Replies

1. Expert Member
   Akos | Oct 10, 2020 09:19am | #1

   Long cantilever like that has to be designed by a structural engineer as the roof will be lifting up the far wall. Both the foundation and roof connections need to be designed for significant uplift.

   You can do shorter cantilevers, typical one is 1/3 out 2/3 in. Your local code should have details on how to do this.

   A ball park for joist sizing is a cantilever is equivalent to 2x simple span. For example for a 4' cantilever you need joist that can support 8' span. This is just ballpark, so check local rules or even better, consult with a structural engineer.

2. 
   johnstrongtree | Oct 10, 2020 03:04pm | #2

   I'm pretending I'm an student engineer. Not do it, just want to learn, look up references on how a very long "wing" is constructed. How do they get a 20' unsupported cantilever span rigid, or to stay flat?

3. Expert Member
   MALCOLM TAYLOR | Oct 10, 2020 05:12pm | #3

   John,

   The principles aren't that complicated, although the calculations are. That's why building codes allow limited length cantilevers following their formulas. But beyond that, as Akos said, they need to be engineered.

   Cantilevered members experience loading in the opposite way than those supported at both ends. Beams or joists suspended between two bearing walls have their top chord in compression, and the bottom in tension. With cantilevers, the top chord is in tension and the bottom is compressed. They also experience two other significant forces: shear and buckling. All three need to be accounted for in any design.

   Those are just the forces acting on the cantilevered portion. You still have to account for the uplift forces on the structure on which the member rests.

   Next time you are in your local Costco or Walmart, take a look at the steel structure overhead. You'll notice the beams are cantileverd over the support columns and attached to the adjoining beam at about the quarter span. That arrangement counterintuitively increases strength.

   Unfortunately, understanding what's in play and how they act, is interesting and helpful theoretically, but doesn't get you far in the practical task of sizing and building long cantilevers. For that both building codes and common sense dictate you need to consult an engineer.

4. 
   burninate | Oct 10, 2020 08:26pm | #4

   Medieval Chinese/Japanese architecture of monumental buildings uses a complex system of bracing called duogong to hold their heavy roof tiles up without metal fasteners, often using the exterior cantilever to balance out an interior cantilever, where upper walls are stepped inwards a few feet from lower walls.

   https://en.wikipedia.org/wiki/Dougong
   https://www.youtube.com/watch?v=JnB3fQTE1XU
   https://www.youtube.com/watch?v=w78Yb_aotH0
   https://www.youtube.com/watch?v=BcNc6nP__6U
   https://www.youtube.com/watch?v=ZmZXlDBN52A
   https://www.youtube.com/watch?v=F2H9UbIApYs

   https://www.youtube.com/watch?v=uG37gQSvrf4
   https://buffaloah.com/a/virtual/jap/dcty.html#Roofs

   Both the remaining Asian traditions of timber framing, and US timber framing, simplify this method a great deal, they add uplift protection, and they rarely attempt to give the lower posts the same flexibility to bend joints in seismic conditions. Without the heavy load of roof tiles and grout, they're much lighter-weight structures that have to worry more about the wind. They're also designed for conditioned space; The original temple architecture, minimally so.

5. 
   burninate | Oct 10, 2020 08:39pm | #5

   If you want a simple 4' overhang, construct some sturdy trusses out of 2x12 rafters (extending past the triangle-birdsmouth-point), 2x6 ceiling joists, and plywood gussets, brace the hell out of them in the gable dimension, and then add a sturdy uplift prevention mechanism to every single joint in the load path between the rafters and the slab/anchors/posts.

   What this will NOT get you is a 12' overhang you might find in those temples, at least not without engineered-lumber joists with questionable moisture-safety. For that you're going to need truss structure extending into the cantilever as in https://en.wikipedia.org/wiki/Hidden_roof

   Anything in that vicinity is going to require engineering, but you may be able to have that performed as part of a custom truss purchase agreement.

6. 
   joenorm | Oct 10, 2020 10:05pm | #6

   4' is not that long of an overhang. Like others have said a 2x12 would be fine and as long as you fasten the building reasonably well it'll work fine.

   The photo attached in the original post on the other hand, is a completely different story. Probably they used steel.