Canopy / Dormer - Tying into Roof Diaphragm

[RFreund]

How do you design a dormer or canapy roof that is projecting from a gable roof.
The canopy/dormer roof seems to be acting like a cantilevered diaphragm/shearwall. So you must transfer shear and the Tension/Compression chord forces.

See attached sketch.
How would you do this if framed with trusses?

Thanks in advance!

EIT

http://www.HowToEngineer.com

 

[msquared48]

Yoou pretty well have it. All you have to do is transfer the T/C from the diaphragm to the roof trusses and major house diaphragm with straps and blocking. It's small enough that cantilevering will be no problem.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[RFreund]

I realized that I'm tying into the ceiling joist however I want to be tying into the rafter. If this force gets large do you look into using straps, Simpson connector or something similar? As for the blocking, this would go between the main house rafters?
How does the shear transfer to the diaphragm?
- say the dormer/canopy is built on top of the main roof. I have seen (done) this where a 1x board is nailed to the main roof following the valley then the valley rafters are nailed to the his board and the sheathing is nailed to it as well in the valley. This can transfer shear but I'm not sure I like trying to use that in my calculation.
Another load path for shear transfer could be to sheath a rafter and rafter tie inside the canopy/dormer and treat it like a gable wall. Then nail the rafter tie (that you sheathed) that would be sitting on the main roof into blocking (blocking is perpendicular to main roof rafters and parallel to canopy/dormer rafter tie) between main rafters.

EIT

http://www.HowToEngineer.com

 

[chicopee]

You may want to double up on your gable roof rafters supporting the frame of the dormer.

 

[RFreund]

@Chicopee - I'm curious why your suggesting to double up the gable rafters?

In the attached I'm assuming that the roof acts as a cantilevered diaphragm (or put another way - a horizontal shear wall). So there is no frame at the gable end. But maybe I'm missing whats behind your comment.

Thanks.

EIT

http://www.HowToEngineer.com

 

[chicopee]

Because the inital installation was not designed with the dormers in mind--I don't think at least, unless you have information otherwise.

 

[RFreund]

Oh, now I see what your saying.
This was just a design concept and example for future referance.

EIT

http://www.HowToEngineer.com