My first post here. I've lurked around, used the search function - but I can't seem to find what I'm looking for. So I'm hoping someone can help me. I am designing a shed for my backyard. It will be used to house a storage tank for water heated by solar collectors on the roof of said shed. I have the rafters and walls designed to bear the loads of the collectors, snow (60#) and dead weight of the roof. The shed wil be 12' X 16' and will set on an 8' x 16' basement foundation. 4' of the 12' width of the shed will cantilever out from the foundation. 6" x 10" pressure treated floor joist on 24" centers appear, by my calculations, to be acceptable for the floor joist. I am interested, however, in building wood I-beams of treated wood to use for the floor joist. I can not find, however, a formula (or formulas) that will give me the data I need to decide on what lumber to use for the chord and flange sections of the I-beams. (My google-fu is usually pretty strong, but on this question I have failed...) For example: Can I just calculate the S and I for each member and add them? Seems I'm missing some of the advantage of the I-shape then. I'm really only adding depth to the member with no accounting of any change various widths of flange would create. Can anyone help me out here?