Felting is a bit 'overkill'. Membrane will do, but I would use a rubber compound sealant on the timber, then you can forget about it. Interlocking tiles good idea from Stoney, if it has to be slate, look at new imitation ones made from weird plastic type stuff, they lock and look right too.
The best way is to use the new Easy Roof System from Permavent. It is really easy to use and will allow any slate to be installed as low as 12.5 degrees. It also improves the overall performance of the roof.
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