Any defense of plywood collar ties??
First time posting.
I’m original owner of garrison house with gable roof; Massachusetts. Now 24 years. Last couple years I’ve noticed the collar ties, attached to every other rafter, in attic (unfinished) are significantly bent and under close examination their ends are even twisting the rafters out of plumb from the plywood sheathing! Collar ties are at approx 2/3 of attic height. Attic floor joists are all parallel with rafters, and nailed to rafters. Then I realized the collar ties are 1/2 inch plywood, cut to approx 7 inch width!! Cannot find any reference anywhere as to acceptability of plywood in this application, which clearly is bending from ambient humidity. I’m about to put 2×4 collar ties at alternate rafters, then cut out the original plywood ties.
Any defense of the original carpenter ? or cautions before I proceed?
Replies
First time posting.
I'm original owner of garrison house with gable roof; Massachusetts. Now 24 years. Last couple years I've noticed the collar ties, attached to every other rafter, in attic (unfinished) are significantly bent and under close examination their ends are even twisting the rafters out of plumb from the plywood sheathing! Collar ties are at approx 2/3 of attic height. Attic floor joists are all parallel with rafters, and nailed to rafters. Then I realized the collar ties are 1/2 inch plywood, cut to approx 7 inch width!! Cannot find any reference anywhere as to acceptability of plywood in this application, which clearly is bending from ambient humidity. I'm about to put 2x4 collar ties at alternate rafters, then cut out the original plywood ties.
Any defense of the original carpenter ? or cautions before I proceed?
I bet the rafters are either not nailed to the ridge board or there's only one nail per rafter near the top.
Leave them. It is far more likely that rafters are twisting the plywood than the other way around. Can't hurt to add more. But these do the job just fine.But obviously your excess humidity up there is the greater problem.
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PS - generally I agree with PIffin. Leave well enough alone but add some more for an additional confort zone.
OTOH, here is an idea. Go up there with a sawsall (or similar) and cut just one in half. Do the 2 pieces pull apart or push together probably pinching the saw as you cut. That would be a way to find out what kind of forces are working on the ties.
And... What's a garrison house?
upper floor is wider than the lower so floor framing cantilevers.The name is from old wood fort garrisons. The attacking enemy would have a harder time climbing the wall
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Matt-good idea to try test cut. I think the rafters are 2x8. I'll get up there later this week to try, and see if there are any other telltale signs of what's going on.
and Thanks to all the other responders; you're helping me be open minded.
Will update later this week.
Steve
Without seeing the house or any dimensions I'll make a generic statement (and maybe a little risky) that a modern house would probably use 2x8 for rafters too. 90% of the standard gable with attic stick framed roofs we build here are 2x8. Also, I'm assuming that your span isn't huge.
I find it hard to believe that you intend to cut a rafter to determine if it is in tension or compression. I'm somewhat flabbergasted but upon second thought...it all seems so normal in this day and age.
It's all making sense to me now...Original Post "Help....I think my roof might be falling in."Helpful answer "Cut some of the members that are holding it up."Reply "Thanks, I'll do that." Ummmm....does anyone else think this is skewed?
Matt used a pronoun, so I guess we could be mis-reading, but I think he was referring to cutting a plywood collar tie, not a rafter
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Piffin, I don't always agree with you on everything, but you are a really smart guy.
This guy probably doesn't have soffit vents.
And if cutting a collar tie causes a roof to collapse, you've got half of the problem solved.
Edited 11/17/2009 4:44 pm ET by excaliber32
I must not be that smart, because I didn't understand any of what you wrote there.
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Paul: You are right. Pronouns are bad. :-)
The idea was to cut a collar tie. The word "rafer" was not in that post.
Edited 11/17/2009 7:15 pm ET by Matt
Okay...lets agree that he's suggested cutting a collar tie. Does that make sense to find out if something is in tension or compression? I musta missed that class.
a little bit.from description, it already sounds like they are in compression tho.
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Thanks for the lesson professor. Can I be dismissed from class now?
Jim, That's not what I thought I said in response to the suggestion. I'm going to cut one collar tie.
Collar tie, not rafter.
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I don't care. Cut it all up to find out if the the ties are in tension or compression. Some of the more brilliant engineer types can answer that without a saw.
I'm sure most of us could just look at the tie and tell, but (a) the OP likely can't, and (b) the collar ties sound to be nonfunctional anyway, so cutting one won't cause a significant problem, especially given that the OP plans to replace them with 2x.
A strong nation, like a strong person, can afford to be gentle, firm, thoughtful, and restrained. It can afford to extend a helping hand to others. It's a weak nation, like a weak person, that must behave with bluster and boasting and rashness and other signs of insecurity. --Jimmy Carter
My suggestion would be to replace them with 2xs, then cut. But...wadda I know?
I would think that 7 inches of plywood should be pretty strong, then again if there is no bracing to keep it straight it could bow. I agree that the rafter is twisting the plywood. 3/4 ply would have been better, 2x4 would have been even better. Then again when you go into an old attic quite often the old 2x4's aren't really sitting any flatter against the rafter either, lots of times youll see that one has come off. Plywood at least you can be relatively sure will maintain it's connection. I've thought to use aircraft cable in the past... haven't really had a real excuse to do this.
Oh - and one other thing... Greetings and welcome to Breaktime!
Sorry for the late salutation but our welcoming committee is down a man... :-(
No need for apologies, I've been pleased with the overall feedback.
I will be getting back in the attic this week for another closer look, but, I do know the rafters are 2x8. Span (horiz) not big, but, with generous slope the true length gets up there. Thinking deeper about the collar ties, my latest suspicion is - being so flexible the collar ties will obviously bend laterally when rafters see wind load; being plywood, under no load or generally at rest, over time they have bowed (I need to measure, but the bow is at least 4+ inches) and developed a 'set' ; then , under wind load the plywood will bend further in the same direction under the compression force ; the rigid nailing of the collar ties to rafters then influences the rafter to twist.
Collar ties (ties near the tops of rafters) are generally considered to contribute little to structural integrity in normal circumstances, and are primarily to prevent the ridge from opening up during uplift situations (strong winds/tornado). As such, they only need to work in tension.
http://www.nachi.org/collar-rafter-ties.htm
Probably plywood is a poor choice, though, since as the rafters sag they'll tend to cause the plywood to compress and warp.
Thanks, DanH, but, I am most concerned though with the rafters , ie the plywood collar ties are securely nailed to the rafters, and as the collar ties bend/warp they are twisting the rafters away from the roof sheathing at the third point of the rafter span?? Looks scary.
Are you sure it's the plywood bending the rafters and not the other way around?
I have a hard time believing that 1/2" plywood can twist with enough force to twist a 2x.
Another possibility is that the rafters are spreading at the bottom - the collar ties (no matter what the material) would then resist the spread where they're connected, which could eventually cause the rafters to twist at that point.
Don
After long look at situation I too found it hard to believe. But, if the cause were the bottom of rafter pushing out and putting the collar ties in tension would not this tend to stretch the collar ties straight? they are quite bent along the length, and there is no discernible sag of ridge?
Collar ties are used to help prevent rafters from sagging. But I doubt they make much difference on 2x8 and 2x10 rafters that are used these days. They certainly did help on the old 2x6 rafters.
I bet they are bowing due to rafter sag.
Rich
What size are the rafters?
I agree thatt he ply does not have the ability to twist the rafters, but collar ties do nothing to resit outward forces. That is done by the rafter ties - which in this case are the ceiling joists.
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My thought was that if rafter ties don't exist or aren't doing the job, then the rafter spread would put the collar ties in tension. If they spread far enough, the rafters could buckle and twist due to the constraint at the collar ties.
It's a moot point in this case, as the OP says they clearly aren't in tension, and there's no noticeable sag.
Don
There is no way that the collar ties could be twisting the rafters. Certainly not if they were half inch plywood. The rafters could twist on their own and they could in turn bend the plywood. Solid wood collar ties would have resisted this more but you are going to have a hard time untwisting rafters that have had 24 years to set in their present position. As long as the rafters are properly sized, I wouldn't worry about it.