Replies

1. 
   mark122 | Jan 27, 2015 06:49pm | #1

   are you being told by a building inspector that you can only cantilever 1'? havent double checked those specs but if your correct on the allowable spans you can cantilever 2' regardless of how many post you add to the minumum requirment.

2. 
   strawmyers | Jan 27, 2015 07:32pm | #2

   Cantilevers are limited to 1/4 of the BEAM span, or 1/4 of the JOIST span?  Give it another look... shoulder be joist span, IIRC.  That would, as stated above, leave you at 2' regardless of the number of posts. 1/4 of 8' is 2'

3. 
   DanH | Jan 27, 2015 09:00pm | #3

   It's almost certainly based on the total (solid) beam length, from the end of the cantilever to the attachment point on the far end.  It's not the number of posts in issue, it's the number of splices in the beam.

4. 
   catmandeux | Jan 27, 2015 09:52pm | #4

   It is the joist overhang that is limited the lessor of the allowable span between beams, or the actual spacing of the joist supports.

   The beam overhang is restricted to LB / 4, where LB is the max allowable span of the beam between supports for continuous beams, or if there is a splice, the distance between post supporting the splice, and the end post supporting the overhang.  Adding additional posts in the middle of that LB space does not affect the overhang.

   I think adding in the addtional posts will create a lot of work.  The 12" Sonotubes do not meet the prescribed footing sizes, so those holes should be undercut to provide the required footing size. It might be easier to dig the 6 larger holes than 10 slightly smaller ones.

5. 
   deadnuts | Jan 28, 2015 10:57pm | #5

   other effective options available by code

   You don't need the additional two footings to achieve the cantilever you desire with double 2x12 beam. I assume you’re adding two additional footings in order to reduce the size of each individual concrete footing. However, what are you saving? Your beam is way oversized for the span (except cantilever caveat) and you’re still digging two more holes and mixing and pouring concrete into them. It's probably less labor and less labor to stick with three posts if you want that size cantilever.

   There is probably a more efficient beam size if you’re wanting 5 posts and breaking your beams in the center. Remember DCA 6 calculates clear span from posts faces. With your indicated layout you could probably get by with a triple 2x8 beam for span (7'-6" allowable and you’d be ½” shy of that) and allowed to cantilever about 19.5". Not exactly the full 20' width you wanted, but close. A triple 2x10 would definitely do it if you pushed your post centers out a bit.

   However, I think the most efficient way to carry your cantilevers and keep your full deck size would be to cantilever your deck joists 2' and go with a triple 2x8 beam. If you used 2x8 s.p. joists 16” O.C, then you can cantilever 2’ and keep your effective joist span at an allowable 10' (can go more, but this works) .  A triple 2x8 beam is allowed to span 8'-3". This would give you a 2' allowable beam cantilever…and keep your posts 2' O.C. Technically speaking,  you would be required to still push the outside post out a bit (in all cases) to achieve the 8’ clear span dimension (that you're dividing by 4); but I doubt an inspector would call you on that. Best to check with your plan reviewer during permit to be sure.