Scottish style roof with sarking choice of membrane & n

[manxsparky]

Hi there folks
First post but I have viewed the forum for a while.

Ok I am going to renew the roof on my single story kitchen extension. Its about 70 years old with slates and battens no felt. Its been kingspanned from the under side earler this year.

I want to cover in sarking board, my choices are 12.5mm ply or 15mm OSB then a layer of breathable membrane then slates direct on top of the membrane there are 275 10X20 slates on the roof and a ridge 4 meters long.
Questions
How many man hours to: strip?, Board and Felt? and Slate?

Whats a recomendation for a breathable membrane

Centre or top nail for sarking and 1 or 2 holes?

And lastly and I think this one might be contentious I am thinking about using stainless screws and stainless cup washers instead of ally nails has any one done this if not whats the reasons not to do so?

Thanks Nick

 

[scottishslater]

normally for sarking board we don't use sheets of ply its normally 150mm wide soft wood sarking layed with a 2mm gap between each length normal felt used for tiling does the job, Leaving the 2mm gap between the sarking will allow your roof to breath and allow the wood to expand and contract etc.Normal style of nailing would be head nail and to cheek nail every 2nd or 3rd course also screws would be a bad idea this would be an absolute nightmare to repair normal practice is to use copper nails.