Larsen truss with veranda (roof and deck)
So I’m considering the Larsen truss for my addition. 2×4 load bearing walls on 16″ centers, 1/2″ plywood on top for shear, poly on that for vapour barrier, followed by about 8-12″ of Larsen truss cantilevered out from the foundation (2×2/2×3 inside with plywood spacer to 2×2/2×3 on the outside), then 1/2″ plywood and wooden siding. Roxul batt inside (for easy utilities) with blown in cellulose on the outside.
I want a veranda on this addition. That would mean tying in the roof and the deck. I will have an engineer review the details but i might as well start with a decent approximation for him to look at.
I’m thinking of tying the veranda roof rafters in to the inner section of larsen truss which is nailed (and maybe locally screwed too around the rafters) into the plywood, and leaving the outer plywood off the truss till the rafters are all placed and supported. If the truss is on 24″ centers and the rafters are too then that doesn’t mean too much fiddling around when i get to the outer sheathing
And for the deck, i’m not sure tying the deck directly back is necessary, so I would probably reinforce the truss at the bottom with some serious gussets back to the main wall. Either with doubling up and lengthening the plywood spacers at the bottom (one on each side of the 2×2) or by adding a 2×4 angled gusset back to the wall. The bottom spacers would be full width here (forget the thermal break in this one small section). Then the veranda supports would be attached to the outer reinforced plywood. I would also rather connect the veranda deck to the wall than stick a second set of piers under the deck and having it float entirely.
Does that sound like a reasonable first cut to start with? Overkill? Underkill?
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Replies
Terry,
The short answer to your question is that the best way to support a deck or veranda is with independent footings. You don't want to support one side of your deck or the porch roof by attaching it to the house.
If you still want to follow your plan of attaching one side of these components to your house, talk to an engineer.
Terry, I think you should work under the assumption that you are going to have to bear on your inner-wall for the deck. Your engineer may come up with a detail that will bring the attachment point out, but it is going to end up a lot more elaborate than increasing the gusset size on the Larson trusses.
ok I'll look into extra piers then. Thanks guys.
I am in the same boat right now also. I am planning on tying the deck joist into the rim joist with a ledger. The joists will go through the truss cavity and I will start my decking on the outside of the siding with flashing to protect the wall from water intrusion. I am going to insulate the inside of the rim board in the basement so that should help reduce heat transfer from thermal bridging.
Chris,
What type of flashing will you use to prevent rain from wicking horizontally along the joists, bringing moisture to the inside of your Larsen truss cavities?
Martin,
I was planning on using metal L flashing at the base of the decking. I am also planning to wrap the joists with Vicor or a similar product and tie it into the Tyvek on the exterior of the truss. A thought that just occurred to me was to wrap the joists that will be on the inside of the cavity in a breathable tape like Tescon Vana. That way it would protect the cellulose from bulk water intrusion but allow the joist to dry to the insulation.
Chris,
Your ideas won't work. Moisture will enter the Larsen truss cavity if the joists run through.
Terry,
I seem to recall from a recent discussion here on freestanding decks that even if all the vertical loads are carried on piers,the IRC still requires them to be connected to the house for lateral stability. It might be something to explore before going too far with the design.
My design has evolved a little further since i last posted, so I may end up just attaching to the outside of the larsen truss. This sounds crazy initially, but my larsen trusses are going to be stupidly strong so I don't think it'll be a problem.
I'll still have an engineer look at it but essentially my walls are going to be 2x4 studs on 16" centers over the foundation, with 1/2" plywood sheathing, poly, then 9.5" I-joists for the truss on 2' centers filled with cellulose, then 3/4" plywood, drainage mat, and board and batten siding. The 3/4" plywood with drainage mat is instead of using furring strips (less annoying to deal with and less labour since i need to put up sheathing anyway), and the I-joists are really cheap and knocks out a tonne of labour building the truss.
Terry,
Do run it by an engineer. I joists are designed for horizontal loading and are generally not warrantied for use vertically.
Oh I understand, and it all has to go by the engineer anyway. This is only a minor point whether i fully support it with double piers or just one set and tie it to the outside and I'll let him tell me which way he wants it. Either way its gonna be a brick 'outhouse' though, which makes me happy. :)