Stair Treads

(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.
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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
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wrote:

if you decide not to suport the middle they will be weak. you realy should add a middle stringer. a dado on the riser may work. 2 layers of plywood would be strong enough but in all of these fixes you are trying to eliminate movement when the steps are walked on. movement under stress will produce squeeks.i would use standard stair tread wheather it be oak or SYP is up to you.oak treads are sturdy but rather expensive. subfloor adhesive or liquid nails should be used. dont be affraid to put the glue in. the more the better..again the best way would be to cut in a middle stringer. it isnt that difficult to do. line each side with a 2x4 for added strength. skeez
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