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Are Sagging Rafters Dangerous?

Dear Tim,

I’m remodeling an older home with a rafter style roof. The rafters are 2x4s, spanning 15 feet, and are sagging a lot. Should they be replaced? Is this a dangerous condition, especially considering snow season is coming? — Rick M., Bellingham, WA

Rick,

Wood is incredibly tough and can take a huge amount of distress before breaking. I recall a restaurant near Banff Canada (bigtime snow country) with exposed rough pole-style rafters that were four- to six-inches in diameter, spanning some 20 feet. They looked like toothpicks. By all accounts these should have failed in the first snow storm. However, it was plain the restaurant had been there many years. The rafters were sagging, but otherwise appeared sound.

Another nice thing about wood: it nearly always provides warning before failure — warning as in excessive sagging, creaking, cracking, etc. I have seen many turn-of-the-century (last century) barns with structural members more resembling pretzels than beams or joists — yet there they stood, taking the load. Certainly, waiting that long before repair is not recommended.

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In your specific case, I offer the following:

While obviously you will need to ensure that your rafters meet local code requirements, assuming a snow load of 35 pounds per square foot, a dead load of 15 pounds per square foot, and two-foot rafter spacing; these rafters should have been 2x12s. (I am compelled to point out you could have easily done this calculation yourself using my company’s software, ding!) At any rate, the 2x4 rafters are grossly undersized. I would consider this a dangerous condition that should be addressed before the next snow flies. Some options follow:

• Provide intermediate support to the sagging rafters via the addition of beam(s) or kicker posts near midspan. These must bear on something solid, i.e. a load-bearing wall or column below that extends all the way to a solid foundation. Be careful you don’t add new load to another member below that is already overtaxed.
• Double the rafters by sistering a 2x6 or larger to the sagging ones. These need not extend over more than the middle three-fourths of the sagging member, unless the saggers exhibit signs of distress at their ends (cracking, splitting). In this case, the sistered rafter(s) should extend over the full length and bear solidly at both ends. Nail or screw new to old at six-inch intervals, using a staggered pattern.
• Check the roof for excessive dead load. It could be that there are too many layers of old roofing piled up. If so, remove the old, saving the wood underlayment, and reroof. This won’t solve the problem, but may buy you a little time.
• As a last resort, you may have to remove and replace the rafters.

Tim K. Garrison P.E. of ConstructionCalc.com has authored books and short courses and lectures on topics relevant to builders. Got a technical or management issue? E-mail buildersengineer@constructioncalc.com. Tim reads every one.

This column cannot be reprinted without permission from the author.

The views expressed in this article represent the personal views, statements and opinions of the author and do not necessarily represent the views, statements, opinions or policies of the National Association of Home Builders. NAHB does not necessarily endorse any of the views expressed by the author and NAHB is not responsible for any direct or indirect consequences arising out of the views expressed in this article.

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