Shimming plywood to ceiling joists

I plan to attach plywood sheets to my basement ceiling (2x10 joists, 16" OC). The application is an extension of a climbing wall (I will be hanging climbing ropes and holds from the plywood).
Every inch (of height) counts, so I want to attach directly to the joists rather than use furring strips. The structure is sound but the lower surface of the joists aren't all at the same height...maybe up to 3/8" difference across the area in question.
I can't use the traditional shim technique, so I was considering building up 1.5" wide strips of material (felt paper?) along the underside of "high" joists to bring them down to the level of the others.
Aside from being a bit strange, is this a really bad idea? I'm thinking that felt paper will also help to reduce sound transmission to upper floors.
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Borrall Wonnell wrote:

I can't see anything wrong with it. Might be easier to put up the ply loosely then use wood shims in the low spots, then tighten all. Even easier is to just put up the ply tight and let it bend as needed. If it doesn't bend enough, no big deal.
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On Mon, 28 Nov 2011 06:09:51 -0800 (PST), Borrall Wonnell

Good idea. Go for it.
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Not to get philosophical but this newsgroup reaffirms my faith in humanity. It is so rare for people to go out of their way to help someone else.
Now, with that silliness out of the way, there seem to be many ways to do the same job. I'm a bit surprised nobody suggested sistering a 2x to the side of the joist to get a flush lower surface. That would be impractical due to wires/blocking not to mention the plywood edges would no longer line up with the nailing surface.
I would have ripped 2x stock *IF* I had the tools and skills to do it. Unfortunately I don't have a table saw, and cutting a straight line with a circular would be a (frustrating) challenge for me. The drywall shims seem neat but aren't available locally. And yes, a little variation in the plywood could make for a more interesting rock climbing environment...but not really interesting enough!
Cheers, Dave
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No, just one way, the right way.

Nobody suggested it because that is the most overly-complicated, wasteful, asinine way to do the job possible.

Here we go with the excuses as to why your overly-complicated, half- baked idea is the only option open to you...
You can get a perfectly viable portable table saw that will do this job and 100's of others for about $100. What self-respecting guy doesn't need more tools?
The big problem with your original idea is that felt paper deteriorates and compresses. This is going to be a load-bearing structure. You want REAL WOOD supporting it.
Use plain old door shims if you are averse to doing it properly and buying a table saw. All you need is a utility knife. Surely you have a utility knife, or can spend the $4 to purchase one.
Two shims in opposite directions to make a level nailing surface. If the gap is less than the thickest part of the shim, cut the thick end of the shim off until it's thin enough.
Use a long straightedge to find the low spots. Slide the shims together or apart until they fill the gap. Attach the shims to the joists with some small finish nails. Drill and screw permanently with the plywood.
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On Nov 29, 1:19pm, snipped-for-privacy@gmail.com wrote:

Bad day?
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re: "cutting a straight line with a circular would be a (frustrating) challenge for me."
It shouldn't be. All you need is a straight edge clamped to the wood to run the saw shoe against.
That straight edge could be anything from a 6 foot level (which you should have for the type of project you are describing) to a store- bought guide such as this:
(Amazon.com product link shortened)
Of course, for $100 you could buy an inexpensive table saw and have it for other projects - as long as you're not building cabinets or fine furniture.
I, for one, got very lucky about 10 years ago and found a full-size belt driven 10" tablesaw with 2 table extensions for $100 in my local paper's Saturday Super Ads. To this day I still feel that it was the best $100 I've ever spent. I can't imagine how I would have gotten through some of the DIY projects I've done without it.
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Worth a try, and yes I have a long level. I managed a 1/4" strip, so there's hope yet. Table saw is a luxury I can't justify at this point (SWMBO).
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