blueman writes:
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.

I finally got around to fixing the tread...

I was just about to cut in through the plaster and lathe on the
underside of the tread, when I had second thoughts about the mess and
pain of repairing it all and having to repaint the entire underside
(since the off-white ceiling paint would be almost impossible to match).

So, I decided instead to probe and clean out the crack first. On
probing, I realized the following:
1. The tread over the crack seemed to be supported either by a piece of
subflooring or by a wide stringer
2. I was able to clean out the crack pretty well
2. The tread itself seemed pretty thick

So, I then decided to mix up a batch of my trusty West System epoxy. I
first dropped in a fair bit of loose epoxy which do to its low viscosity
mostly dripped down through the crack -- my hope being that it wouldn't
all be lost but would server to bond the tread better to the stringer
and/or subfloor piece. I then added some colloidal filler and filled the
crack.

This seemed to really solidify the tread and so far is holding well (and
also looks pretty good).

If this repairs doesn't last, I will go back to my original plan of
cutting in from below -- but meanwhile it seems to be working both
physically and cosmetically...