Building stairs (not to code, but safe)

Building a few stair units for my next set and don't want anyone to die. (too much paperwork)

Stair units are 4' wide (with the exception of one that is 3'6"), either 8/10 rise or 8/12 rise, and built with 2x12s and 5/4" treads. I'll be using three stringers (code would technically call for four, but this is going to be used by one actor for two weeks)

The longest is an 8-step unit (96" horizontal run). Risers are 3/4" ply and will be screwed into the backs of the treads for additional support.

Can I get away without a center support? Designer says we can, but he's a designer, not an engineer.

Would it help to attach some stiffening inside the stringers? I only have about a 5" throat so I could do 5" strips of ply glued and screwed, but that doesn't seem like it would add much. Thoughts?

Just did this very thing a few weeks ago for "Much Ado About Nothing". 2 stringers were not too shaky, but that's also because we were using 3/4 ply treads. 5/4 treads would stiffen it up even more. Unless there are dance numbers on the stairs!

I think you'll be fine without the center stringer.

Sounds fine to me. They will be much stronger than the cellar steps in the wife's aunt's house which have been there since 1940.

Thanks for the feedback. I did it with 2 stringers and didn't like it (for reasons described below) so I added a third

The reason I didn't like it was because my stock of 5/4 bullnose stair tread is all standard 11.5", so when doing 12" runs I had to add a 1" spacer at the back. Normally, I screw through the back of the 3/4" riser into the back of the tread to support the middle of the step, but with a 1" spacer to go through I didn't trust it quite as much.

I took a quick shot of the 8-step 8/12 unit. Its overkill :)

As the father of an actor/singer/dancer, I say overbuild it.