Cantilever: how much support?

4831 Views 9 Replies 3 Participants Last post by CanadianRockies, Dec 30, 2019

CanadianRockies

Discussion Starter · Dec 28, 2019 (Edited)

We're planning to build a tiny house with the living room cantilevered 20" over the trailer tongue. There will be 3.5"x3.5" floor joists 24" o.c. supporting the cantilever, and they will have 7'-long clear spans between the ledger beam and the wall posts/studs. (See images, below.)

The above image also shows how a 20" bumpout over the trailer tongue will help support the middle half of the cantilevered floor. The living room floor will be 10' wide, and the bumpout will be 5' wide.

My question is whether the bumpout plus the floor joists will provide sufficient support for the cantilevered living room? Or should I also add a pair of knee braces to the corners of the cantilevered floor? If I need knee braces, then should I construct them with 2x4s, 4x4s, or something else? (See image, below.)

The cantilevered floor will support a 6.5'-high (at the peak) gabled wall, so it won't be load bearing except at the two ends (where the knee braces could go, if necessary). The rest of the cantilever will support a wall of closets, so fairly uniformly distributed, static weight. The roof probably will have 6" eaves and 6" rakes. Any more information needed? Thanks for any advice you can offer.

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CanadianRockies

Discussion Starter · #3 · Dec 29, 2019

Nealtw said:
Usually the cantilever is limited to 1/4 of total length so that looks like you are fine...

I've seen a rule of thumb that indicates 1 foot of cantilever for every 4 feet of clear span, but I've also seen a (probably more outdated) rule of thumb that says 1 foot of cantilever for every 6 feet of clear span. So, in the latter case, I should limit my cantilever to 14 inches instead of 20. But I suspect the bumpout support probably brings the 20 inches into range even here. I'm leaning towards not using knee braces. I don't think they would interfere with the tow vehicle's turning radius, but I'd rather not find out the hard way.

Nealtw said:
...but i would think about making the edge beams out of 2x6 with shorter studs in the wall below.

I'm not sure what you mean by "edge beams." If you mean the two beams at the top of the two walls that run parallel to the four center 4x4 floor joists, then those already are 2x6s (as is the rim joist that runs over the bumpout). The wall studs are 2x4s that are notched on the outside to accommodate the 2x6 "edge beams." See detailed image, below.

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CanadianRockies

Discussion Starter · #4 · Dec 29, 2019

Another question. I'll be using joist hangers to attach the 4x4 floor joists to the ledger beam. I expect some downward load on these hangers from living room furniture and people. But I also expect some upward load on these same hangers caused by the weight of the closets applying a downward force on the cantilevered portions of these joists.

Is the upward force likely to be greater than the downward force? If so, then should I turn the joist hangers upside down? My guess is that the hangers are going to keep the joists firmly in place no matter which direction they are turned.

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CanadianRockies

Discussion Starter · #8 · Dec 29, 2019

Nealtw said:
When we build a bump out for a window seat we run the side plywood into the wall and trap it between two studs and that usually gets 5/8" floor plywood instead of the osb we are using on the rest of the house.

The side walls of the structure above this floor can be sheeted like a stress wall which would help support this corner.

I can do 5/8" plywood without any problem. I'm just curious why these bumpout sides get special attention. It doesn't seem like they are under any undue stress.

Nealtw said:
I would differently make the beam on the outside a beam, that would be at least 2 plys of the 2x6.

I had assumed a single 2x6 would be able to handle the downward load from the floor along an outside edge. Unlike the 4x4 beams in the middle of the floor, the 2x6 receives lots of center support from the wall studs. What am I missing here?

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CanadianRockies

Discussion Starter · #10 · Dec 30, 2019

Nealtw said:
You are supporting the corner, that requires a beam,a single is not a beam.

I might be missing something here, but it seems to me that the corner is supporting the floor. With all those wall studs supporting the 2x6, it's not going to bend very much even if i's a single one.

The reason I'm trying to avoid doubling up the 2x6, is because it will create a 9-foot long thermal bridge. With a single 2x6, I can put more insulation into the wall, which is a major consideration up here in Canada.

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