I might have to post pics of this later, but I figured I would get started for now. I'm not an engineer......

I have a 1965 detached 2 car garage that measures 22.3 ft wide by 21.3 ft deep. Currently the ceiling joists span top plate to top plate across the width of the garage on 26-29 inch centers. They are original 2x8, cracking, and sagging upwards of 3-4 inches. Further, the joists above the garage door have been notched to allow for the door opener pull and track assembly.

I know I need to replace the joists. My question is, what size boards and how best to go about it? My first thought was replacing them entirely and adding joists to bring everything to 16 in centers. But, is my width (22.3 ft) too long of a span? There will be no storage above the joists, so load will be minimal, but the roof is hipped and is currently braced down to the joists. Would I have better luck trying to jack up the current joists and sistering a new 2x10 to each one?

You'll need at least a 2X12 for that clear span. You might be able to use some engineered I-beam joists, but the won't end up being much smaller than that.

Call GP and have them size I joists for you.

http://www.buildgp.com/wood-i-beam-joists

Can I cut a wood I joist on an angle to make it fit across the top plate but into the roof line angle? I've got a very shallow roof angle with a max of only 6 inches to fit any structure across the top plate (and going down from there).

Awc.org has an easy span table calculator. Select structural 2x12 SPF can span just over 23' at 16" centers.

is it just the joist sagging that is bothering you? could you just add a strongback down the center on the topside of the joists and then cross brace to the rafters?

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Can I cut a wood I joist on an angle to make it fit across the top plate but into the roof line angle? I've got a very shallow roof angle with a max of only 6 inches to fit any structure across the top plate (and going down from there).

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I wouldn't do this because it can compromise the strength of the ends and you just end up risking failure at the ends instead of the middle. If you need to do that, I would use a regular 2X joist and fasten the cut upper portion to the rafters with some 3/8" or 1/2" bolts. Does that make sense?

Thanks RTT. That makes sense. Look like I'll just have to go the conventional lumber route.

These are ceiling joists, not floor joists, so AWC table C-1 would apply. It shows that 2x8's on 24" centers can span around 20 feet.

So I've decided to go with 2x10 conventional ceiling joists. As to the original joists still remaining, after I install the new joists, is it worth trying to jack up the old sagging ones and sistering them with lag bolts to the new structure, or should I just rip them out. To add to this, currently my garage door tracks and opener are attached to a few of the old sagging joists...so that will be annoying to deal with regardless of the outcome.

They are dead weight and they are sagging far too much- yank those suckers out.

Don't forget to install new joists crown up and to use screws!

For that span you need 2x12 especially if you are going to brace the roof to it. You need 2 upright stiff backs ~8 ft from the outsude plate that consist of a 2x6 laying flat and a 2x12 upright to brace it.

I have a similar problem. Thanks for the help! As for the why, it may have been built with wood shingles. Now that they have fallen out of favor and replaced with asphalt, the weight of the roof is much greater.

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For that span you need 2x12 especially if you are going to brace the roof to it. You need 2 upright stiff backs ~8 ft from the outside plate that consist of a 2x6 laying flat and a 2x12 upright to brace it.

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2x12s are going to be a no go. The pitch of the roof is too shallow to allow for a 2x12 to be cut at and angle to fit in. I've only got 6 inches between my top plate and the underside of my roof sheathing.

The current roof bracing is minimal and, as I see it, anyhting is more structurally sound than what I have now.

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I have a similar problem. Thanks for the help! As for the why, it may have been built with wood shingles. Now that they have fallen out of favor and replaced with asphalt, the weight of the roof is much greater.

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That is most definitely the reason...the houses in our neighborhood were all originally built with wood shingles so most all purlins and purlin bracings were done with 2x4s so guess what, you can see all the roofs sagging. Some to the degrees of like 6-8 inches. Ours was, so I had framers go in and re-support it as needed. Now I'll dance and jump up on my roof whereas before, I was afraid to tippy toe on it.

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For that span you need 2x12 especially if you are going to brace the roof to it. You need 2 upright stiff backs ~8 ft from the outside plate that consist of a 2x6 laying flat and a 2x12 upright to brace it.

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2x12s are going to be a no go. The pitch of the roof is too shallow to allow for a 2x12 to be cut at and angle to fit in. I've only got 6 inches between my top plate and the underside of my roof sheathing.

The current roof bracing is minimal and, as I see it, anyhting is more structurally sound than what I have now.

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What pitch roof ?
What is nornally done is you pull up ~4 inches or whatever clears at the back of the plate where the joist stops and then put a backside angle across the top of the joist so it stays under the roofline.

The pitch is 3.5/12.

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What is nornally done is you pull up ~4 inches or whatever clears at the back of the plate where the joist stops and then put a backside angle across the top of the joist so it stays under the roofline.

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Got it. That was my plan.

Should I plan to use lag screws through joist and into truss?