Timber Framers Guild Forums General General Forum Questions Cantilever design w/sips

Experienced builder...lots to learn about timber framing and sips. I am designing a hybrid and I want 2 or 3 of my TF floor joists to run through the sip panel so I can have a cantilever window seat in the upstairs bedroom. Should I A: drop the girder so the joists run over the top and out the wall. B: Leave the girder out and have pockets in the sip panel with some of the joists setting in pockets and some continuing through the panel.
After I have the timbers outside the sips what should I do? Will the sip companies send me the 5 small panels it would take to enclose the small popout and then stick frame a roof for it? The house will be in interior Alaska so I would like to make the envelope as tight as possible. Any info would be greatly appreciated

Scott Dillard

Sorry, Scoots, I gotta talk to Derek here...

Derek, how long have you been doing this for?
Maybe you're a little away from it all there in Montana?

Oh my....you really are a traditionalist...

Well if you truly don't know, then I'm not going to be the one to tell you. You won't like it.


Ahh, Scoots...I can't tell you anything either...not because I don't want to, but because I don't know....
Good luck, though

You should not even consider running the timbers through a sip wall.
Support the window seat completely from outside the sip enclosure.

I agree with Jim:

In a cold climate, I think it's a mistake to have a timber penetrate the insualted wall, so that one end is hot, the other cold. It'll never seal, it provides a way for bugs to get in. It might start to rot out there, and then creep into the house. CB.

I agree with you guys on wanting to keep them separate, this will also help in my vapor barrier. So how do I support a cantilevered section above? Corbels attached to the Sips. There has got to be another way.

First of all you have to have good communications with the sip supplier.
In the house design that I'm currently working on we have two porches (one front one back) outside of the sips enclosure system.
We are supporting the outer edge of the porch with posts to their own footings. The inside edge is being supported by a bracket hanging off the side of the foundation below the sips' bottom edge.
With a space between the porch decking and the house siding on the surface of the sip.
At the roof line the sip manufacturer will embed a horizontal block in the sip behind the osb. This block will have a mate outside the osb, an outer block (being a rafter ledger) and it will be lag screwed to the inner block. And if needed the inner block will have a support running down inside the sip to the foundation.
The lag will only penetrate the outer osb. And will be protected from the weather by the porch roof, so that the possibility of moisture will be removed. And the possibility of condensation from the hot/cold action will be reduced.

Here is a rough cross section drawing of the deck support at the foundation wall:


I call it a rough drawing as the bracket itself hasn't been specified yet, and of course it will bolt directly to the foundation not to the 2" foam.
And the z flashing may not be installed to allow a space for any debris to pass through.

Jim Rogers

Hey Jim, that is really good information. Thanks!

I do not know much about sips. Do the sips have 2x dimensional lumber inside of them on a set center to give them there structural rating? If they do, could you not attach the ledger directly to them? Of course I do not know what kind of rafters you are using for the project.....

I guess, my question is what is the design makeup of the sip?

As far as Scoots cantilever (for a balcony I think) I guess the trouble is connecting the level joists to the sips, then supporting with braces six feet of balcony (balconies in my opinion should be no less than six feet) posts and a roof. Whew!

Scoots, what about recessing the balcony into the house say four feet, and have 6 feet protruding? Of course all of it would be "outside", but there may be a way to get better support. I dunno, interesting topic.

Sorry Scoots, not a balcony a window seat.

No



You'll have to check with your local sips supplier. But most don't have anything other than osb foam osb. My supplier will put pieces inside the sip for things like cabinet nailers, baseboard heater attaching, and for special jobs like ledger nailers for porch rafters. All of these are horizontal nothing on an angle...
They can put in vertical stuff if you need it...

Jim Rogers

This cantilever stuff has got me thinking.

Ok, the angle between a brace and joist if it is cantilevered. (without sips, just timber frame)

At what acute angle does the brace become inherently weak (all things being equal). I realize that as the stated angle becomes smaller you probably lose strength with each degree.... The brace would be tenoned into the joist that is cantilevered as far out as possible without sacrificing blow out of endgrain.

The greater the angle = more strength right?

I realize that are a lot of variables with this question but in general is there a point that you don't want to pass regarding the angle between joist and brace? Thanks for any thoughts.