Rafter to Wall Connection
Rafter to Wall Connection
(OP)
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!
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!






RE: Rafter to Wall Connection
RE: Rafter to Wall Connection
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
RE: Rafter to Wall Connection
Yes, the rafters would tend to kick out if not anchored sufficiently to the plate over the joists.
Mike McCann, PE, SE (WA)
RE: Rafter to Wall Connection
@Manstrom - Seriously?
A standard gable roof where the rafters lap the ceiling joists and the ceiling joists lap each other and are fastened properly is significantly stronger than a roof with a structural ridge.
RE: Rafter to Wall Connection
It can work, just not as good. would never use it today on anything i design.
RE: Rafter to Wall Connection
DaveAtkins
RE: Rafter to Wall Connection
The 100 lbs or so rated lateral strength of each 16d nail in a more traditional joint, or securing the (missing) collar ties gets used up quick.
Depending on how things actually failed all that is missing are some Simpson connectors.
That detail is sure a lot easier to insulate nicely and ventilate than when rafters and joists both rest on the top plate.
RE: Rafter to Wall Connection
Kind of stinks when your arguing with someone who's father writes your paycheck.
RE: Rafter to Wall Connection
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Rafter to Wall Connection
I can't understand so many Engineer's obsession with ridge beams. I always try to find ways not to use them if it is not too complicated. If you use a ridge, you have the costs associated with the materials and chasing the loads all thru the house. Maybe even have to add a crane to the mix and you end with a roof that is not as stiff, IMHO, as a series of well constructed triangles.
RE: Rafter to Wall Connection
RE: Rafter to Wall Connection
RE: Rafter to Wall Connection
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Rafter to Wall Connection
This part of the roof DID NOT have a structural ridge beam. It only had a 2x12 ridge board that was used for rafter attachment.
Tmoose, you hit the nail on the head(no pun intended). The outward thrust forced a failure at the joint causing a roof collapse. There were collar ties every third rafter. These were installed about 1/3 down from the ridge. In some locations there were shear failures of the bottom of the rafters at the birdsmouth cuts.
XR250, Nice detail. That's the way this should have been constructed in my opinion.
I wasn't sure if this was a one of a kind structure to be built this way or if it was reasonably common for the area.
RE: Rafter to Wall Connection
This type of construction is very common in older buildings, particularly residences.
Mike McCann, PE, SE (WA)
RE: Rafter to Wall Connection
I'm not surprised that these are seeing failures.
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
RE: Rafter to Wall Connection
Can you elaborate on this?
EIT
www.HowToEngineer.com
RE: Rafter to Wall Connection
Is this the house that was in the news because the roof slid off.
http://www.concordmonitor.com/home/15823349-95/con...
RE: Rafter to Wall Connection
RE: Rafter to Wall Connection
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