"David Alexander" wrote:
I have some enclosed stairs 39" wide with only two stringers
....
I plan to glue and screw two layers of 3/4" plywood for the treads.
This will give me roughly the same height as the old 2x's. The front
edge of the tread will sit on top of the riser below for support.
Will that be sturdy enough considering the span and lack of center
stringer? I'm thinking that I need to support the back edge of the
treads as well and I've had a couple wild ideas for how to do that.
....

David, I agree that 30+ inches is a large span for a tread. Do the 2x
treads feel bouncy now? I suggest using a "lock joint" made by extending
the TOP tread layer about 1/4 inch or more into a 3/4" wide dado in the
riser. This avoids having to make the riser drop below the tread bottom and
provides lots of glue surface. After all the glue dries the 1 1/2 in tread
and lock joint will be strong and stiff. Another alternative is to cut back
the LOWER tread layer and glue and nail in a rail that is, say 3/4 by 3".
This is similar to your idea of adding a "joist" under each tread. This
second alternative has the advantage of not involving the riser in the
tread stiffening system. You could even layer the rail by face gluing a
second 3/4 by 2 1/4" piece to it, or by using a piece of lumber and cutting
a rabbet along the top edge. This would provide a double step lock joint
giving even more glue surface. In every design I am assuming your cuts will
be straight and smooth so you can use woodworker's glue to provide strong
bonds. I think your idea of using biscuits will work fine also, although I
believe a double row would be overkill.

In conclusion, I would avoid notching the two existing stringers to
accomodate extended risers or tread rails or tread joists unless those
stringers are MUCH beefier than normal. Such notches create a stress point.
If you extend the risers down, notch the risers so the extended part fits
BETWEEN the stringers. Like wise, any tread rails or joist supports should
fit between the stringers. It is my belief that your laminated 1 1/2"
treads will be plenty strong, but without any middle stringers, they will
have an annoying bounce and a disconcerting tendancy to bend under load. I
think your "wild" ideas are on the right track, er I mean the right-angle,
but are over engineered. The main goal is to stiffen the tread. You could
probably get by with just a butt joint between the tread and riser if it is
true and square and glued up well.

I like your picture. What did you use to create it? Here is a reference on
treads, risers, and nosings you should look at before you finalize your
design:
http://www.access-board.gov/adaag/html/adaag.htm#4.9

I hope these suggestions help. If any of it is unclear, request a better
description. Let me know how the project goes.

jj