"Jay-T" writes:
I have a hunch that when you open it up from underneath you will be
able to see what your options are in terms of fixing the crack and
supporting the tread. For example, you may find that you can add a
supporting strip on one end and screw that into the stringer on the
side.
Very wise advice...


You mentioned that there is wood lath and plaster. I assume that the
wood laths will run across, from stringer to stringer, parallel to the
tread. So, you should be able to cut out a whole section all the way
across, and that should be easier to patch in one piece with sheetrock
rather than trying to cut a small hole and patch that. I usually use
a handheld sheetrock saw to cut the plaster along a line in between
two pieces of lath. Then I knock out the plaster in the section of
plaster that I cut out and then take out the underlying laths that go
all the way across. That avoids cross-cutting any of the laths which
helps prevent vibrating the laths and causing unwanted plaster to come
loose around the outside of the hole/opening that is being cut out.

Thanks for the suggestions -- patching will probably be a little more
difficult than just sheetrock since the bottom of the stairs are
contoured rather than flat -- so probably I will need a combination of
sheetrock base and plaster/setting-joint compound fill.

I have also used Gorilla glue from underneath to glue and seal cracks
and the joints where the treads meet the stringers and risers, etc. I
like it because it expands and fills the area and seems to stay just
slightly flexible rather than brittle like regular wood glue.


Personally, I have been using more high quality (West Systems) wood
epoxy and less Gorilla glue since in my experience, I have found that:
- Polyurethane glue makes a mess with all the foaming which requires
careful cleanup/scraping
- My understanding is that the filling portion of polyurethane glue is
mostly dried "foam" that is not nearly as strong as epoxy which when
used with the right filler can be a super-strong structural filler
- Finally, clamping is less critical with epoxy since you don't have to
counteract the expansive force of polyurethan.