SLATE: MINIMUM ROOF PITCH
The Australian Standards makes no recommendation for the minimum roof pitch for slate. It has references to roof pitches as low as 17.5 degrees in relation to sarking requirements (Table 3). One would have to assume that slates fixed on roofs with less than 20 degree pitches would need to be in very benign climates with large headlaps. In such circumstances an under-surface membrane / sarking would be a critical design factor.
Welsh Slate's brochure on Roofing Slate (downloadable from www.welshslate.com ) indicates a minimum rafter pitch of 20 degrees.
Our suggestion is not to contemplate using slates on roofs with a pitch less than 20 degrees and then only for big format slates eg 500x300mm. At low roof pitches, a building will be vulnerable to high rainfall, high winds or a maritime location that combines both.
Inspire composite slates which are manufactured with a convex camber may be used down to 14 degrees. Unlike genuine slates, when Inspires are nailed they flatten the slate so that there is a positive downward pressure on the slate. This reduces the movement of moisture up the slate by capillary action. It is a very thoughtfully made slate by engineers who understand the principles of slate roofing.