[special-K]

I assume it's framed with horizontal 2xs across posts, not studded up like the house. Is T-11 only $30 now? I bought some a couple years ago and it was $48 a sheet, which was not an inflated cost like the plywood was. Plywood was as much or more.

I don't like T-111. It's destine to rot, especially in this use. Let me guess, is the bottom all rotted out? I guess any wood will do that, but the T-111 seems to really sponge up the water. Unlike lumber, it's laminations of thin wood. So the glue can fail and the thin layers rot quickly. I'm not sure it's any cheaper, but try finding a local sawmill and do board & batten barn siding. Make sure you stay 8" or more from the ground. Another option to consider is the metal roofing/siding used on pole buildings. Again, may not be cheaper but it's do once and forget about it