Larsen Truss on the Roof?
Tim_O
| Posted in General Questions on
At some point I started on Instagram, came across a post by Robert Swinburne on his modified Larsen Truss that he calls the “Swinburne Truss.” I started down the rabbit hole of differing methods of Larsen truss construction. What I came across is the following post used by a Canadian builder where they use the Larsen truss concept across the roof. Has anyone used a technique like this on the roof? Maybe there is not much advantage vs using deep Ijoists or other methods of achieving a cathedral ceiling.
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When I read that my first thought was, "isn't a Larsen truss on a roof the same as a roof truss?" So I read the linked article. This is what they wrote:
"We also apply the Larsen Truss concept to roof truss extensions. See above where the plywood vapour barrier method has been used on a retrofit project, then rafter extensions, engineered/manufactured by a local company are installed to maximise attic insulation on this storey and a half home."
So they're using them as extensions on an existing roof, that makes sense. We've been talking more and more here about over-roofing, which is what I'd call that. It seems like a good solution for an unvented roof
Yes, maybe it only really makes sense in this case where it's a retrofit. But I was thinking maybe it's a decent alternative to using dimensional rafters and then instead of having to put a thick layer of foam on top, you could do this. Especially on a room/house that only needed say 2x8 or 2x10 rafters where you can't fit much insulation.
Tim_O,
I think you are asking whether it makes sense to use deeper roof members than are necessary for purely structural reasons, so you can use more insulation in new construction?
It's done all the time. Raised-heel trusses, I-joists, 2"x12"s when 2"x6s" would do. One very frequently posted question here on GBA is how to insulate roofs that have been framed with no plan in mind, and have inadequate depth for anything but spray foam. Increasing the depth would almost always have solved the problem.
Usually by the time we get it the roof has already been put on, so we're talking about furring the rafters from below. Although I don't see why this technique couldn't also be done from below.
My thinking is more of is this a decent alternative to deeper roof members. And avoiding the need for a ridge beam for example if using I joist rafters.
As DC mentions, I've seen it done from below, and it makes good sense. I was just curious if anyone had done it from above like this.
Larsen trusses are generally not structural. They are only there to hold insulation, I can't see how it would allow you to remove the need for a ridge beam.
A structural ridge beam is always needed for a gabled roof if you don't have collar or ceiling ties bellow.
Only because you could use dimensional lumber with a ridge board and rafter ties. My understanding of using I-beam rafters is they require a ridge beam. Maybe not a big deal here really.
Ecocor uses a similar roof system, with I-joists above the structural sheathing.: https://ecocor.us/enclosure-systems-2/
Interesting, so they use I-joists above the 2x12 roof to achieve R80+. They do a sarking membrane over the Ijoists and then another layer of structural zip. I guess doing this larsen truss setup, you still need a vent channel below the second layer of sheathing.
Do all the Robert's have trusses named after them? I've heard of the "Riversong truss"— a variation on the Larsen truss—never Swinburne.
Haha, good point! I forget what is special about Riversong's truss but Swinburne's is pretty clever, using vertical 2x6s on the exterior with 3"x3" squares of plywood screwed to a narrow edge of the 2x6, then using those as tabs to fasten the 2x6 through the sheathing (in his case, board sheathing) to the structural framing. It provides a 3/4" thermal break and space for batt or blown insulation.
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