Everything about slate roofing

Discover the unique Brazilian slate from PIATRAONLINE (Nero brushed and riven, Multicolor brushed and riven) designed for slate roofing. Here are some technical data to be considered for purchasing.

For avoiding any misunderstanding, we will use the next definitions:

• roof framing – the structural frame of a roof and usually made of metal, wood, concrete. Roof framing depends, among other things, on the type of roof and its weight.

• roof boarding – the rigid element of a roof; supports slate tiles and is made from wooden planks, placed on the rafters. Recommended length of wooden planks is minimum 1.2 m, sufficient for 3 rafters. Workers will move on a line that connects the rafters, not between them. A waterproof foil is installed over the roof boarding.

• slope – defines the steepness of a line.

Slate roofing is actually 20% from the global tile roofing market. Slate are fixed with glue or nails (copper is best, 38-63 mm length). For strong wind areas, tiles fastening in 3 points is highly recommended. Although installing them with adhesive results cheaper, there is a risk of moisture between the slate and roof. Besides, with adhesive, roof will dry slowly after rain and you have also issues with melting snow. Installing slate roofing with adhesives is then recommended in drier areas, with perpendicular joints technique as standard. 

Technical data for slate roofing:

• Water absorption rate below 0.3%

• Calcium carbonate concentration below 3% to avoid the outcome of acidic rains

• Breaking module (flexural resistance) of approximately 600-650 kg/cm ² for superior wind-resistance 

• Low iron ore concentration (pyrite, hematite) to stay away from oxidation  

There are many size and weights available for slate roofing tiles (most common: 600 x 300 mm). Based on thickness, you can use 4 mm tiles (lighter) or 5, 6, up to 9 mm (heavy plates). For wide slate tile roofing, lay out the widest at the base of your roofing. PIATRAONLINE offer slate tiles of 300 x 150 mm size and 7-8 mm thickness.

Slope of slate roofing starts at 20%. Anything under this slope is considered flat roof, therefore fixing slate tiles with nails will no longer ensure waterproofing. Roofing made with PIATRAONLINE’s slate tiles require a minimum roof slope of 30%. Slope is closely related to tile exposure. Slope is measured as the ratio between roof’s maximum height and its opening. E.g.: a roof with 3 m height and 6 m opening, has a slope of ½, corresponding to 45°. Slanted roofs belong to cold roof category (low vented roof). There are 2 types of cold roofs: attic roof and no attic roof.

In the covered area of fixing tiles with nails by roof boarding, you will have 3 tiles overlapping, a requisite for waterproofing and snow proofing. The overlay area depends on roof slope and tile size. According to calculation a 20% slope needs an overlapping on an area with a minimum length of 115 mm, and for 35% slope, required overlapping is 75 mm. 

Example for Nero slate, riven and brushed finish, 300 x 150 mm size, medium weight / tile = 600 g. According to balance of forces on a inclined plane, theoretically, for a 30° slope, there is a balanced load distribution, half is supported by roof boarding and half by fixation system. You can calculate again based in roof slope and obtain essential info on nail size but also for roof boarding. For such tiles, minimum roof slope is 30%. It can be estimated that for a normal exposure, number of tiles / sqm in this example is 60 (with 3 tiles overlapping), which results in 720 kg total weight, for 20 sqm roofing. Rafters for roof framing, roof boarding and the other roof elements will be resized accordingly. Tiles for slate roofing can also be used for exterior cladding and for this installation you have to use adhesive and a notched trowel to “comb” the area.