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?

T1-11 is either plywood or OSB (Oriented Strand Board) with grooves cut
vertically to simulate wood planks. The term "T1-11" apparently refers
to "Texture 1-11", a description of its appearance.

Plywood is, as you state, several layers of wood glued together. The
grain is perpendicular in alternating layers, making the sheet very
stable and resistant to size changes due to humidity.

OSB is "flakes" of wood glued together. The grain of the flakes is
alternated in different layers to make the panel stable, similarly to
plywood.

The T1-11 I've bought has been slightly lighter than the same thickness
plywood because of the missing wood in the grooves.

Hardboard siding is available with pressed-in textures that look a lot
like T1-11, but it's more fragile and more easily damaged by water. I
once had a house with this type of siding, and it's a lot of trouble.

Trivia:
Fiberboard comes in LDF (Low Density), MDF (Medium Density), and HDF
(High Density). Pegboard is typically LDF, painted or hidden parts of
cabinets is typically MDF, and Masonite is a common brand of HDF.

--
Steve Bell
New Life Home Improvement
Arlington, TX