Sistering Joists for old Garage Roof?

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Old 10-11-19, 01:57 PM
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Question Sistering Joists for old Garage Roof?

I have a 1949, "terrace" (built into a hill), cast concrete garage with roof joists that are 2x8, 13.5" on center, with 14' spans. The joists are covered with diagonal ship lap, no plywood, then sheet roofing. I know that these joists would not meet Seattle building code now, and I plan to load the torch down bitumen roof with some plastic pallets with planted containers.each summer. (I will clear each winter, and we typically have no snow loads here.)

(BTW, I have been "farming" on this roof for the past 2 summer. I'd just like to get a handle on keeping the roof from failing, before it happens. You can see the garage roof in "farm" mode in the first photo with coffee bean bags filled with dirt & plants on top of foil insulation on top of plastic pallets.)

I'm on a tight budget. A roofer is helping me with a new surface on the garage, but I'm concerned about dead loads. Do you recommend "sistering" the 2x8s? If so, can you please explain how you do it? If you don't recommend this, what other ideas can I explore?The inside of the garage is unfinished, so I can see where the joists are currently sitting on the concrete walls.
 
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Old 10-11-19, 02:13 PM
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One easy and inexpensive thing you can do is change where you place your plants. Right now you have a row of plants on the left that are probably all bearing on one or two joists. Then there are several joists with nothing. Then you have a group of six plants in another group bearing on a couple joists. Then another group of three plants in the middle of the span.

Better would be to place your posts in a row down each side of the roof near the outer wall. This would put the weight close to the support of the garage's walls and spread the weight out evenly so no one joist is carrying too much load.

Avoid putting heavy items in the middle of the roof which is mid-span for your joists, the worst place for weight. Avoid putting plants close together in groups which concentrates their weight on a few joists
 
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Old 10-11-19, 02:40 PM
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PD I like what you said. But you didn't answer the question as to the best way to sister the joist. In spite of the piping and electrical being in the way why can't new joist just be place between existing ones or next to them? Or am I not seeing things correctly?
 
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Old 10-11-19, 03:41 PM
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Sistering is better than nothing.
I am not a structural engineer so am not qualified to say if this is the best option.

First I would check to see if they are bowing down now.
Also check for ant rot.
If yes then how much?

Before doing this I would remove the weight from the roof and give it a couple days without the plants.

Then get 2X8 that are long enough to to fit and sit on top of the walls.
Fir would be the preferred wood.
Then construction adhesive wavy line along the the length of the rafter.
I would then stitch the two together using screws unless you have a power nailer.
Just be sure they are tight together.
I am not a big fan of pounding away on old construction with a hammer.
 
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