I’m building a summerhouse that is open at the front and enclosed on 3 sides, therefore adequately ventilated. The roof is a pitched roof and on top of the rafters are 22mm x 150mm x 5m boards (like sarking boards). There is no insulation in the roof nor is it required. I would be laying a breathable membrane onto the boards, meaning the membrane is laid flat as there is no way to introduce any sagging for water vapour. The tiles are concrete tiles also. I would then fix the battens as normal, but it would mean there is no air gap at all between the membrane and the boards. Water in effect could collect and sit and against the battens. Considering this is an outdoor garden summerhouse, with lots of open space and ventilation under the roof, is water vapour or rain something I should be worried about? Should I counter batten or is that overkill?