We purchased an older home that had an existing carport with a flat roof. The former owners had built an additional bedroom on top of the roof using the existing structure. The support for the bedroom consists of 2"X 8" floor joists 24 OC covering a 20' span. The ceiling height will not allow us to go to a 2"X10" floor joists. We were planning on building a knee wall to support the existing joist while adding either another 2"x 8" or laminated beam to the existing joists. Thank-you for any help.