I'm building a structure that will have a "flat" roof. Currently the structure is 7x13. I'd be adding 13x13 to the structure for a total of 20x13. keeping with the architecture of the main house, I'm looking at beams under 2x6 t&G under 1/2 in ply. Thinking about running 3 glulam beams 20ft would this work? what size would I need. The two outside beams would be sitting on the headers, the center span would run the length. Another option is to run across the 13 ft spans every 6ft. thoughts?