Strength of Trussed Rafters


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As I continue to ponder the layout of my new small shop, and find myself running out of floor and wall space for machines and storage, my eyes constantly wander up to the rafters of the garage. Of course I won't be hoisting any machines up there, but I can't help but wonder how I can utilize that space...there's plenty of it for storing woodstock, seldom-used tools, and other miscellaneous crap, even though it's not the most convenient area to access.

Obviously trussed rafters can support a tremendous amount of compressive force, considering they hold a roof, but I'm specifically curious about how much weight the bottom chord can handle, given they are only held together by a splice plate. I found some formulas online for determining strength of trussed rafters based on their construction, but they went right over my head and I don't think any of them addressed weight capacity of the bottom chords in particular.

I know they can handle a considerable amount of weight because I've seen people hoist out car engines with them before, but I was just looking for some other real-world experiences from people who have utilized their trussed rafters in their shops. How much weight do you guys have up there? Did you reinforce your splices? Any opinions are appreciated!

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I've been in constuction 30+ years and I have never seen a splice plate fail, I've seen the roof cords broken by trees crashing across the roof. As far as loading the attic space if your ceiling starts deflecting your living on the edge at that point. That being said you will be amazed how much you can store in an attic space, and if your storeing lumber the weight is distibuted across all the bottom cords that support the length of the boards. Put your heaviest objects close to the cross webbing because thats where the truss is the stongest. Hope this helps, Dave

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Dave has the right of it for sure. One thing I would do if I had to put something that was heavy and small in one spot is put something under it to distribute the weight over more bottom chords a five gallon pail full of metal would over time have some effect on just two bottom chords as an example. The bottom chords are designed to accept some form of cladding so having weight there is factored in on the engineering side of the design. Lumber should be no problem as it will naturally distribute the weight but balance out the load to be safe maybe, having a few hundred board feet of hard wood all on the same side of center might do something over a long time. All in all if you avoid extremes over long time periods and you should be safe. The other thing is snow load a factor here, maybe not where you are but if it is you may overload your roof had two roofs collapse here a few years back due to snow only an if there were additional load under the roof this would accelerate that process. Those are all the extremes I can think of hope that helps.

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For real world experiences in my first shop, I had 2 pick up loads of tinkering and shop stuff including 300-500 bf of lumber with no problems. A shop I built for a friend I helped him store a huge amount of stuff besides lumber. Both cases there was no sign of failure or damage. In both cases there was 5/8" plywood laid across the cords for a floor.

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I had a shop with a 2x4 truss structured roof that was built over the garage. I hung an 10' "I" beam across the bottom boards of 5 trusses and added a hoist and trolley. This was installed crossways over a 5' x 9' trap door in the floor.

I used that rig to lift many heavy loads from the garage to the shop. The list includes all of my power tools such as a unisaw, a jet 15" planer and an 8" jointer (not all at once). The "i" beam itself also weighed almost 200 lbs.

I never had a problem in the 7 years I had that setup.

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