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Truss uplift

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nealtw

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More than likely in the winter it is too dry up there.
I have been reading all the "experts" are saying it is the top member aborbing moisture. The long expansion from very dry to very wet is .1%
So if the top cord is 40 ft you might get to 1/2 inch. I figure if the roof pitch is 12/12, some one could do the math if side c is 1/4 longer and side a is constant side b will be.
If you believe that, it has to get very wet in the attic in every house in the country because the gap is about average.
If they are correct, in the summer you could go paint the top cord and stop moisture absorption by a great %age
 

bud16415

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I’m thinking it is temp in the winter the bottom stays warm.
 

nealtw

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That is part of the "experts" argument.
So you are thinking the top cord shrinks like steel. Find a chart for that.
That might be a better argument.
 

nealtw

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But the Epoxy people will tell you that it is the same as wood.
I am not even try the math to figure it out.
If the top cord on both sides of the roof shrink at the same rate with cooler temp, that would lower the center of the roof and push the upright members down so it would counter any swelling going on from moisture. yes, no?
So you would subtract one from the other making moisture even harder to believe.

I think someone has just grabbed the first hand full of straw they could. They claim insulating the roof sheeting will solve it, wonder if I can find something to prove that.
 

slownsteady

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I know "zero" about trusses, so let me ask the dumb question: why doesn't this happen to stick-built houses? After all, once it's all nailed together it is essentially the same thing???
 

nealtw

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I know "zero" about trusses, so let me ask the dumb question: why doesn't this happen to stick-built houses? After all, once it's all nailed together it is essentially the same thing???
Hand framed rafters may come into contact with a ceiling joist over the wall but that is about all.
The tie in the top third of the rafter ties the two together so between the tie and the wall the rafter could do what ever it want to with out having any effect
on the ceiling joist which is sitting on and nailed to a bearing wall in the middle.

A truss may have 3, 4, 5, or six members that join the ceiling member to the rafter member.
 

slownsteady

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So in a stick-built house the ceiling joists on the top floor are totally independent from the roof structure. But in a truss construction, the bottom member of the truss is the ceiling joist?
 

nealtw

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Yes,, samples, they all have names but for most framers they are just called trusses with just a few extra word, like girder, jacks and corners'
They are delivered to the top of the house in bundles and no one cares what design theyy are just do they fit and are they in the right order.

truss_types.jpg
 

arnoldat30

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Venting in the attic should be fine, at least as far as I can figure. About 50' of ridge vent and 2 gable vents. Back soffit is all vented, about 62', and front is half vented. Rafter vents on both front and back the whole way. Figured it all up when building and it was okay numbers wise. I have an email in to the truss manufacturer and designer to see what they have to say about it.
 

mudmixer

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Truss uplift varies on the climate. A big difference geographically.

In the cold, dry north the uplift effects come from the bottom cord that is buried and covered with insulation. That minimized the temperature effects, so the diagonals a much colder in the winter if there is good ventilation above the ceiling insulation. - This leads to the diagonals and top chords becoming shorter and pulling up the total truss upward, causing the cracks in dry wall joints.

It is worse on the interior walls because the temperature effects are a little less than the exterior walls there the trussed a held in place better.

It is a little different where the temperatures and are more constant.

Any truss will move with loads, temperature and moisture.

Dick
 

nealtw

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Truss uplift varies on the climate. A big difference geographically.

In the cold, dry north the uplift effects come from the bottom cord that is buried and covered with insulation. That minimized the temperature effects, so the diagonals a much colder in the winter if there is good ventilation above the ceiling insulation. - This leads to the diagonals and top chords becoming shorter and pulling up the total truss upward, causing the cracks in dry wall joints.

It is worse on the interior walls because the temperature effects are a little less than the exterior walls there the trussed a held in place better.

It is a little different where the temperatures and are more constant.

Any truss will move with loads, temperature and moisture.

Dick
That is the theory. Have you seen any studies that prove it or tell you what the force is? I can't find anything that sounds scientific.
 

arnoldat30

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I can see it in my hallway, it runs perpendicular to the trusses. The drywall lifts where the wall and ceiling meet. Comes back down in the summer. You can just see the tape pulling, doesn't leave a crack, most people probably never see it. In hind sight, there are a few things that I would do different.
 

nealtw

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I can see it in my hallway, it runs perpendicular to the trusses. The drywall lifts where the wall and ceiling meet. Comes back down in the summer. You can just see the tape pulling, doesn't leave a crack, most people probably never see it. In hind sight, there are a few things that I would do different.
In the hall way there should be no screws in the ceiling close to the wall.
2x6 backing on the wall between the trusses would have been a good idea.

For the main room I think your best bet would be to make a header in the openings, not connected to the truss and then a molding only nailed to the lower wall.
 
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