That doesn't look like trusses.
They look like otherwise unsupported two by four rafters?
On what ....... 24 inch centres?
Any snow load? Human load (while working on roof?
If any are going to be cut best get some advice; if not to be cut roof
structure will be much the same as it is now!
Doubling up as mentioned by some will also help to stiffen the roof
around skylights which should reduce flexing.
My last house had a few skylights. Didn't have a reinforced rafters.
I was the original owner and lived in that house almost 20 years.
Did not have any problem anything to do with sky lights. Now I live
in this house custom built in '94 per our plan. We did away with sky
lights, we have all glass all season sun room now like a green house.
If the roof is on engineered trusses, modifications are needed. Allay
your doubts by getting sound professional advice (architect) and
asking questions at your building inspection department. We can't give
you good advice based on so little information. Good luck.
My guess is he's talking about the roof deck or the ceiling below the
skylight. It does weaken the structure slightly but I don't know what
sistering the rafters is needed. For a truly accurate answer, you'd have to
know the span, the skylight size and the rest of the construction details.
I'd check to see if the skylight manufacturer has suggestions.
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