Hi all, 

new here and hoping I might get some advice.  I am relatively new to framing and building but finally able to take on a project at our new house.  

I am building a flat roof garden room which is 3.5m x 4m.  The height will be a maximum of 2.5 meters, so I want to maximum headroom my building a roof that is relatively thin (head height is more important than retaining huge amounts of heat)

i have been going through span tables, and would like to use 2×4 for the roof as my foundations can’t go very low, and this will be the only way I can retain a minimum 2.1m head height.  

the maximum clear span for a 2×4 seems to be around 1.8m, so I am looking at options for adding an RSJ steels I beam running across the 4m width of the roof supported by the walls.  This would reduce the roof spans from 3.3m to Around 1.5m.  This would be within the 2×4 dead load rating for flat roofs based on some Local council Span tables. 

I have looked at glulam the etc but steel seems the simplest way to go.  

would there be any obvious issues with adding a 120×75 steel, and then sitting the roof timbers on to that?  

also to complicate things I have a front door which is 2.6 wide, so to transfer load down through the stud work either side and prevent future sag, I was planning to add a 3m steel above the door, as the maximum span for the front opening seems to be close to the limits of span tables when using a 5×4.

hopefully that makes sense, and would love to hear thoughts on this.  I am not too worried about heat loss through a nominal roof thickness but it will be using 80mm insulation and won’t need electrics in the roof . 

kind regards, 

william