I used it on my house in 1977 when it was popular. It was the Douglas fir type. It was indestructable. When I added on in 1990, I used yellow pine. That was the biggest mistake of my life. It delaminated, wouldn't hold color, and generally fell apart. In 2000 I put stucco on the entire house and the Fir plywood was still like new. It does matter what type you use. I never sealed the vertical joints and never had a problem. The horizontal joints should have flashing to prevent water from getting behind the lower sections. I often cut narrow pieces, but because I had machinery to cut the lap joints, this was never a problem when I joined a small piece to a full sheet, or pieced in a piece. If I was butt jointing two pieces, I would definately use a paintable silicone latex caulk. I don't see it used much anywhere anymore. Most homes have manufactured stone in this area.

Thanks for your comments. Question 3. above refers to having the rusted z-flashing in place, but covering the rusted z-flashing with a trim strip caulked at the top, so no more rusting. That shouldn't take that much caulk, and caulk is cheap compared to residing (which uses caulk, too *smile*). The trim strip cover for the existing z-flashing is the least labor and materials intensive approach, and I'm stongly considering doing it as an interim thing to see how well and how long it works. I do have a quote out for replacing the T1-11 in this one bad spot with new T1-11 and z-flashing and priming and painting it. I'll see where that comes in, to see if it makes more sense than the interim solution I'm considering. I also have a quote coming for vinyl re-siding, but I'm expecting that to be quite expensive. Although I may decide to go that way for the long term, even though it isn't wood *smile*. Thanks again.

T1-11 is about as simple as siding gets, hard to imagine that any halfway trained carpenter can't fix it. Just replace entire damages sheets ("patching" isn't worthwhile) and make sure a z-flashing is used at the top edges to divert water to the outside at joints. A "siding contractor" will almost certainly try to sell you something different and more expensive, like vinyl siding. Just look for an ordinary carpenter.

Yes. If you haven't already figured this out, there are numbers of how-to-do-it posts here starting with the "Featured Answer" at the top of this thread. Because Houzz displays only part of the Featured Answer, you want to click on the faint "read more" line to get Houzz to display the rest of that detailed post by Luna123456. Also, there are something like 1.1k additional posts in this thread, most of which are not initially displayed. On my computer, I have to display all the posts before I can do a full search of the thread. Houzz makes you look below the "Featured Answer" to find the fine print which currently reads "see 1037 more comments." You may have to click a "See more comments" several times to get Houzz to display all of the posts. Then you can do a terms search (CTRL-F calls up a search box on MS Windows computers.) That will get you the posts with recommendations on specific brands of detergents and oxy liquids to use with DIY refilling.