Gap between sarking boards at ridge?

[aa44]

Can somebody please tell me how my ridge is supposed to be built? I think that I have got to grips with most of the roof but the ridge still baffles me.

My new house will have a slate roof. As we are in Scotland the roof will have plywood sarking boards so the roof construction will be:

slates
battens
counterbattens
membrane
plywood sarking boards
50mm ventilation gap
insulation between rafters
vapour control layer
plasterboard

There will be ventilation in the soffits to get an airflow under the sarking boards and there will be some sort of ventilated dry ridge system at the top.

My question is how big should the gap be between the top edge of the sarking boards at the ridge. Presumably they should stop just short of the ridge on each side to provide a ventilation path. The roof will be constructed with attic trusses so there is no ridge board.

Assuming that there is a ventilation gap between the sarking boards at the top, should the membrane on one side be lapped over the top to the other side or does this defeat the object of the ventilation gap?

If you could help to clear my confusion then I would be very grateful.

Thanks
Andrew