2x rafter to purlin joint

[higashiyama]

Hi guys
Ive been searching for a while now trying to find a simple strong way to attach 2x rafters to purlins with no luck. I'm hoping a bit of collective wisdom here might help me. I've got a log builder here in japan making us a frame out 12in spruce logs. It's about 30ft square , 2 story, 12/12 pitch roof, with the wall plate, ridge beam and purlins cut on the plane of the roof. The plan is to use 2x12 @16" rafters. Local building code snow load is about 130lb/ft but by U.S. or Canadian stds this seems way low. Annual snowfall here is over 50 feet!

The log builder says that they usually just toe nail the rafters to the  top plate, purlin and ridge. With rafters sitting on a 45deg slope common sense tells me they are going to want to slide right off the roof under their own weight, let alone with 10ft of snow up there. Toe nails don't seem enough to hold them in place?
The log builder just wants to do it the way he always does it, so when I suggested ripping a horizontal cut on top of all the beams, nail a 2x4 on top and birds-mouth the rafters to remove any horizontal thrust, he made all kinds of noise about it being too much work. So trying to work with the way he wanted to do it, I thought maybe we could use joist hangers on the plane of the roof and saw-cut the rafters so they sit down into them. I talked with Simpson strong-tie engineer about this and he thought that was a bad idea(wouldn't work) and suggested angle nailing plate(a34,a35,HGA or HL) on each side.
Don't really want to get an engineer involved,  anybody know a there better way to make the joint between rafter and purlin other than toe nailed? Or am worried about nothing?



 

[beenthere]

At the ridge (peak), put gussets on both sides of the rafters that meet there. Will tie them together so they cannot slide down.
Can see those gussets on my 2x12 rafters that meet at the peak in this photo.


Welcome to the Forestry Forum. Click your forum name and it will take you to where you can add at least location to your profile, as it will help with answers.

Expect you meant 2x12 x 16 feet, not inches.

[Jim_Rogers]

First of all with metal roofing there load "duration" is going to be short. As the metal roof will get hot in the sun and the snow will slide off. So it won't be up there very long.

Next you could be thinking too singular. What I mean is in order for a rafter to slide off the log purlin the nails have to shear. And not only the nails on this rafter but all nails on all rafters that are connected to each other by the roof decking. The entire roof decking ties everything together making the entire roof stronger.

And the joint at the ridge where the rafters meet the rafters from the other side of the roof has to fail as well. Could it happen? Yes, but would it happen, most likely, no.

If you need to sleep at night buy the bracket that Simpson suggested and put them in yourself.

Jim Rogers

[jaciausa]

Engineer might be a good investment. Do you have a framing plan?

[WmFritz]

First of all with metal roofing there load "duration" is going to be short. As the metal roof will get hot in the sun and the snow will slide off. So it won't be up there very long.

Jim is right. I built my cabin with a 12/12 pitch and metal roof. Not only will the sun warm the roof enough to clear itself; even on cloudy, overcast days and at night, I've never seen the snow get deeper then about 4" and then it's own weight will cause the snow to slide off.

We've had 2" an hour at night and  it wakes me every time that snow let's go. I have 12" rafters packed with insulation and radiant floors, so I doubt I have significant heat loss through the roof.



  

 

[Longstreet]

I agree with everyone else in that I don't think you have anything to worry about.  Further, I think you are misunderstanding the forces in play with a ridge beam.  There is no meaningful force which will cause your rafters to "slide" down the roof at all.  With a ridge beam, all of the stress is straight down, not out.  Both your plates and ridge beam are level to the ground (hopefully), and therefore the weight on your rafters is perpendicular to the plane on both, not at an angle.



The gussets mentioned above are primary used to combat uplift and to keep the rafters aligned.  In my area you could also use a ridge strap.  With standard ceiling you could substitute for collar tie.



If you talk to the Simpson people again, I'd ask about the adjustable rafter hangers.  They should be right what you are looking for if you want a bracket.

http://www.strongtie.com/products/connectors/LRU.asp

[WmFritz]

Wow Longstreet, look at you.
Your first post with diagrams and links.  :D

Welcome to the Forestry Forum.

[jaciausa]

The wall plates should be horizontal, an that would help. The finish ceiling boards if any could also lock it to each purlin on the uphill side. I would be more interested and fearful of how the perlins are fastened!

[higashiyama]

Thanks for everyones input. Nailing and a gusset at the top looks like probably be enough. This isnt  the first time Ive been accused of worrying too much.
All the responses got me thinking and I dragged up my old high school physics just to see what kind of force we talking about. Rough and ready, disregarding friction - if the live load is 130psf, rafter spacing 16in(1.33ft) and horizontal projected length of rafters 18ft(incl eaves), then total load for each rafter is 18*1.33*130= 3112lb. Then for an inclined plane, the component of gravitational force parallel with the plane of the roof(45 deg) is
cos 45deg = Fparallel/3112lb
=2200lb.
Im sure there is other factors like friction that would lower this number but if thats the maximum load it could be, then by old rule of thumb 1 nail= 100lb, then about 20 nails over the length of rafter would be plenty I would think?