If the roof has not leaked, there is no limit on replacement. Shingles should last about 20-25 years. Anything less than that could indicate you have may rotted decking so new shingles are not holding. Also, if the roof is shaded, your roof will not dry properly and also can get moss growing.

Drywall diy guy,
Thanks for answering.

But I didn't ask my question clearly. I should have asked: how much nail-pulling stress and nail hole "density" can plywood take before the plywood will no longer hold more nails?

I would go ahead and bite the bullet and replace the sheathing. You need to seek out a reputable roofer who will warrant their install. Shingle mgfr's warranties are typically not worth the weight of the paper they are written on but do have install guidelines that must be met. A good roofer outfit will exceed those minimums and cover their install work as well. The claimed life expectancy of shingles that mfgr's throw out there is dependent on geographical location. No shingle is going to stand uo to severe hail storms. Here in the mojave desert, you are lucky to get 15 years out of a "claimed" 30 year shingle. A 7 year average in your case is real extreme and along with the hail, sounds like incompetent installs.

It's not the nail holes and nail pulling that are going to be the issue - it is the potential water damage, delamination, rot, etc from poor roof installs. The condition of the plywood should be assessed prior to reroofing.

Thanks to all for replying.

The roof is not leaking, so I'll wait until more roofers are available. It's a crapshoot, regardless of license, BBB information, contractor warranty, recommendations, polo shirt, whatever.

I'm anxious to see how the shingle manufacturers maintain their quality control. One manufacturer did admit to faulty shingles and paid a few dollars.

Thanks again.

"The condition of the plywood should be assessed prior to reroofing."

And AFTER stripping.

Looking from the attic only shows massive damage, there can be a lot of damage under the shingles that does not show from an attic.

Consider a metal roof, a cool metal roof.

Interesting!
No one has suggested that you ask a local building official, if there are restriction on the number of re-roofs allowed for your structure.

Roofing material has a weight per square(100sq.ft.)
Your roof structure has a design live and dead load.

Here on the truly left coast you are allowed a max. of three layers of roofing, because of the weight.

Snoonyb,
"three layers of roofing, because of the weight".

Huh? the shingles and felt have been removed and replaced each time.
Did I misunderstand your comment?

snoonyb, no one mentioned it because that wasn't the question. OP wasn't talking about layering of shingles, but was referring to removing and replacing shingles and the effect on the plywood underneath, if I understood correctly.

In places with winter, you aren't allowed more than 2 layers of shingles because of the additional weight of snow and ice.

I'm sure that post would have been useful if you had posted it over a year ago when the OP first asked this question. I'm sure they have done whatever they were going to do by this point.
Why did you revive this old post?

wufpack registered today to respond to an old post. Might be a Carolina (Chapel Hill) graduate.

Thanks to all of you for your comments, concerns, and humor.
In case I wasn't clear in my original post, two roofs were replaced because of either age (20+ years) or weather (hail). The others were crappy shingles (admitted by the shingle manufacturer) and/or bad install.

Since I now have a fifth roof, and am hoping to live long enough to have a sixth, any info is useful.

The latest roofer, recommended by the insurance company, said I didn't need to replace any plywood, even though two sheets (at different sites) were bowing outward. Even I knew better.

Don't know how to judge roofers...

Most roofers do not understand the concepts underlying roof sheathing. The last roofer I used insisted that roof sheathing panels must be butted as tightly together as possible, which is absolutely wrong.
I could not get a link to post, but Google the words
"Buckling of Wood Structural Panel Sheathing" and open the PDF document that should be as the top of the list. You, Mr. homeowner, will have to be the technical expert on the site.

The nails are not fatned in any precise position.

Evey one n a while you might hit the same hole with a new nail, but is is almost instantly apparent when it goes in with almost no resistance.
You just add another nail a fraction of an inch away and move on.

Iy you really believe there is an area with excessive nail holes, just move the first shingle row down an inch when starting.