Fitting Fascia boards

[Hail]

Hi guys

I have fitted a flat roof with 6 inch furrings. One end of the roof joist section is around 250mm and the other end is 365mm. The plan is the fit in the fascia, and then install the classicbond sure edge trim on top of the fascia (using EPDM).
My question is, does the fascia board need to cover the entire wood? or can it cover a portion of it? The sure edge trim will cover about 75mm of the joist, so I can probably just about get away with 300mm fascia boards.
I can get 350mm boards, but it'll have to be online (so hefty delivery charges), and they are much more expensive hence why I am exploring other options.

 

[^woody^]

The trim will need to be packed out the depth of the fascia, so you'll need something for that. Plus, 10mm is not a lot of cover, and you may well find that you don't get that in reality. Plus, the capping will ripple slightly in the heat, leaving a gap if you don't have a lot of cover.

So you need deeper fascia.

But, there is another recent thread about having the fascia sloping with the roof. If you do that, then you won't need deeper fascia.

https://www.diynot.com/diy/threads/fascia-board-on-flat-roof.482867/