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I have a 1929 craftsman house with dimensional 2x4 rafters on 24" centers. They're about 13.5 ft long, spanning 11.5 ft each (measured horizontally, wall to ridge beam) at a 7:12 pitch. About 25% of them have diagonal 2x4 bracing tying them to either the center interior wall or ceiling joists (also 2x4) over interior walls. The top of the rafters were angle cut to sit flat against the 1x6 ridge beam, then nailed through the ridge into the end of the rafters. (I know, definitely not up to current standards.) Over the years, 3 consecutive rafters split at the top end (maybe a tree branch fell on the roof?). I've sistered the top 6 feet on 2 of them with additional 2x4's, screws, and plates and plan to do the 3rd one soon. The roof decking is 1x6's and 1x8's nailed on top of the rafters. Besides a few knots and a couple split decking boards, the are no apparent soft spots or sags in the roof. It currently has 2 layers of 3-tab shingles.

I need to re-shingle the roof. I want to strip the existing shingles, add deck and ridge vents, and go back with an impact resistant dimensional shingle with a long warranty but I'm concerned about the weight. I've read that some can weigh over 400lb per square (100 square feet). A couple roofers I asked out for bids also said I would have to add 15/32" plywood or osb decking to ensure the nails have something to hold to, not empty knot holes or gaps between the current decking boards.

I'm in the central United States, Kansas City. We normally get 6-8" of snow on the ground, but occasionally up to 24-30" of heavy wet snow. We also routinely get 60-80 mph wind gusts with thunderstorms, sometimes higher.

Will the 2x4 rafters support the weight of the dimensional shingles, possible additional decking, and snow? Since not all the roofers recommended the additional decking, would that increase the roof strength, or is it an unnecessary added load?

  • The OSB will definitely make the roof stronger. It will also increase the load, but not that much compared to wet snow and/or wind gusts. – Martin Bonner May 13 at 14:02
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Your roofers are correct in adding plywood or osb board I order to maintain the roofing warranty. If there is a “blow off” the manufacturer will not honor the warranty if the nails are not installed correctly, including the minimum required thickness of the roof sheathing, which is 1/2”. (1x6 boards is not acceptable because of the number of gaps between boards means the nails could miss the lumber and be installed in the gaps, and thus no withdrawal holding power.)

We design for the exceptional not the standard. So, your occasional 24” deep snow will translate to 30 - 35 lbs. per square foot Live Load plus about 15 lbs. per square foot Dead Load (roof sheathing, roofing, etc.).

Your 2x4 roof joists (even the older larger ones like you have) will only span about 9’, depending on the species and grade. However, the occasional diagonal 2x4 brace will help, but your roof joists are over stressed.

I’d also be alarmed about the connection at the ridge. High winds could “suck” the joists off the roof with just nailing. I’d use Simpson joist clips to hold the roof together.

  • I'm not challenging your answer, but do you know what safety factor was used or how much the joists are considered over stressed? I ask because in the last 89 years only those 3 joists show any signs of trouble, it looks like they broke several years ago, and showed no movement since I bought the house 6 yrs ago. I'm considering a roof with a 50yr warranty and want to make an educated, calculated decision on the roof upgrades so it'll hopefully last the full 50 years. – Eric Simpson May 14 at 16:44
  • @EricSimpson I understand. I’d be confused too if my house had withstood 89 years of stress with minor problems. However, now your adding about 1.5 lbs. per square foot of sheathing plus heavier shingles...but removing two layers of shingles. When we design we worry about the extreme for loads AND duration. We use a safety factor of 1.8. Also, we assume the loads will be there longer than 30 days. If loads are removed after 7 days you can increase the resistance by 2 and if it’s there less than 14 days you can increase the load by 50%. Also, we assume the joists will be “stiff” without – Lee Sam May 14 at 17:03
  • Without much deflection. Wood can deflect quit a bit before failing. You can eyeball the joists to see if they are deflecting a lot. Also, the joists that you’re having trouble with are probably the ones that don’t have a diagonal brace. If so, you could add a few diagonal braces to help. BTW, make sure you screw the new diagonal braces into place rather than hammer. Hammering could split old lumber. Then, with clips, I think you’ll be fine. – Lee Sam May 14 at 17:08
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    It’s been standing since 29 I don’t think the roof is over stressed, the wood used back then was clear old growth with at least 3x the rings (and on some 5x) that are today’s standard for building grade lumber or #2 with only 6 rings on a 2x4, so today’s load calculations don’t compare for pre 40’s homes. – Ed Beal Jun 12 at 17:52

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