Warm roof and Installing osb plus cutting my own firring strips.

19 Feb 2020
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United Kingdom
For my bottom deck I have 8x4 sheets of 18mm OSB for installing on my 150mm insulated warm roof onto 9x2 joists spaced 400mm apart. When installing my osb onto my firring strips, how many nails(with the nail gun) should i use for each board. The reason i ask this is when im screwing my 200mm screws into my joists, my thinking is if i have to many nails in the firrings, it gives me more chances of screwing onto of the nails which could cause issues having to find a new location for the screws/thermal bridging caps and end up having to many holes in the osb for when installing the EPDM. Or am i overthinking this. Also should i leave a 12mm gap around where the OSB meets the block on the parapet and 2mm between each board glued.

I cant seem to get any firring strips local or delivered and am now thinking of making my own. The 9x2 firring strips i was wanting to purchase was 95mm tappered down to 0mm and are 4.8m in length! From one end of the building to the other side, i have 5.1m to create a slope. I can purchase 9x2s in 5.4m lengths and could cut slope in one go instead of two. My taper is going to be for every meter i will drop an inch. I have read 1/2 inch is acceptable but prefer the larger fall for any movement in the building or discrepancies in the timber or workman ship i might not notice.

I dont have experience cutting firring strips in this length so i dont know wether to use my dewalt table saw or just a circular saw. Any recommendations on which tool to use.
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