Rafter to Wall Connection

[sema79]

Hi everyone,
My company is investigating the failure of some roof framing after the recent snowstorms. The rafter does not connect directly into the wall plate. Instead the joists bear on the wall, continuous 2x4 runs across the top of the joists and the rafters bear on the continuous 2x4. There is a rim joist that also takes some of the load from the rafters. As you would expect the rafter forces are pushing the rim joist away from the joist. I'm from Florida and have never seen framing like this in 5 years of construction and 10 years of engineering. When I saw this my response was why would someone do something like that and think it was ok. I have attached a sketch of the condition. Please take a look at the sketch and let me know what you think. Is this detail something that is or was acceptable in the north?

Thanks in advance!

 

[manstrom]

RFreund,

To elaborate on my post last week.

If you have ever sat down and down the statics for a true rafter roof, you'll see that the thrust forces are very large. The connection from the rafters to the collar tie is very large.

For example, a 24' wide roof with a 6' height using 2x8@16" o.c. has a thrust force of about 720# per rafter. That is a lot of nails (more than IRC requires). T=wl^2/8h. Last time I checked the IRC, you need (3)8D nails. Hopefully this has changed since it fails by a factor of about 3.

Additionally, the proper design of this rafter is rather complicated. It is essentially a truss with a very long tail. The proper design considers a beam column with varying conditions along its length. It is a difficult design to do by hand.

My example is for a simple roof. Factor in multiple hips, valleys, etc and it just doesn't work. I have seen plenty of rafter roofs with a healthy dip in the ridgeline. I cringe whenever I see a cape cod style house with a shed roof addition because I know it just doesn't work statically.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller