birdsmouth vs Simpson VPA2 on 12:12 roof

Started by LatinForLiar, September 26, 2017, 07:36:19 PM

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LatinForLiar

Planning a 12:12 gable roof on a 10x20 building. For such a steep roof and such a short span, the rafter tables allow for 2x4s (easily), however, the depth of the birdsmouth cut is going to be severely limiting. Based on my rough calculations, I get to to about 1 5/8" of bearing with a 2x4 before I go past 1/3 the depth of the board. Even worse, 3/8" of the bearing will be on sheathing (smartside).

I'd like to have more bearing than that. So, it seems like my options are to over-build and use 2x6s or 2x8s or use the Simpson VPA2...unless I'm missing something.

Obviously the VPA2s are pretty expensive, but I'll be saving money by using 2x4s, and saving time (I think) not making seat cuts. Has anyone used VPAs, and want to weigh in?

ChugiakTinkerer

I've not seen those connectors before.  Looking at the Strongtie web page they look to be adequate for your project.  You would likely be putting in tie down plates anyhow, so you're only adding a little more to the cost and time for installation using the VPA instead.  The time saved from not cutting the birdsmouth notches will be a nice bonus.

Not having the direct support of the top plate supporting a notched rafter means the VPA is distributing the load.  That makes me nervous, and I'd want to check the load calcs to be sure it is adequate.  Assuming a 24" eave and 24" rafter spacing, each rafter is supporting a tributary load area of 2' x 7', which at 30 lbs total load on the roof amounts to a load of 420 lbs.  If you look at the load numbers on the Strongtie site, it shows that with Spruce/Pine/Fir the connector can support a download of 870 lbs.  Looks good to go, assuming uplift and lateral design loads are adequate.
My cabin build thread: Alaskan remote 16x28 1.5 story


JRR

Would the H1Z seismic connector do the job .....at less expense?   Avoiding a birds mouth, but allowing the addition of a vertical 2x "hip" to the rafter; with the hip-end resting on top of wall plate?   Two "hips" could be used when space allows.

ChugiakTinkerer

I'm sure the H1 can carry some download load, but they aren't designed to be load bearing.  If you look a the installation images on the Strongtie site, it shows a flat connection where the wood of the rafter, truss, or joist is resting directly on the top plate.  Also, there is no download force provided in the H1 specs table.  If the H1Z ends up not being strong enough then there would be deformation of the connector and/or the rafter.  In a light duty shed that never sees a load on the roof even close to the design load, it might be good enough.  It looks like about a $4 differential in price, so for all 22 rafters that amounts to something in the ballpark of $100.
My cabin build thread: Alaskan remote 16x28 1.5 story

Don_P

the uplift hangers wouldn't provide the min 1.5" bearing (on framing, sheathing doesn't count) the vpa does provide that in the adjustable seat. if there is no overhang you can make the seat cut larger, if there is an overhang you can't notch a 2x4 going on to provide an overhang anyway. I haven't used them nor would I, the same money would buy a deeper rafter.

this connector doesn't remove the need for ties or a structural ridge  ;).


ChugiakTinkerer

Any update on the use of the VPA2?  This would make my sister's shed project go a lot faster, just hoping to hear some feedback.
My cabin build thread: Alaskan remote 16x28 1.5 story

LatinForLiar

Mostly happy with the vpa2s. A few random thoughts: I've cut birdsmouths before and wasn't great at it, so the greatest benefit was avoiding the hassle. Once you install the brackets, you make your plumb cut for the ridge board and that's it. Pretty straightforward. Also, i was also able to use 2x4s rather than 2x6s, which saved $ and was easier for a solo builder,, though this also limits the amount of insulation I can use. Another drawback is that the shape of the brackets makes blocking between the rafters difficult. The brackets also make it difficult to tie ceiling joists to both the top plate and the rafter. I will update my project page, and hopefully that will explain it a little more.

GaryT

Just a thought:  about a dozen years ago I built a 10X16 storage shed for us.  Roof pitch is only 8/12 though.  I made my own trusses out of 2X4's, with 1/2" plywood webbing at the joints.  Used Simpson ties of some sort to tie the truss to the top plate.  A dozen Vermont winters and storms later, not a problem with the shed anywhere.  Was pretty quick and easy to construct.  I'd do it again in a heartbeat.   Built a form to make the trusses on the floor of the shed.  Very quick and easy.  And no rafter notching.
Gary

LatinForLiar

One additional hiccup. The brackets cannot be placed at the very edge of the wall, or at least, not in a way that the rafter itself falls on the edge of the wall. This would have been fine if I was notching and using outriggers, but I wanted the last rafter flush with the wall so I could run sheathing all the way up. I ended up installing one rafter 1.75" in from the edge, and then sistering another rafter beside it, with shims to get the proper spacing.


ChugiakTinkerer

Thanks for the feedback on those connectors.  For the end rafters, if you had framed the gable walls with the peak built in, rather than flat, then you could just set the end rafter on the top plate, like this:


That would also prevent the hinge in the wall you get with an open ceiling.

I'm helping my sister cut bridsmouth notches today for her shed.  I'm wishing we had opted for the VPAs, but the downside of building in the gable peak on the end walls is you are committed to the rafter height you designed for.  With the flat wall you can change your design at the last moment without having to redo any work.
My cabin build thread: Alaskan remote 16x28 1.5 story