As a follow-up to my recent post on ledger boards I want to add some information about why you aren’t allowed to attach a ledger board to any kind of overhanging part of a house.
Here’s a quick sketch of a typical foundation, sill plate, some floor joists, and the rim joist. Now imagine attaching the deck ledger board to this rim joist. The load goes through the rim joist, straight down to the sill plate and directly into the foundation. Clearly this load path is very strong and it can handle an immense amount of load. As long as the ledger board is attached well this load path can handle all the load you can throw at it.
Now here’s a sketch below of what things look like if the house is framed up so that there’s an overhang – some of the joists cantilever over the foundation. Now imagine attaching a ledger board to this rim joist. Now the load path has to go through the rim joist, through the connectors that attach the rim joist to the joists, through the joists horizontally, and finally down through the sill plate and foundation. First, don’t underestimate the problem of this load path going through the connector hardware between the rim joist and floor joists. This connection was never designed to handle that kind of load. Then the load has to travel horizontally, and then turn down into the sill plate and foundation.
While it’s possible to build a house so that you can hang a deck ledger board from a cantilever like this, it would take an engineer to design everything, and no house in the recorded history of time has been built like this. Don’t do it. And if you see a deck built like this, be assured that it’s wrong and not safe.