Hi Perce,

We have T1-11 siding that is looking the worse for wear: some rotted
patches, some delamination. I don't know what is underneath it -- just
the studs?
What would be involved in replacing it by Hardie-Plank? *Not* installing
the H-P over the T1-11. Insulation between the joists, plywood over the
joists, then Tyvek housewrap, then the Hardie-Plank?
Would it be possible to replace one panel at a time, or would I (or a
contractor) have to do a whole wall in one go?

T1-11 is typically nailed directly to the studs. In addition to it's
function as siding, it also serves a structural function as sheathing.
Basically, it provides diagonal reinforcement to prevent the wall from
racking, and it ties the wall together (top plates, studs, bottom plates)
to hold the building together in earthquakes or strong winds.

Hardi-plank (or any horizonital lap siding) by itself will not provide any
kind of structural strength.

The best solution would be to repair the areas that are rotted or
delaminated with new sheathing the same thickness as your T1-11. Then you
can install your housewrap and siding of your choice on top of that. It
will be less work and your building will be stronger.

Anthony Watson
www.mountainsoftware.com
www.watsondiy.com