Roof Truss uplift

[henryd]

I am an architect and have designed a 2 story house in Northern Pennsylvania. The roof is designed with roof trusses and the builder says that the climate is too cold to use roof trusses and the bottom chords will uplift so he wants to stick frame the roof. Only problem is there are no continuous internal bearing walls on the second floor to support stick framing...all the roof loads are on the exterior walls.

To conceal any gapping that may occur I know how to detail the drywall and the wall to truss connection with slotted truss clips. But my question is:

Would having the chords of the trusses made with larger sizes help avoid truss uplift? My idea is that truss uplift is caused by moisture differentials between the top and bottom chords so a larger member would not be as affected as a smaller one. Does this sound reasonable?


Thanks

 

[BadgerPE]

Trusses are no problem. They can make them extra deep at the heel if need be for insulating purposes (I'm in WI and we usually specify 15" energy heel for R-38 attic insulation). Also, you are going to have uplift regardless if the roof is truss or rafter framed so that should be thoroughly accounted for in your design through the use of clips, strapping, tie downs, etc.

Never heard of truss "uplift" due to moisture differential between chords. Maybe someone else has.

 

[dik]

If truss uplift is a problem... due to humidity and change in member length; it can be a real problem. I'll try to dig up a paper I have on this...

Dik

 

[msquared48]

I think he sees a difference in plate heating/moisture content between a warm and a cold roof - the placement of the insulation.

With trusses, the insulation is normally placed on the bottom of the truss, between the bottom chords. The airspace between the insulation and the roof is cold.

With stick framing, the insulation is placed between the rafters and there is less air to get cold.

Both scenarios need to be vented to the outwide.

Personally, I see no real difference for any "uplift" due to seasonal differential swelling of the plates here to be concerned over either, especially if the areas are well vented. Not providing adequate venting could create the situation he is talking about though.

You could always place the wall sheetrock first, holding it down from the ceiling by 1/4", and place the lid sheetrock after. That way, with a flexible joint, any vertical movement will be less noticeable.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

Found a couple of articles...

 

[msquared48]

Yup.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dik]

and another...

 

[dhengr]

No, that won’t appreciably change or improve the problem, because it doesn’t change the mechanics of how and why the problem happens. Given the stiffer bottom chord, it would just stress the joints and nailer plates a little more to lift the bottom chord. The way to tolerate the problem is as you suggest, allow the bottom chord to move up and down w.r.t. the top plate of walls below, with the slotted clip angles. Then don’t nail ceiling sht. rock to the bottom chords within 18 - 24" from the walls, but nail it to blocking on the wall, along the wall. This allows the sht. rock to flex over the 24" length and tolerate the truss movement. Put a molding at the wall/ceiling corner to hide the inevitable small cracking at the corner. Also, watch the termination (jamb) end of any wall that goes to the ceiling; stop the wall under a truss with your typical detailing, and have the next truss be 18 - 20" further from the jamb, so the sht. rock can flex in a similar way, away from the jamb, to that next truss.

Really good attic venting, and a perfect vapor barrier at the ceiling often settles the problem down. You have to stop all air movement up into the attic around such things as, vents, chimneys, piping, electrical boxes, lighting cans, all wall plates from below, etc. etc. Some of these should have insulated boxes built around them, or be foamed so they are sealed. It is also important to counsel homeowners that they must control the humidity in the house, for any number of reasons. In cold regions you just can’t run a house as if it is a greenhouse. Alternatively, have the attic be conditioned space, and insulate right under the roof sheathing.

 

[MiketheEngineer]

1. "Float" your ceiling to wall connections. Detail is easy to find
2. Got to TPI ( Truss Plate Institute) for articles refering to this

Can be handled very easily and done ALL the time - any part of the country!!

 

[henryd]

"Can be handled very easily and done ALL the time - any part of the country!!"

Thanks...that's what I told the client. It's easily dealt with..certainly not worth redesigning a new structural system.

I don't believe truss uplift is just a problem in cold climates as I have heard of truss uplift happening as far South as Texas. I have been designing housed for over 35 years and only seen it happen once where the owner went for spray foam insulation on the underside of the roof sheathing with no attic venting and also had a monster whole house de-humidifier. In that situation I think the bottom chord became very dry, shrank and arched up.

Thanks to everyone for all your help!

 

[bylar]

I don't know if it is a temp difference or moisture difference. I never had a problem in Florida but when I moved to NC it was a problem. I had a hard time teaching the drywall hangers to float the ceiling and the wall corners. they thought I was crazy but once they floated the corners I no longer got ceiling seperation and crack at wall corners

 

[dik]

It's mostly a moisture difference... and is fairly well documented...

Dik