I had built a loft which isn't full height above my two-car garage. That's all we could afford to do, we kept the same roof instead of bumping it higher. The garage is a nominal 20' x 22' & the loft is a nominal 13' x 22'. We used a structural engineer, and I asked for a capacity capable of supporting the weight of multiple motorcycles. The engineer did the calcs and gave us a front-of-the loft steel box beam, 1/2" wall thickness, 4" x 8" welded into steel flitch plates cast into the masonry side walls. The beam is sandwiched on both sides by a 2" x10" (ripped to 8" to match the box beam dimension) wood beam to allow easier attachment of the loft floor joists, which are 2" x 8" & 18" o.c. It's sheathed with 3/4" plywood. The structure has no stairs to access the loft, not-even an attic 'pull-down' stairs. For access, I use an A-frame ladder, but since it's used for storage, it's not an everyday trip I make.
One concession I made to getting heavy things up & down to/from the loft, I thought about what would make an easy load-lifter? The roof ridge beam is also a 1/2" wall thickness box beam with plates welded to it for the ceiling joists. If it was an I-beam girder I could use an overhead trolley on the lower surface flange of the beam, but due to the box construction, that was out. I even went as-far as-to buy a short-web I-beam with the idea to bolt it to the underside of the box beam at the ridge, but I discovered that between the I-beam girder, the trolley, and the rigging, I would lose precious headroom, and be unable to lift much over 3' in height onto the loft deck. So I went a different way. I adapted a 'cherry-picker.' I'll post that up in a new thread.
Yes, you may be able to use the free design skills of your local supply store, but here in Florida, you don't get a permit for structural work like that without having a professional engineer (P.E.) submitting sealed plans for the structural calcs. Considering what's at stake, you don't need a catastrophic failure so using the services of a structural engineer is a matter of safety for you and anyone around the structure. I am a plans examiner and a Life Safety Code inspector, and I've seen a lot of stupid, dangerous things done by amateurs and professionals.
In the event of a failure of a structural component, the plaintiff attorneys supoena the design documents, including the plans review, the inspections, product approvals for the components right-down to the fasteners, and then a forensic engineering team reviews them with compliance to the building code at the time of construction. This is typical of what they may find: contractors substituting cheaper fasteners or structural materials which are not rated for the level of weather exposure or load can cause the entire load-bearing system to fail, possibly with tragic results. Protect yourself, get the professional services, it's $ well-spent.
