joist size for 4.6 meter flat roof

[marra]

Hi all. Hope someone can help on this. I am in the design process of Building a workshop at the end of garden. Due to the permitted allowance I can only go 2.5 meter high, so internal headroom after roof and floor is tight.
My building width is 4.6 MTR wide and shall be looking at a grp roof to Finnish. For the roof joists can someone advice on the size, I was hoping to use 200x50 at 400 centres. ( hope I've worded that correctly ) . If I Need wider such as 220 or 250 , would 200 But narower centres be OK ?
Also I would like the slope to be little as possible to gain all headroom, so was thinking of a 100 mm drop at the run off side. Would this be OK ? And regarding the firing strips, could I not just run a circular saw down the sides of joists ?
Thanks for any advise.

 

[roofspecialistsLondon]

Hi 8x2's is fine for that

 

[hanvyj]

That's a slope of 1:45 which should be okay. What surface are you planning? I'd say add some more slope if it's anything that could sag like corrugated sheeting material.

 

[marra]

The roof will be topped with 18mm OSB with GRP glass fibre build . I thought of tin box but costs got out of hand with all trimming and the sag was a consideration.