Porch stair tread span ?

[Arky217]

Using treated SYP 2x12's for open stair treads (no risers), what would be the maximum span between stringers without excessive bounce ?
( just 2 stringers, no middle stringer)

I feel fairly confident 3' would be ok, but not sure about any wider.

Thanks,
Arky

[Don_P]

No openings allowed larger than 4"... risers are pretty much needed. I always use a 3rd stringer even at the minimum 3' width... you've got joists no more than 2' on center and the stairs take more abuse and concentrated load. Think of it as giving another fail safe if nothing else. Stairs are not a place to skimp.

[Arky217]

No openings allowed larger than 4"... risers are pretty much needed. I always use a 3rd stringer even at the minimum 3' width... you've got joists no more than 2' on center and the stairs take more abuse and concentrated load. Think of it as giving another fail safe if nothing else. Stairs are not a place to skimp.

Really ?; I don't think I've ever seen risers on porch or deck stairs. Not that it applies to the rural area where I'm building, but is that a code issue somewhere about greater than a 4" riser opening ?

True,  2x joists are generally limited to 2' on center, but then again, you're not generally putting 2x12's across them. A SYP 2x12 is pretty stout; I doubt you would feel any bounce at all with a 3' span (unless you were pretty heavy).

Actually, my reason for asking is that the box stores have the stringers and treads precut, and the treads are 2x12's available in 3' and 4' widths.

The stringers are a real deal; about the same price as if you bought a regular treated 2x12x8'.

And the treads, although about twice that of buying the 2x12's alone, have the fronts rounded, have grooves for traction, and most important, both the stringers and treads are treated after they are cut, and with a higher concentration than a regular treated 2x12.

If I go with the 4' treads, I was wondering how far I should inset the stringers to minimize bounce.

Of course, I could just spring for a third stringer and not fret about it 

Thanks,
Arky

[dug]

I'd spring for the third stringer.

[davidj]

By the time you cut out for the triangle for the tread, a 2x12 stringer is more like a 2x6 at the thinnest point.  Probably worse than a 2x6 if you don't select the 2x12 carefully, as 2x12s have bigger maximum allowed knots than 2x6s.

[Don_P]

The 4" maximum space is code nationally, inside as well as outside. It applies to not only the stairs but also the spacing between railing elements as well. The wording is something like "not able to pass a sphere larger than 4". There is an exception for the triangle formed by the tread and bottom rail going down a set of stairs where it allows up to a 6" sphere. This code changed from 6" everywhere in about 2000 if I remember right. It was introduced to prevent infants bodies, which are smaller than their heads, from passing thru. Risers also help strengthen the set. I think we have the current stair code visual guide posted in the references forum which shows drawings of all this.

I'll agree that a 2x12 on that spacing will not break, I think the code reads that a tread must be able to support a #300 lb point load and I'm right certain we could prove it capable. The third stringer helps keep the treads flat as they weather... ever seen a 2 stringer set with a "sagging" tread? It's usually a wet/dry problem and the third one will help hold the treads in line. The main thing though is that there is one more attachment up top and as David points out sometimes a stringer has a weak spot. The third one is pretty cheap insurance.