Pitched roof and sarking

[S1llyB1lly]

Does anybody here have a good working knowledge of sarking boards in roof applications?

I'm thinking to go with the following construction on the pitched part of my warm roof (top down):

1. Cupa Heavy 3 slate
2. Counter battens / battens
3. Breathable membrane
4. OSB3 or sarking boards
5. PIR insulation between rafters
6. Ceiling plasterboard

Sort of similar to attached image, except I am adding battens / counter battens to attach the slate to.

My questions are:

1. What size gap should I leave between the insulation board and the sarking board, if any?

2. Should the sarking / OSB boards be butted together or should there be a small gap between each?

 

[palaceray]

Normally you leave a minimum of 25mm gap between the insulation and boarding. You can leave a small gap if you wish but I am not sure it is essential.
Usually we fix the counter battens through the boarding into the rafters and fix the membrane on top.
How big is your roof ?

 

[S1llyB1lly]

It's not that big, pls see pic below:

The area in red is the flat roof of my flat and the two areas outlines in green are are the pitched sides.

http://i990.photobucket.com/albums/af24/superiorsounds/Flat Roof Plan 2_zpsgthe598h.jpg