mm wrote:
I've looked for a definition on the web and it seems that T1-11 is
hardboard. But I've also seen it called plywood. Which is it?

I thought they were very different. Plywood is iiuc 3 or more layers
of wood, where each layer comes from putting a log on a big lathe and
taking off a continuous strip of wood, each layer deeper into the log
for every rotation of the lathe. That's why one sees a repetitive
pattern on plywood, because if that part of the tree is 16 inches in
circumference, every 16 inches one sees a different level of the same
branches, etc.

Hardboard otoh in wikipedia is described "Hardboard, also called
high-density fiberboard, is a type of fiberboard, which is an
engineered wood product. It is similar to particleboard and
medium-density fiberboard, but is denser and much stronger and harder
because it is made out of exploded wood fibers that have been highly
compressed. Consequently, the density of hardboard is 31 lbs. or more
per cubic foot[1] and is usually about 50-65 lbs. per cubic foot. It
differs from particle board in that the bonding of the wood fibers
requires no additional materials,[2] although resin is often added.
Unlike particleboard, it will not split or crack. It is used in
construction and furniture."

I think T1-11 is heavier than plywood of the same thickness. It seems
to be hardboard. Right?

Thanks.
Hi,
My cabin built in '97 used this material as exterior horizontal sidng.
T1-11 is hardboard. Ours has natural wood color with grains Ver very
hard. Driving nail on it is not easy. I used screws when I had to hang
or attach something. It came with a 20 year warranty for color and still
it looks like just installed. Better fire resistant than plywood as well,