I'm retrofitting my 70s ranch house in a snowy area to handle heavier snow loads. The house has 2x6 rafters every 16 inches, 3/4 plywood sheathing, and 2x6 collar ties (16 feet long) that sit mid-roof, lapped on a bearing wall. I've read that these collar ties can go under extreme compression with heavy snow loads. I'm considering adding blocking and sheathing above these collar ties to prevent deflection, plus plywood gussets glued to the rafters for extra support. However, I've seen conflicting info about collar tie placement, so I'm here to get some advice on my plan.
3 Answers
It's important to focus on strengthening the lower rafter ties first. If your collar ties are 2x4s, consider replacing them with stiffer materials or even switching to steel tension rods. Adding blocking and sheathing is good, but make sure to use the right screws or nails to prevent buckling. I'd recommend consulting a structural engineer to size everything properly for the snow load you're expecting.
Actually, collar ties are under tension when you have a heavy snow load, as they help prevent the rafters from spreading apart. It sounds like you're right in thinking about adding blocking and sheathing, but for a stronger solution, you might want to consider reinforcing the connections with gussets and ensuring you're tying more of the roof's load back to your bearing wall. Just remember, each roof is unique, so having an engineer check your setup would be wise.
From my experience, collar ties mainly just hold the tops of rafters together against high winds. If you’re concerned about compression under snow, think about removing the collar ties altogether and sistering the rafters with 2x8s instead. That way, you can put collar ties in the top third of the roof like current guidelines suggest. Also, check out the IRC for specific info on roof framing.