Hi, 
I have a house boat (aka a floating cabin that is not navigable) and am looking for suggestions to increase the structural support of the roof which also supports a deck. 

Importantly, this is NOT to code, code is NOT required and I understand that anything I do is at my own risk. 

Currently there are 2×4’s  with a bowed cap spaced at 18″ OC and span a width of 10′. The bowed cap is used to create a sloped roof to shed water and is 2″ high in the center and tapers to 1/2″ on the ends.

I am looking for the easiest way to increase the structural integrity and load bearing of the roof. I cannot use 2×6’s as headroom is limited. Some have suggested sistering an additional 2×4 or two (doubling or tripling up on the 2×4’s) and potentially running a block line down the center. Of course weight is also a consideration as it’s a floating home. 

Questions 
– benefit of sistering 2x4s
– benefit of blocking?
– are there other materials that would increase the integrity without compromising head room  (aluminum or steel c-channels) ? 
– other thoughts?