Fastening plywood gussets
Hi, I will be fastening plywood gussets to my 2 by 6 rafters to deepen the bays for additional insulation in my attic. The plywood is 3/4 inch thick. Gussets will be 11 inches long, mounted onto 4.5 inches of the long side of the rafter and allowing me to deepen the rafter by a further 6.5 inches. I have 3 questions.
1. How to fasten the gussets? Naively was thinking of 2 inch long screws. Then I read screws are more likely to break than nails. Should I use 5d or 6d box nails? Use maybe 3 nails per connection with rafter?
2. How wide to make the 11 inch long gussets? I was thinking about 6 inches. Is that width strong enough?
3. How far apart to space them? I was thinking of about every 3-4 feet. Putting them on alternative sides of the rafter.
The gussets will hold 2 by 2s to which drywall will be fastened. Pretty basic questions, I know. I have not done this before. I do appreciate your views.
Best,
Matthias
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Replies
Matthias,
It's true that common nails are stronger in shear than most screws -- certainly stronger than drywall screws, which you shouldn't use. Add glue if you want some piece of mind. I would recommend 4 to 6 nails per joint.
Below are some photos from Dan Kolbert's JLC article, Building a Tight House, to give you an idea of dimensions that work.
[There are two images below; the top image has a caption to the right, and the lower image includes two photos. Click on each image to enlarge it.]
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Matthias,
Both of the easiest choices, framing nails and deck screws, have more than enough strength in shear for your purpose. If you have a nail gun I'd use 2" ring shanks. If not use a cordless drill and use deck screws. The number of fasteners is determined more by wanting to keep the gussets tight than any strength concerns. Off the top of my head, I'd use 2 in each end of a 4" wide gusset, and space them every three feet.
Thank you, Martin and Malcolm. I appreciate your responses. Matthias
Hey Matt P,
Did you happen to take any pictures of your installation you could share ?
When I did this I didn't like how the gussets created gaps in the insulation (I was using fiberglass batts), so I installed one long gusset using scrap osb. I also was concerned about the gap between the rafter and the dropped 2x, so I filled that with 1-1/2" foam board. Blown in cellulose would solve those issues.
I used a similar method to create a cathedral ceiling and strengthen my 2 X 4 rafters. I used 12" square 7/16" OSB gussets 4" OC. I attached the gussets with GRK truss head cabinet screws. I considered the screws main purpose was to clamp the gussets while the urethane construction adhesive cured. before placing the second set of gussets I filled the space between with 1 1/2" XPS. (Sorry, it was before GBA.) I insulated the new trusses with three layers of high density fiberglass batts, mostly salvaged from the original ceiling.
The wavy edged EPS sheets are my air chutes. They are spaced away from the sheathing with 1 X 1 strips tacked to the rafters. Being an old house the rafters were neither evenly spaced or parallel. Making the edges wavy allowed me to make them slightly over wide and friction fit them easily. If a spot was too tight I just snapped off the offending bit. I put the shiny side of the foam up to act as a radiant barrier. A side benefit of the waves is that I got one extra chute out of each sheet of EPS.
Douglas Higden
I did not take any pictures. But I can attest that the void created by the gussets were a pain that I am sure subtract substantially from the value of the assembly. I wish I had used the approach that Joel described.
Matt