Code of practice for the application of bitumen mastic for waterproofing of roofs
1967 Edition

Per IS 4365, suitable substrates for bitumen mastic include:

• Horizontal and gently sloping surfaces that are smooth, even, and free from irregularities (Clause 4.3.1).
• Vertical or steep timber slopes exceeding 30°, where an isolating membrane is mandatory between the timber and bitumen mastic to prevent adhesion problems (Clause 4.4.1). This membrane should be affixed using galvanized nails conforming to IS 723.
• Surfaces must be clean and thoroughly prepared to bond effectively with blown or residual bitumen (Clause 2.2).

Key considerations:

• The substrate surface should be uniform and crack-free.
• Timber surfaces require an isolating membrane.
• Appropriate bonding bitumen selection depends on local temperature and slope.

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This approach ensures proper adhesion and durability of the bitumen mastic waterproofing layer.

IS 4365 recommends the following for installing isolating membranes on timber roofs:

• Use a hessian-based type 3 felt as specified in IS 1322-1965.
• The membrane must be securely nailed to the timber surface.
• Galvanized round-headed nails as per IS 723 with an 11 mm diameter and 20 mm length should be used.
• Nails should be spaced no more than 150 mm apart.
• Overlaps must be at least 100 mm at the ends and 75 mm along the sides.
• This membrane installation is compulsory for roofs with slopes over 30° between timber and bitumen mastic.
• Timber boards should be well-seasoned, minimum 25 mm thick, tongued and grooved or tightly butted, rigid, and dry before membrane application.
• Adequate ventilation between the roof deck and ceiling must be provided to prevent fungal growth.

Summary Table:

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This installation safeguards against moisture ingress, prevents direct timber-bitumen contact, and extends roof lifespan.

According to IS 4365, the recommended bitumen mastic thickness and number of coats vary based on roof slope as follows:

• For horizontal roofs such as terraces or balconies subjected to foot traffic:

  • Apply two coats:
    • First coat minimum thickness: 10 mm
    • Second coat minimum thickness: 15 mm

• For slopes exceeding 30° and vertical surfaces (excluding timber):

  • Either two coats with a total thickness of at least 13 mm
  • Or three coats with a total thickness of at least 20 mm

• For flat surfaces and gentle slopes:

  • Apply bitumen mastic evenly ensuring the base has a slope of at least 1 in 60 to facilitate drainage

Thickness per coat depends on the maximum practical thickness achievable while the mastic remains warm and plastic (refer Clauses 4.8.1 to 4.8.5).

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This ensures proper waterproofing durability and efficient drainage.

IS 4365 outlines the following procedure for remelting and handling bitumen mastic on-site:

• Utilize a mechanical mixer suitable for the site conditions and roof area.
• Break bitumen mastic blocks (conforming to IS 3037-1965) into pieces and arrange them in layers around the mixer's edges proceeding towards the center.
• Gradually heat the mastic to approximately 200°C, ensuring the temperature does not exceed this limit to prevent deterioration.
• Continuously stir the mixture for uniform consistency.
• Add coarse aggregates only after the bitumen has fully melted, mixing thoroughly until evenly dispersed.
• Transport the molten mastic in small batches using buckets coated with fine inert dust (excluding ash or oil) to avoid contamination and sticking.
• Position the mixer close to the application area to minimize cooling during transfer.

This method maintains the bitumen mastic’s workability and performance on the roof.

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To boost durability and decrease solar heat absorption on bitumen mastic roofs, IS 4365 recommends:

• Spreading light-colored mineral aggregates or pea-sized gravel uniformly over the warm bitumen surface, bonded with bituminous material to increase solar reflectivity.
• Installing cement concrete flooring tiles (as per IS 1237-1959) over bitumen mastic primed with bitumen primer (IS 3384-1965), suitable for roofs experiencing continuous foot traffic. Tiles must be fixed using hot bituminous bonding with carefully sealed joints and edges.
• Applying decorative finishes such as aluminium paint or coloured bitumen emulsion paint compatible with bitumen mastic.
• Preparing the surface by rubbing warm bitumen with clean sharp sand sized between 300 and 850 microns using a wooden float to create texture and keying.

These finishes improve weather resistance, durability, and thermal comfort by reducing heat gain.

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Key dimensions to note:

• Tile joints should be approximately 25 mm wide.
• Space at wall upstands should be around 75 mm.
• Sand particle size for rubbing ranges from 300 to 850 microns.

This ensures a long-lasting, reflective, and serviceable roof finish in line with IS 4365.