Larger timber instead of doubling up joists for trimmers?

16 May 2016
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United Kingdom
Hi all, I am building an extension 4m x 7.5m. We have changed the design from a pitched with velum to a flat roof with roof lights so can’t use the original structural calcs for a pitched roof.

I want 3 roof lights 2mx1m running in line with the joints across the 4m span (3.7m clear span). The span tables show for a 4m span with normal loading I could use 47x195 c16 at 600 centres. I then believe the normal routine would be to double up joists around the roof windows as trimmer joists as the roof lights would block one joist.

As timber is worth more than gold at the moment and I have found a good deal on some 47x250 (2x10) c24 I was wondering if I used this (which is complete overkill) would I still need to double up trimmers or could I just use one joist around the windows? Where can I find out or check this info? Any sources anyone knows of? Can’t seem to find trimmer joist span tables or anything of the sort. Thanks!
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