slope (at least 1 in 100) for positive drainage, which is often overlooked in construction.

Failures most commonly occur at junctions, such as with parapet walls, pipes, and drains. Meticulous detailing and execution in these areas are non-negotiable.

Ensure the concrete surface is completely dry, clean, and free of loose material before applying any primer or waterproofing layer to achieve proper adhesion.