Replies

1. 
   TrimButcher | Jan 06, 2004 08:55pm | #1

   Your four posts form a square, which means that you've got overhang on all four sides, which can't be done. Assume your joists run North-South, you can't have overhang on East-West sides. You could if your deck was square (beams would extend fully from east to west, but posts could be inboard), but not an octagon. An octagon on 4 posts would require east-west rim joists, and possibly more joists depending upon size, to be unsupported by beams, only by blocking from inboard joists. 

   Won't meet code.

   Regards,

   Tim Ruttan

1. 
   Kyle | Jan 06, 2004 10:51pm | #3

   Not sure why this wouldn't work. As long as the beams that make up the square are sized right. Wouldn't let deck hang over to much, probably 2' max. Then I would double or triple the diagonal joist (as shown), because it will be what holds the joist down.

   Most cantilever rules of thumbs are designed for second floors of houses. With no roof being supported I wouldn't have a problem with this.

   It would probably be good to double the longest joist that is hanging over also.

2. 
   UncleDunc | Jan 06, 2004 10:41pm | #2

   There was a cross shaped house in FHB many years ago that used cantilevered trusses to support the arms of the cross. Imagine the pound sign (#) with the ends closed in as a plan view. The four posts go in the four corners of the central square. In the magazine the foundation was a square concrete structure the same size as the middle square of the cross, but it could just as easily have been four posts. Once you've got those four beams in place, all it takes to make an octagon is to run beams across the open diagonals.

   The trick is getting the crossing beams past each other and still carrying the load. The house in the magazine used open web trusses so one set of trusses could be threaded through openings in the other set. Shouldn't be too difficult to do the same thing for a deck, if you can get somebody to engineer it for you. Another possibility would be to use welded steel so the beams wouldn't actually go through each other, but butt into the sides of the intersecting beams.

   The only way I can think of to do it with solid lumber would be to have one pair of beams sitting on top of the other. Joists would sit on top of the lower beams and on joist hangers on the upper beams. If the joists were shorter (vertically) than the beams, some blocking would be required to get everything to come out even. The total vertical space required for the framing might be more than you'd like.

   Actually, there is another way starting with the same concept, one pair of lower beams and one pair of upper beams. That is to add blocking on top of the lower beams so all the joists would be in hangers. The joint between the lower beams and the blocking would be at least poentially a hinge point, and I don't know how much effort you'd have to make to overcome that. Plus that approach would use more wood.

3. 
   geob21 | Jan 07, 2004 01:05am | #4

   You need to decide the orientation of the decking first. This will determine the joist and post layout.