Stair treads

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.
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"David Alexander" wrote in message

Your drawing looks fine, but I would add to it. A 2"x4" on edge (not flat like being used for a tread) between the stringers at each tread. You have a 39" opening, minus the stringers (3") gives you 36" between stringers. This is the minimum allowance when using 2"x treads before going into a 3rd stringer. Since you're altering the tread strength, and you only have 2 treads to do, adding the 2"x on edge surely would give you enough support.
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"Gunner" wrote

3rd
Should be "Maximum allowance" = 36" between stringers when using 2"x material for treads.
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I built a stair several years ago and used two layers of 3/4" plywood glued together for the treads. My risers are a single 3/4" plywood member that sits on top of the lower tread and below the upper tread. I used biscuits to fasten to the upper tread and screws through the lower tread into the riser in three places. My stairs are formed with 2 steel "C" channels for the stringers and the treads sit on steel angles that I welded to the "C" channels. There is no central support at all and the stairs are more solid than any I've walked on.
RB
David Alexander wrote:

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