GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Rafter tie question

Chris R | Posted in General Questions on

Live in Maine have a colonial house with unfinished full attic, 8:12 pitch roof., 20 years old.

Looks like when roof was framed they basically decked over the floor of the attic (second floor ceiling) with plywood and then placed the rafters on top of this deck with a birds-mouth cut on edge. Ridge is non structural; no collar ties.

I recently pulled up all the plywood, and removed existing fiberglass insulation to blow in cellulose at a greater depth, covering the top of the 2×10 floor joists which I presume act as rafter ties. I found that the floor joist was not a continuous 2×10 but rather two separate pieces of wood overlapped about 4′ on a load bearing wall, toe-nailed to top plate of bearing wall. They were not nailed to one another. There is cross bracing for the joists

I nailed them to one another with a dozen or so nails where they overlap. I also added collar ties 1/3 of way down from ridge. Since I am not putting the plywood back down due to increased insulation depth, I am wondering if it will create any problems with spreading. Was the plywood playing much of a role keeping the overlapped floor joists/rafter ties from spreading? Now I’m wondering if I should go back and try to really secure the overlapped 2×10’s together rather than just a few nails – bolts or metal strap?

Maybe I just am worrying too much, and maybe if it has been fine for 20 years with plywood in the attic it will be just as fine without plywood. And yes, I did air seal everything and put insulation baffles in.
Any thoughts would be appreciated.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Riversong | | #1

    Nice to hear from someone who knows the difference between a rafter tie and a collar tie. Few carpenters do.

    If the plywood overlapped the lap joint in the joist pairs, then it probably was helping create a tension assembly. To perform as rafter ties, the joist pairs should have been nailed together, but a dozen nails per lap should be more than sufficient for this connection - the toe nails were woefully insufficient.

    The weak point, however, in the rafter thrust triangle is the birdsmouth connection to the plywood sublfoor on top of the joists. There would only be toe nails at that connection, too. In a conventionally framed roof, the rafters would lap the floor joists at the eaves and be side nailed. So it's that connection that is worth strengthening, which can be done with metal angle plates and heavy duty screws (not drywall screws) or small lags.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |