Cleft oak shakes are very durable as they are made from the heartwood only . They weather to a nice silvery grey colour and should last 30 to 40 years under normal conditions . They do need a steeply slopeing roof ( greater than 22 degrees ) so that the rain runs off quickly and they will last longer if they haven't got ivy or other undergrowth growing over them which reduces the air flow .
The shakes we usually make are 13 inch(325mm) or 16 inch(406mm) long in random widths from 3inch (75mm) up to 8inch (200mm) . The 12 inch shakes are around 3/4 inch (19mm) thick tapering down to nothing and the 16inch shakes about 1 inch thick tapering to nothing . ,but can be longer if you insist , it just becomes more expensive as the shakes become thicker so you get less per disk and it becomes harder to find the required length of knot free wood .
the number of shakes to the square metre depends on the 'exposure' of the shake . For steeply sloping roofs or buildings where there isn't permanant habitation you can overlap the shakes by a half . For lesser slopes or when you need more waterproofing two thirds overlap is better , but uses a lot more shakes.