I apologize if this topic has been addressed else where. My quick n' dirty search hasn't turned up any in-depth results.The Project
Most builders seem to use a butt joint to connect their roof spars to their sidewalls. The spar butts up against the sidewall and is screwed into the end-grain from the outside using a 3.5-inch deck screw. Some people apply glue to the joint as well.The Problem
I'm interested in alternatives. The butt joint has the following weaknesses:
- It can be difficult and time-consuming to position the spars to predrill the screw holes.
- Using glue is (arguably) useless and messy.
- If you don't predrill correctly, the spars are prone to splitting from shock impact.
- Intrinsically, the joint only works well in tension. In shear (standing on a roof, for instance), the entire load is borne by the cross-section of the deck screws. And the joint provides almost no racking resistance whatsoever.
- Using a 3-inch deck screw makes you feel good inside, but I'm not convinced it's any better than a shorter screw. According to the following chart ...http://www.zillarac.com/Portals/0/Docum ... ll-out.pdf
... and when downgraded 25% for the end grain effect, you're looking at 80 - 150 pounds of pull-out strength per inch of thread. The thing is, the countersunk head of the screw will rip through the sidewall long before the screw pulls out.The Solution
Now, the butt joint has a long history and works fairly well. And it's certainly easier than cutting dovetails! But I'm interested in something that's FASTER to assemble and STRONGER over time, particularly for 4WD use.
Looking for ideas.
- Replace the deck screws with lag bolts?
- Some sort of structural connector?
- Mating wood joint?
- Extra bracing?
Thanks for the help!