Code of Practice for Laying and Fixing of Sloped Roof Covering, Part I: Slating
1968 Edition

According to IS 5119 Part 1, the recommended slate sizes and thicknesses vary based on roof pitch: common slate dimensions are 60 x 30 cm, 50 x 25 cm, and 40 x 20 cm with thickness ranging from 5 mm to 10 mm. The thickness variation among any batch of 20 slates should not exceed 25%. Roof slopes of 22°, 27°, and 30° correspond to slate sizes of 60 x 30 cm, 50 x 25 cm, and 40 x 20 cm respectively. Additionally, the bottom course of slates projects 7.5 cm beyond the boarding and is laid as double slates with overlapping joints. Metal gutters are used in valleys with slates overlapping by 7.5 cm to ensure effective water drainage and durability.

Per IS 5119 Part 1, slates must be fastened securely on battens (reepers) using flat-headed galvanized wire nails approximately 2.5 mm in diameter and 3 to 4 cm in length. Nails should penetrate the batten by at least 2 cm or two-thirds of its thickness and preferably be coated with anti-corrosive paint; copper or aluminum nails and screws are acceptable alternatives. At junctions such as chimneys, galvanized steel sheet flashing along with a double layer of bitumen felt is used for waterproofing. Valleys should have lead or galvanized steel gutters extending 20 cm on each side, with slates cut and overlapped by at least 7.5 cm over the gutters, edges dressed for a tight fit. This method ensures strong anchorage, effective water shedding, and long-lasting slate roofing.

IS 5119 Part 1 specifies the use of waterproof mortar or similar waterproof materials to grout joints at junctions between the roof and walls. A double layer of bitumen felt is required, especially under rainwater gutters and recommended for critical buildings. Wooden battens (reepers) are affixed on top of the bitumen felt to support the slates, with thicker battens placed beneath the middle of slates to ensure even contact. Slates are inserted into walls to a minimum depth of 5 cm at junctions, and a drip moulding approximately 10 cm above the roof surface is provided to prevent water ingress. The bitumen felt extends at least the length of one slate beneath the bottom course at gutters, securing effective waterproofing and slate support.

To avoid water penetration at junctions, IS 5119 Part 1 mandates the use of galvanized steel sheet flashing at chimney-roof intersections, backed by a double layer of bitumen felt for waterproofing. At wall junctions, slates must be embedded into the wall by at least 5 cm, with a drip moulding installed about 10 cm above the roof surface to divert water away. All joints between the roof and wall should be sealed with waterproof mortar or an equivalent product. In the absence of gutters or flashings, these measures are critical to prevent leakage and maintain the roof’s integrity.

Inspection standards per IS 5119 Part 1 include verifying that slates are rectangular and uniform in size, commonly 60x30 cm, 50x25 cm, or 40x20 cm, with thickness between 5 to 10 mm. Thickness variation among any 20 slates should not exceed 25%. Slates must be uniform in color and texture, free from white patches, and brittle rather than tough. When struck, they should produce a ringing sound and withstand a drop from 1.25 meters without breaking. Water absorption must be no more than 2% by weight after six hours immersion, and water rise on edges should be under 12 mm without swelling or flaking. At junctions with walls, slates must be embedded at least 5 cm deep, with drip moulding about 10 cm above the roof surface and joints grouted with waterproof mortar to prevent leaks. These criteria ensure the durability, watertightness, and structural soundness of the slate roof.