Grading Roof Slates - Flatter Roofs
The best slaters will carefully grade every slate before it goes on the roof. They grade slate thickness into three groups (thin, medium & thick) using the thinner slates at the top of the roof and thicker slates near the eaves. Usually only a small percentage of slates are bowed or twisted; these are set aside for "cuts" on hips, valleys and verges.
A professionally fixed roof using graded slates will have a wonderful harmony from one slate to the next and will contain no "kickers" – bowed slates that sit up and ruin the uniformity of the roof line.
In general, roofing slates are applied to steeper pitched roofs eg 26 degrees or more. Synthetic slates eg Inspire (www.inspireroofing.com ) that are deliberately bowed when manufactured have a slight camber may be used down to 14 degrees. This creates a downward pressure on the slates when nailed that reduces capillary action of moisture moving up the slate.