need to provide drainage under some decks boards on a raised horizontal felted roof.Question is can you buy treated timber batten 2" x 1" with semi circular holes in the bottom edge to allow water to drain through.If so what is it called

You can buy tanalised batons but not sure what you mean with regards semi-circular holes for drainage.

do you mean like a first floor balcony? Ie laying batons straight onto the felt roof, cross ways so the decking planks go down the slope?

One solution would be to put the bottom batons at an angle so that rain would run sideways off it. Assuming there is a gradient to the roof.

[Edited on 23/4/10 by JoelP]

like a series of railway arches cut into the underside of the batten so that the water can flow through it.

ive not seen owt like that, but you could easily add a few grooves carefully with a chopsaw.