What is the minimum overlap required when joining bitumen felts?

A minimum overlap of 75 mm is required for longitudinal (side) laps and 100 mm for transverse (end) laps.

What type of bitumen is used for bonding the felts?

Industrial blown type bitumen conforming to IS 702, typically applied hot at a rate of 1.2 kg/m².

Is a primer necessary before laying the bitumen felt?

Yes, a cold applied bitumen primer conforming to IS 3384 must be applied to the completely dry base surface to ensure proper adhesion of the hot bitumen.

IS 1609

: 1991

Code of practice for laying damp-proofing treatment using bitumen felts

CurrentSpecializedCode of PracticeArchitectural · Waterproofing and Damp-Proofing

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This code specifies the standard procedures and requirements for executing damp-proofing treatments (DPC) in buildings using bitumen felts. It covers surface preparation, application of primer, laying and bonding techniques, and specific requirements for overlaps to prevent capillary moisture ingress.

Code of practice for laying damp-proofing treatment using bitumen felts

Quick Reference Values

Side lap for bitumen felt75 mm (min)

End lap for bitumen felt100 mm (min)

Bitumen primer application rate0.27 to 0.32 litres/sq.m

Hot bitumen bonding application rate1.2 kg/sq.m per coat

Levelling mortar thickness for rough surfaces12 mm (min)

Practical Notes

Ensure the concrete or masonry base is completely dry, clean, and free of dust before applying the bitumen primer to prevent debonding.

Trapped moisture during the laying process can cause blistering; laying should be avoided during rainy or highly humid conditions.

For rough masonry surfaces, a leveling coat of cement mortar (1:3 or 1:4, min 12 mm thick) must be applied, troweled smooth, and cured before commencing damp-proofing.