Reinforcing sagging rafters.


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Old 10-06-04, 08:25 PM
ggs
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Reinforcing sagging rafters.

I bought a ranch house last summer built in the early 1970's that has a 4:12 pitch gable roof with 28 pairs of 16 foot long 2x6 rafters 24 inches on center, a 2x6 ridge board, and 2x6 joists. There are a few flimsy collar ties (looks like 1x4's ?), maybe averaging every 6 rafters apart, but no other supports to this roof frame. The roof was sagging in a few places and had developed an unrelated leak, so it was replaced last fall, including approximately 20 new 2x6 rafters and all new 5/8 inch plywood sheathing (original was 1/2 inch and much of it rotted) (have a Cobra ridge vent now and am in the process of increasing soffit ventilation at every rafter bay). Some of the old rafters that were removed were so curved they looked like giant rocking chair legs. The new rafters improved the sagging areas of the roof greatly, but not to the point where it's perfectly flat. Since all of this happened, I have learned more than I ever wanted to know about roofs, including that 2x6's are not supposed to span 16 feet, no matter what grade of wood, without some support along the way. I also learned not to use the same home inspector again (he is supposedly an engineer no less!) and that the veteran roofers on the crew that did my roof made no mention that 2x6's wouldn't be strong enough to span 16 feet without extra support. My question is what is the best way to strengthen the roof frame from within the attic to prevent any further sagging of the rafters? The house only appears to have 1 interior bearing wall, running directly under or close to and parallel to the ridge. The supports below are concrete block piers and doubled up 2x10's in the crawl space. Would 1x6 or 2x6 cross ties on every pair of rafters be useful in stopping further sag? If so, how far down from the ridge should they be attached? Also, if the rafters don't perfectly match up at the ridge, can or should a cross tie be used at all? Also, how many/ what type of nails or screws should be used to attach them? My other idea to supplement the cross ties was to have 2x4 knee studs (not sure if this is right term) attached to the side of each rafter, going straight down, and attached to the top of a flat continuous horizontal 2x4 running perpendicular to and on top of the joists. Since there would be no bearing wall directly under these knee studs, I thought of placing them closer to the eaves where I would think (but certainly don't know) the joists are not at their peak load (i.e. closer to the exterior wall and not at mid-span). I'm not sure if this is a sound idea or if I would just end up with sagging joists too. I figure this cross tie/ knee stud combo could split the 16 foot rafter span in 3 sections and as long as they are all less than around 8 feet, I should be pretty safe in not overloading the 2x6's to the point of sagging. Then again, I'm not even sure if cross ties prevent rafters from sagging at all. Some sources say they do, but most don't mention that as the reason to use them. The 1 thing I really don't want to get involved with is purlins and tying them to the bearing wall. I'm no engineer, but I think the single row of block piers and double 2x10's in the crawl space are under enough of a direct load, supporting the floor joists above. I just don't want to risk anything with them. Sorry for being long winded, but I wanted to provide as much information as possible. Any advice on this subject would be greatly appreciated. Thank you very much.
 
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Old 10-06-04, 08:36 PM
Doug Aleshire's Avatar
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ggs,

So what don't you want to do?

It seems that you need someone to look at this - structural engineer who knows their stuff!

There could many options here to suggest but if you intend to make due with what you have - get some professionals eyes on it. It would be a risk here to suggest anything as a "fix" not knowing or seeing more.

Alternative - tear off and install new roof system - trusses - exterior wall load bearing. Cost would be high but trying to make all the adjustments as needed with a 4/12 pitch means terrible working conditions which drive labor price up. In fact, I would be against anything less than trusses as you need to get material in there to work.

Thoughts?
 
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Old 10-09-04, 05:45 PM
L
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ggs,

Like Doug said, you need to have a good structural engineer look at this -- someone who can actually lay their eyes on it. At 16', 24" O.C. you are overspanned with most dimensional lumber, and you haven't even told us if you are dealing with a snow and/or wind load for this roof. And what kind of roofing?? (You mentioned 5/8" plywood sheathing, which is overkill, and really a waste of money, so I'm assuming it's comp.)

The "BEST" solution would be trusses, based on what you have told us. And I'm thinking that they wouldn't be that much more expensive than trying to correct what you have in place already. One way to find out -- ASK!!
 
 

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