I have acquired an old loading dock built in 1930 that has toe-bearing rafters in addition to undersized purlins. As you can imagine, many of the rafters have sagged. Also, because of the toe-bearing design of the rafter, the building has no eaves.
There are many articles about avoiding toe-bearing design. But there aren't many articles about remedying the situation in retrospect. Here is a drawing of the existing situation. The small white block in the middle of the rafter is a purlin.
So here are some questions and thoughts:
Inexpensive Fix - The articles state that you could use some type of joist hanger to possibly fix the problem, but it's not clear to me at all what type of joist hanger would help me. This would be a low cost way to fix the issue. I could:
- Jack up the sagging rafters
- Sister up a new rafter, and
- Use a joist hanger to help prevent cracks in the re-entrant corner.
Is there a Simpson Strong-tie (or some other brand) hanger that would help solve this issue?
Medium cost Fix - I would really like eaves, so I was thinking another much more involved approach might be to:
- Support the existing rafter with a false wall,
- Lower the wall by half a block, and then
- Sister up a new rafter that has a proper birdsmouth cut that will allow the rafter to sit on the heel.
The sistered rafter could now extend beyond the wall, giving me an eave. Here's a picture of what I am thinking about:
Edit: Ugh, it's not clear from the angle I posted, but the sistered rafter is sitting on it's heel on the lowered block/sill plate. It's not hanging in the air.
A properly engineered sistered rafter can take the load from the purlin on down, can't it?
Edit: Based on a comment, here are more details: The existing rafters are 2x6, 24" centers. The pitch is 4/12. Just from visual inspection, the re-entrant corner on the birdsmouth appears to be right around half the thickness of the 2x6, so it seems like it's effectively a 2x3. I believe the purlin is about 9' from the birdsmouth, but I would need to remeasure that, if it's critical.
Most expensive fix - Replace the entire roof with prefabbed trusses. Seems like overkill, but that sort of depends on the answers to the questions above.