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    © Mark Saville 2006  

Wood Truss

 
 
 

Truss Uplift or Arching is an annoying little problem that may send chills through a homeowner if it's the cause of cracks in the ceiling. In this case prevention is the cure. Truss uplift is caused by the top chord of the truss attracting more moisture than the bottom chord which is protected by insulation and the heat from the interior of the house. Compounded by that fact that in winter in colder climates the top chord freezes, expands and forces the bottom chord to arch up taking whatever is attached to it, in this case the ceiling.

 

 
 

An important first step to guard against truss movement is adequate ventilation. Remember, a minimum of 1 square foot for every 300 square feet of attic space. Attics can be a great place to develop condensation which will be picked up by the wooden truss members. It's important to keep the moist air vented to the outside.

Do a tight job of the ceiling vapor barrier. As we strive for air-tight houses, a good job of keeping the envelope tight is a must. If you don't then any little pin-hole in the ceiling vapor barrier will transfer moisture from the inside of the house into the attic.

Truss to Wall Bracket

The image to the left is an example of a bracket used to secure the trusses to the partitions without the danger of the bottom chord lifting the wall up when uplift occurs. The slot in the bracket lets the nail slide up with the truss while still holding the wall in place.

This will stop the trusses from lifting the whole wall and creating a space between the bottom of the wall and floor.

 

 

 

 
 

Using Resilient Bar is a sure method of guarding against truss uplift. The photo to the right is an example of resilient bar fastened to the bottom chord of the trusses and in turn the drywall is screwed to the bar. This lets the whole ceiling float and if there is any movement in the trusses the resilient bar will flex keeping the ceiling where it is. It also provides a more straight ceiling making the drywall taper's job a whole lot easier and a much better job of the finishing.

 

 

resilient bar
Vapor Barrier is not installed here to show a clearer picture of the resilient bar. The vapor barrier is installed under the resilient bar.

Below is another method of guarding against truss uplift. Some builders have actually gone back to stick framing their roof structure but for strength trusses can't be beaten. I can't count how many times I've seen a sagging roof because the stick framing was not enough to carry the snow load. Plus trusses give you more flexibility because you don't need any bearing walls when you use clear span trusses. If precautions are taken as layed out in this article, you should have no problem.

 

 

If using resilient bar is not practical for you and you'd rather fasten your drywall right to the trusses then Truss Upliftyou can use an alternative method to guard against truss uplift or arching. with this method you screw the drywall to backing nailed to the top of the wall and don't screw it to a truss closer than 16 inches from the wall. This way when the trusses move upwards the truss closest to the wall can lift without lifting the drywall. The trusses that are further away from the wall will lift the drywall but it'll flex enough to prevent cracking at the wall.


© Mark Saville 2006 Reprint with Permission only.

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