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To Sister Or Leave Alone...

4K views 8 replies 7 participants last post by  JA Boomer 
#1 ·
Hi All,

I am finishing my garage. It was built in 68' and is an oversized two-car about 26' wide. The ceiling joists are a 2x4 trussed design with the two joist spans meeting in the middle of the garage. It's 24" on center construction with plywood and nails for the "plating" on the truss connections. I'm drywalling the whole thing including the ceiling and I am debating sistering the middle portion of the joists (where the two spans meet) before I do this.

Pros:

-Would be stronger (I think).

Cons:

-Harder to insulate the attic with the reduced spans in the middle (it's 24" on center right now).
-Drilling more holes in existing joists probably not a good thing.
-It's been standing for 45 years so why bother.

My friend is a structural engineer and he recommended not touching it, primary because by putting more holes in the existing joists you could actually weaken the structure. But I'll be putting screws into it when I install my lighting junction boxes and drywall anyways, and I was going to add some brace supports as well .. so why not sister it with plywood (to match the width of the existing truss plates and put a 8 foot 2x4 on either side, PL premium'ed and screwed together. So the middle 8 feet would be like a laminated beam .. kinda.

Is this overkill, a good idea, or actually potentially dangerous?

Thanks in advance for the advise!
 
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#5 ·
His main concern was screwing more holes into the existing joists could weaken the existing structure. If I have to screw into the same existing joists to mount my octogonal juntion boxes and drywall then why not sister it anyways to make it stronger?

A picture is always nice so we can see what your seeing.
Pictures attached. Hopefully this will help!

As far as I know, even a truss can not have a joint in the middle. Truss is getting strength from smaller pieces, not reinforcing a cut joint in the main pieces. Yours sounds home made and any reinforcement is a guess.

Your friend should be hired and paid. That way he can at least take the responsibility.
I'm not sure that it's home made, I think a contractor built the garage. There is definitely a joint in the middle of every joist as you can in the pics - located in the middle of the garage.

This may sound cheap or stupid. But I am not willing to pay someone to look at this. Just going to try and gather as much information as I can over what the prudent course of action should be. And proceed.
 

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#4 ·
As far as I know, even a truss can not have a joint in the middle. Truss is getting strength from smaller pieces, not reinforcing a cut joint in the main pieces. Yours sounds home made and any reinforcement is a guess.

Your friend should be hired and paid. That way he can at least take the responsibility.
 
#7 ·
My garage has a factory-constructed trusses which hold up a second floor. They are made from two bye four material and all have a joint in the top and bottom chords. So yes, in that type of truss construction joints can be ok as long as the truss is prerly specced out. Ron
 
#6 ·
Couple of terminology issues. You have trusses, not joists. A truss consists of individual elements arranged in a series of triangles. Each element is either in tension or compression only, there is no bending moment on any of the elements. This occurs because the joints between the elements are considered to be pins, allowing free (theoretically) rotation of the elements. And that is what you have. And they appear to be well constructed, using techniques consistent with the age of the structure. Most trusses built today use pneumatically installed gang nail plates, but there is absolutely nothing wrong with the method used to build your trusses.

As to sistering, I am not clear exactly what you want to sister, and why you want to do it. I understand you want to drywall the underside of the trusses, presumably for appearance. The extra weight of the drywall is not likely to be a problem for the trusses, but without a structural analysis (which you have indicated you are not interested in paying for) it is impossible to say for certain that the trusses will not become overloaded due to the additional weight of drywall. Certainly if you planned to use the trusses for storage support, you would want an analysis first to see if the trusses could handle the load, since trusses are typically designed only for the actual imposed load, which in this case is just the weight of the roof and wind loading.

Before altering a truss, you need to think very carefully about unintended consequences. For example, each joint in a truss is designed to act like a pin. You could inadvertently strengthen a portion of a truss and make the joint rigid, in which case you would have a space frame, not a truss. Load distribution in a space frame is very different than in a truss, since moment can be transferred across a rigid joint, meaning structural elements can pick up bending as well as compression and tension. This can radically alter the behavior of the truss in ways that are outside the original design characteristics. Structural engineers earn their living analyzing structures like that, it is way beyond DIY to understand the mechanics of a space frame versus a truss.

Conclusion: If it were my house, I would not touch those trusses. Adding drywall is a risk, but probably OK, since it is only a few pounds per square foot. Best of luck.
 
#9 ·
Thanks for your responses guys. I guess I have my answer. My structural engineer isn't worried about a single layer of drywall from either the weight or screwing perspective. So I will not proceed with sistering the trusses. Actual makes my life easier.
 
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