UpNorth

A little house is on my drawing board, and its main roof cannot be spanned with single length rafters. Given the snow load and the 18 foot clear span, I will see if an arrangement with flush beams at mid-span can work.

It all hinges on whether some girder trusses I propose to bury in the cheek walls of a shed dormer can pick up the sidehung reactions from the beam connections.

One pic here shows the roof with its shed dormer, and the other shows the structural for the roof scheme I am hallucinating. I did not bother to show the truss with beefy chords or webs, and I don't know how many plies this might take. Two-ply girders are shown.

We've not done this with trusses before. Have you seen anything like this?

The truss engineer will get this soon, but I thought I would ask here.

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framerman

How about a parallam under the dormer walls? Or TJI rafters? I know that makes things more complicated for you, especially money wise.

__________________
If one advances confidently in the direction of one's dreams,
and endeavors to live the life which one has imagined,
one will meet with a success unexpected in common hours
~Henry David Thoreau

UpNorth

Thanks, but even the heaviest of I-Level's 16-deep I-joists barely make it for us, and the centers need to be 12. That's too much wood.

Half-spans like what I've got in the pic, and all we need is 2x10s on 24s, SPF#2.

It would not take much steel to do what I'm thinking about, this thing in the cheeks, but I'm hoping the truss whiz can make it work with wood.

Warren

I would be surprise if that truss configuration will be adequate. Keep us posted.

__________________
"Stop wasting lumber. It doesn't grow on trees ya know! Oh wait, it does."

framerman

Sorry, I just noticed you're in Lake Placid. Massive roof loads right? Over 100?

__________________
If one advances confidently in the direction of one's dreams,
and endeavors to live the life which one has imagined,
one will meet with a success unexpected in common hours
~Henry David Thoreau

Birch

we know....

.... yes ...

kpatrix

It seems like that design would make the main ridge carry the truss that the other roof is bearing on... I don't build in snow loads but I imagine that the dormer side walls could be built in a make your own truss assembly with LVL's and three quarter plywood that would make that design fly as long as the top ridge is supported adequately.

If the ceiling is vaulted then give it a timber underframe but use solid sawn wood or LVL. Metal gusset plates in "modern" trusses will ruin that design anyway.

UpNorth

Thanks for the replies. I'm now thinking of giving the shed eave a little subtle eyebrow swoop, a warp. We see this done occasionally around here. Looks real good with exposed tails, very small fascia, and standing seam terne-coated steel roof.

The center fireplace structure in the house is a wood-framed affair, finished in "phony stone," such as from Eldorado or O.C. Its framing provides rock solid support for that ridge.

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jhark123

I think the truss guys would make that work here (probably a triple ply girder with lvl bottom cords), but we only have a 25lb snow load.

UpNorth

Here is a new scheme for the arrangement, one in which an LVL "girder rafter" goes into/under the cheek wall of the shed dormer, and from which hang the doubled sawn-lumber flush headers for the sawn-lumber rafters.

Read the fine print and you can see what the loads look like. The starting point for the load analysis is our 90 psf ground snow load, with the numbers all crunching through ASCE-7.

The JLC "Field Guide - A Manual of Best Practice" is giving me recommended nailing for the rafter-to-ridge, and other rafter-to-stuff, but is saying nothing about hangers.

Would your AHJ require hangers for this roof frame at:

Rafter-to-structural ridge?

Rafter-to-flush-midspan-beam?

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framerman

Well, that is what I suggested. Though the 3½x20 I'd change to a 5¼ or even better a 7 parallam unless you only have a 2x4 wall there. Try not to make the framer nail it together unless he's OK with that. I prefer them to come out all ready to set, I can't stand nailing the bejesus out of microlams, though some may find it easier to get it in place using singles.

Now you have a detail there where you trim the top edge of that beam. That's not going to be necessary, or at least it wouldn't with me. I see why you may want it like that, but I'd rather have the beef on the beam carrying everything. You can easily cut that top plate from the dormer to sit on top of the beam.

Your ridge is carrying some heavy load so you have to figure out how that ridge is staying put. A 2x ridge is fine, but usually there's a point load directly under the beam connection. I'm not sure what your interior design is like, where structural loads are going, etc. You may end up with a microlam ridge, not sure.

What about microlam rafters? That would eliminate all these mid span beams I would think, even in your heavy snow load area, though cost would be a big consideration.

__________________
If one advances confidently in the direction of one's dreams,
and endeavors to live the life which one has imagined,
one will meet with a success unexpected in common hours
~Henry David Thoreau

Warren

Why does that rafter have to be 2 piece? 17' 10" span, seems like a 2x12 at maybe 12" o.c. might work?

__________________
"Stop wasting lumber. It doesn't grow on trees ya know! Oh wait, it does."

jhark123

Exactly, you never specified what the snow load is in your area. According to the 2006 IRC Rafter span table you can do a 2x12 DF #2 at 12" oc for a 17' 6" span (this is for a 70lb snow load). You can go 18' 8" with #1 DF. It seems to me that this could be done with conventional framing depending on your snow load.

framerman

Lake Placid area....a whole lotta snow

__________________
If one advances confidently in the direction of one's dreams,
and endeavors to live the life which one has imagined,
one will meet with a success unexpected in common hours
~Henry David Thoreau

UpNorth

Well, after working with this house design, I decided that the roof is best framed using engineered trusses almost everywhere.

Hand-framed rafters, 2x10 #2 SPF on 24" centers, are used on the rearmost plane of the main roof, where we need max overhead room for a pair of bed alcoves in the loft, and the rest of the main roof is a truss arrangement, using a parallel-chord two-ply girder that functions as a ridge, offset from the actual roof ridge so as to allow easy chimney chase routing.

I positioned the girder truss "ridge" to sit directly atop the two-stud pair that is the "frontside" king and jack for the gable window openings that are mirrored in each tall gable, thus we'll need no structural headers over those window openings, which are centered on each gable.

I realized this could be done when looking at the full house section, and seeing how much excessive overhead I had in the single under-roof vault. Dropping the ceiling down for a better spatial feel and scale, gave me plenty of overhead between ceiling and roof for truss depths. The cheek wall frames for the dormer simply sit atop doubled trusses.

The pics are worth another thousand words.

And if you ask, "why won't a modified scissors arrangement work, and then you won't need the girder?" the answer is, "the scissors at its size is way, way over the transport size limit."

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SNC

Yes we do stuff like this a lot, a truss can be built for just about anything.
You may end up with a double or triple thick truss and 2x10 post to put your hangers on.