One of the 12" wide treads on our main staircase has an ~18" longitudinal
crack in it. As per an earlier thread, the consensus seems to be that
these treads are 150 yr old heart pine.

Here is a link to a pic:
http://96.252.37.8:8080/public/1stFlrTread.jpg

I am not concerned about the visual look of the crack (if anyting it
adds "character") but the area under the crack is not well supported and
gives a bit when you put weight on it -- in fact, the absence of such
support may be the proximate cause of the crack. The crack has probably
been there for many years.

Since replacing the tread is not easily feasible given the old wood, I
would like to repair the tread.

Now removing the tread would not be easy since 2 balustrades are
toenailed into the other side of the tread and removing the entire
bannister and balustrades *seems* to me to be a big job.

So, I was thinking it might be easier for me to access the underside of
the tread from the alcove underneath the stairway by cutting through the
plaster and lathe on the underside of the stairway. My idea would be to
expose enough area so that I could slip another board underneath the
tread to span the crack and add support.

To secure the support, my thought was to cover it with glue (either
polyurethane or epoxy). Assuming that I won't be able to get good
clamping pressure from below, I would run some temporary small gauge screws
through the face of the tread to temporarily clamp the tread and support
board together. Alternatively, I could try to expose more of the stair
underside at the risk of creating more mess and a bigger plaster repair
job.

Before, I go cutting into the plaster and lathe, I wanted to get advice
on whether this is a good approach along with any additional advice or
pointers. Of course, I am open to *any* other better suggestions.

Thanks