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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.

enter image description here

So here are some questions and thoughts:

  1. 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:

    1. Jack up the sagging rafters
    2. Sister up a new rafter, and
    3. 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?

  1. Medium cost Fix - I would really like eaves, so I was thinking another much more involved approach might be to:

    1. Support the existing rafter with a false wall,
    2. Lower the wall by half a block, and then
    3. 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:

enter image description here

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.

  1. 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.

  2. Better/cheaper/easier ideas?

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  • $\begingroup$ A variation on option #2 would be to add the sistered rafters at a slightly flatter angle so that they can rest on top of the wall without removing any blocks. You would get a slight bend in the line of the roof, which may or may not be an aesthetic problem. My house has dual-slope roofs in several areas as the result of remodeling, and I really don't see it as a problem. $\endgroup$
    – Dave Tweed
    Apr 27, 2016 at 15:32
  • $\begingroup$ Regarding whether the sistered rafter approach is feasible, dimensions certainly wouldn't hurt. $\endgroup$
    – Air
    Apr 27, 2016 at 17:21
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    $\begingroup$ Ah, 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. $\endgroup$
    – alfreema
    Apr 27, 2016 at 21:17
  • $\begingroup$ @Dave Tweed If I were to sister new rafter tails at a different angle, are you thinking that the new tails would have a birdmouth and take some of the load, or is this just a non-bearing fix to give me eaves? Can it accomplish both strengthening the existing design as well as giving me some overhanging rafters? $\endgroup$
    – alfreema
    Apr 28, 2016 at 2:47
  • $\begingroup$ Yes, they would certainly be load-bearing, running from the purlins to the wall, with proper "birdmouths". They would remove the load from the lower halves of the existing rafters. $\endgroup$
    – Dave Tweed
    Apr 28, 2016 at 11:01

1 Answer 1

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OK, so after having a couple more roofing gurus come out, I just had one with an "ahha moment" that seems so obvious now looking back on it. So far this seems to be the best solution to my particular problem, and may help somebody else in the future with a toe-bearing rafter situation.

The idea is simply build a new roof on top of the old one.

enter image description here

By running new 2x6 rafters from the ridge down over the existing sheathing, I get the overhang I want, get my R-19 insulation, don't have to try and take the sag out of the old rafters, don't have to worry about an expensive demolition. I will plan on shimming the new rafters over the purlins where the old rafters have sagged, to make sure the new rafters are supported by the purlins from Day 1 and don't start sagging too.

Some questions that come to mind with this solution are:

  1. will the existing steel ridge beam handle the weight of the new roof and any snow loads?
  2. how best to fasten the new rafters at the tail end?
  3. can the new rafters remove some stress off the old rafters?

Anyhow, thought I'd share this contractor's simple, yet clever solution.

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  • $\begingroup$ If this is your answer, be sure to return to accept it as the answer (you need to wait a while). And if you actually want answers to those questions, you'll need to ask them in a separate question. $\endgroup$
    – Wasabi ♦
    May 3, 2016 at 20:33
  • $\begingroup$ I was thinking I would leave it open until my structural engineer gets a chance to look at it and either accepts or rejects it. $\endgroup$
    – alfreema
    May 3, 2016 at 20:58

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