That AF&PA Deck Construction Guide #6 is a very good guide for the intent and thought process of designing and building a good, structurally sound, deck. I think the 1500lbs. is something of an arbitrary number, picked by committee, to force fly-by-night deck builders to do something in the way of tying the deck to the building in a meaningful way. Doesn’t the same code section wave this criteria if the deck’s connection and stability is checked by a real Structural Engineer? Obviously, someone has to check the deck for stability, if we care. Remember, a lot of DIY’ers and deck builders thought it was perfectly o.k. to attach a ledger over vinyl siding, foam insulation and sheathing with 3 or 3.5" gun nails, and a few 4" lag screws. After all, the deck did hang there after they were done building it.
The LL for decks should be at least as great as those LL for the interior spaces they are accessed from. That is, a residence 40lbs./sq.ft., a bar, frat. house or restaurant something more. But, should we sum that with snow and drifting or roof slide snows too? The lateral loads are not very well defined for decks, we know about wind and EQ. Without privacy walls and the like wind is usually manageable. Make some account of the total face areas of the framing members to the wind, and potential uplift. It is a fairly light structure, but all the mass is concentrated at the deck level, and the whole structure is on 3 or 4 tooth picks out at the outer edge. The biggest unknown is the people loadings, and most deck failures have involved these. The extremes, of course, are four or five drunk football linemen swaying in unison to some music, out at the handrail, to impress the cheerleaders; or a group of people rushing to the handrail to see a deer in the back yard. What are these loadings, not particularly easily defined? Pay attention to the orientation of the deck. A 20' long deck on a ledger, and projecting out 8-10' from the bldg. should not be much of a problem. But, the same deck connected to an 8' ledger and projecting out 20' from the bldg. is quite a different animal. Is the deck connected to the bldg. of several sides and protected by the bldg.
On new construction an intended deck attachment is easily accommodated, with some extra blocking, and proper floor sheathing nailing, etc. On an existing bldg. it is more difficult because you don’t know how the floor sheathing was nailed to the rim joist, or the joists for that matter, and you are unsure of how to get that lateral load back into the floor diaphragm. The ledger should really be fixed directly to the rim joist, maybe through the sheathing, but not through added layers of material, where the fasteners end up cantilevering to far. That pocket should be flashed up behind the drainage plane, and include end dams, all draining out under the ledger. Then there should be a second flashing installed over the ledger and behind the drainage plane. Finally the ledger is fixed to the rim joist, and some size and spacing proposals have really complicated that process. These prescriptive requirements are almost always needed to keep idiots from killing other people. When the decking is parallel to the bldg., I fix the last piece of decking under the threshold on precut wedges, atop the joists, to make that deck piece drain away from the bldg. and the ledger. Also, flash the tops of joists and built-up beams with peel-n-stick or some such to keep water from accumulating and siting at these locations, they never dry out.
