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Dear Daniel,
You would do well not to wait. A 15-second calc using my handy ConstructionCalc Power Span Table tells me your rafters should have been 2x10 (assuming 24-inch spacing, Doug Fir #2, 25 pounds per square foot snow load and a comp roof). Collar ties are not your answer, unfortunately, because they will not give the rafters the additional vertical support they need.
Collar ties mainly keep rafters from spreading apart sideways. They’re of little help for a vertical sagging problem. It sounds like your ceiling joists are already doing the job of keeping the rafter ends from thrusting outward, thus new collar ties would be redundant.
You have two choices. Plan A (see sketch below) is the preferred alternative: Add support to the rafters via vertical or diagonal 2x4 “kickers” nailed (screwed would be better) to the rafters, as
close to the rafter’s mid-span as possible, extending to a bearing wall at the ceiling level. If there are no bearing walls (walls with footings below them), we’ll have to go to plan B.
Plan B would be to sister (place side by side) an additional 2x6 to each existing rafter, thus doubling the rafter’s width. I would suggest screwing the two together with three-inch wood screws at about nine-inch spacing, staggered pattern.Trying to drive nails into old wood can really jar things, something you definitely don’t want, especially at the rafter heels. The new 2x6s can be 10-feet long, centered over each existing rafter. They don’t have to extend over the full length of the existing rafters because the problem is a bending problem, occurring only near the middle. There is no overstress problem near the rafter’s ends, so we don’t have to beef them up. My book, “Basic Structural Concepts for the Non-Engineer” explains bending theory for anyone wanting more of the whys and wherefores.
One last thing. I suspect the heel connections of rafter ends to ceiling joists are grossly weak. This is the connection in any triangular truss where the most force occurs, and thus explains why gang nail plates are always largest there. To expect several face nails into overlapped 2x’s to develop the required strength is expecting too much. However, because the wood is old and prone to splitting, it may be advisable to leave things be, unless visual inspection reveals slipping, cracking or other signs of distress. The reason Plan A, above, is the preferred alternative, is because adding additional vertical support will greatly relieve the stress on this critical heel joint.
As you are well aware, the devil is in the details, and there are many I’ve not addressed. Don’t be shy about calling in a local engineer to fill in the blanks.
Thanks Daniel for a very good question!
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.
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Dear Builder’s Engineer,
