Recommendations for use of polyethene film for water-proofing of roofs
1979 Edition

IS 7290 (1979) outlines guidelines for using polyethylene film in roof waterproofing without specifying exact types or thicknesses. Generally, low-density polyethylene (LDPE) films are favored for their flexibility and robustness. Thicknesses commonly range between 200 to 500 microns (0.2 to 0.5 mm), with 200–250 microns suitable for standard roofing applications and 300–500 microns preferred for heavy-duty or highly exposed roof areas. Films should be installed with appropriate longitudinal and end overlaps sealed using cutback bitumen to ensure watertight joints and secure adhesion.

Prior to polyethylene film application, the roof surface should be adequately smooth to prevent damage to the membrane. It must be dry and free from loose debris. A bituminous primer is applied uniformly, avoiding excess that might cause slipping or wrinkling of the film. Temperature conditions should be moderate to prevent softening of materials. The film must be laid carefully without overstretching, pressing it firmly with a cloth pad to eliminate air pockets. Workers should use appropriate footwear such as canvas shoes or go barefoot to avoid puncturing the film. Transition areas, like corners and vertical surfaces, need to be protected with fillets and cement plaster to maintain membrane integrity.

The standard recommends the use of straight-run bitumen as the primary bonding material in accordance with IS:73-1961. For bonding the polyethylene film to the roof or between layers, bituminous substances are employed. A cold cutback bitumen, also conforming to IS:73-1961, may be applied over the film at temperatures not exceeding 50°C. Surface preparation requires the substrate to be dry and smooth; primers are not to be applied on wet surfaces. Excessive bitumen application should be avoided to prevent film wrinkling or displacement. The film must not be overstretched during laying, and it should be immediately covered by a bituminous layer and pressed firmly with a cloth pad to ensure firm adhesion and to avoid air entrapment. Black films must contain at least 2% carbon black with particle sizes below 0.06 microns, while natural films require UV stabilization.

Overlaps must be oriented and sealed carefully based on roof type. For flat roofs, the film is laid perpendicular to water flow with overlaps facing downward to prevent water ingress. On sloped roofs, film layers are applied along the slope, whereas on curved shell roofs, laying the film across the slope minimizes excessive laps in valleys. All overlaps should be firmly pressed onto the bituminous layer beneath and sealed using cutback bitumen applied over the lower film layer. Where laps in valleys are unavoidable, an additional strip of polyethylene film of sufficient width should cover the joint. Care should be taken to avoid overstretching, wrinkles, and air pockets, and protective measures like fillets and plaster should safeguard film at horizontal-to-vertical transitions.

To protect the polyethylene membrane, a sequence of finishing layers is advised. Begin with applying bitumen primer at 0.3 to 0.5 kg/m², allowing solvents to evaporate fully. Next, apply a hot coat of straight-run bitumen (minimum 0.7 kg/m²). The polyethylene film is then laid with overlaps sealed using cold cutback bitumen adhesive. Over the film, a cold cutback bitumen top coat of at least 1.0 kg/m² is applied, followed by a blinding layer of fine dry sand ranging from 0.5 to 1.0 kg/m² to shield against mechanical damage and UV exposure. Optionally, the surface can be finished with tiles, cement concrete, or patent stone for enhanced durability. The installation orientation varies by roof type, with films laid at right angles to flow on flat roofs and along the slope on inclined roofs.