(I posted this in alt.home.repair but I thought I'd ask the folks in
here too since I know a lot of you do this kind of work.)

I am about to rebuild some stairs and I'd like some feedback on
whether my ideas for the treads are going to work or not.

I have some enclosed stairs 39" wide with only two stringers on the
outside edges. Adding a middle stringer would mean some major
construction due to the way things are laid out. Currently, the
treads are 2x lumber with carpet. Naturally, removing the carpet
reveals that the 2x's are not flat, or straight, or in any way
suitable for my new treads. I'd like to use plywood to make new
treads for stability and flatness.

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.

One idea is to have each riser drop below the back of the tread and
cut a dado in the riser that the tread can slip into at the back.

Another idea is to use a double row of biscuits to join the back of
the tread to the face of the riser.

I've also thought about modifying the dado idea and just gluing on a
ledge that the tread sits on rather than milling a dado. The face to
face glue up for the ledge should be very strong.

Then there's the idea of mounting "joists" across the stairwell just
below each tread.

The question is, do I really need to do this or will the 1-1/2" of
plywood be stiff enough over time given that the front edge will be
supported?


Here's a picture I threw together for the first section of stair that
I'm redoing.

http://www.pobox.com/~DaveA/treads.jpg

It's only a 16" stepdown with two treads. This one I could easily
rebuild with all the stringers I want but I'd like to use it as a
model for my main stairs so I'll have my process down when I get
there.