Do I need a third stringer for my deck stairs?

A 2X6 maximum span is 24" , so you definately need a 3rd stringer. I have 3 stringers on my deck stairs and the treads are 5 feet wide.

Did you ask you local building inspector's office if they have any spacing requirements for stair stingers?

Phil

My 2 deck steps are about 4' wide using

If you were using 2x12 Douglas fir or yellow pine for treads I might say OK---

But using 2x6's, likely 'white wood' from the borg, you're nuts if you don't protect yourself from a personal injury or a lawsuit by adding a stringer.

Jim

A few years ago I was building a deck with steps very much like the OP's, except using 2-by cedar and with wooden cleats under the ends of the steps. Doing it to code, permits, inspectors, the works. Local code very specifically said 3' of 2x material was ok with no stringer. When I took the permit app in I deal with a couple of older guys who seemed to know what they were doing; one of them drew a line down the middle of the steps and said, "you need a stringer there". I said code didn't say so, and he said, "yeah, but it'll bounce, put in a stringer." So I did. Lot more work, since the steps were faced onto the two side stringers I hadn't done any of the design work on cutting risers and steps out of a stringer (it was a non-stock rise/run), and had to get a cedar 2x12. But it made a nice job.

Bottom line: if you're wondering about, put it in. Bouncy steps will leave a shoddy impression in people's mind, even if the rest of the job is top-notch, and designing a stringer for a custom stair is an interesting and satisfying exercise.

Chip C Toronto

My rule of thumb is that I don't leave a 2x6 unsupported less than

Try this; take a couple of 2x4s and lay them down on some flat concrete spaced 36" apart, outside to outside. Lay a 2x6 across them cut at 36". Now step on the middle of the 2x6 and see if you feel good about the flex. If you do, then go and get about 50 lbs of groceries in your arms and step on it again. If you still like it, ask your wife to step on it with you. If you still like it, then go for the unsupported span. If you don't, add the middle stringer.

Sounds wide, I'd use a middle stringer.

So why are you debating using one? The cost of adding a third stringer is minimal and you already know in your mind that without it, it will be 'springy'.

Add the third stringer. It won't need any fancy cleats, etc, just cut to fit under the treads.

Harry K

Thanks for all the input! Yes, some of you are asking the right question...why am I set against not doing the third stringer, since its minimal cost and work to add it and will make it more safe? Well, originally I wasnt crazy about the asthetics of having the third stringer visible between my steps, since they will be spanning from closed stringers on the ends and will not have a riser board attached, but its not the end of the world. An injury on my deck would be MUCH worse. Anyway, thanks for all the help!

I struggle with this issue from an engineering perspective and I'm hoping another engineer can set me straight.

Deflection of a simply supported beam with a concentrated load is: y=3D (F L^3) /(48EI)

Using an L/480 stiffness criteria gives an allowable deflection of 36"/

A standard 2x6 has an I =3D 24.1 in^4 and E =3D 1.6 e6 psi

Therefore the allowable force on a SINGLE 2x6 would be: 2975 lbs ! That's a 10X safety factor for a 300 lb man !

Spread the load over two boards and the allowable load would double.

Given these calculations, why would a third stringer be even remotely necessary?

I'm no engineer, but my SWAG answer would be because it isn't a static load. Hence the term, 'springy' feeling. I know from experience that an end-supported 2x6 3 feet long will indeed flex under a walking man, probably not enough to be dangerous, but enough to feel creepy. (temporary construction steps while the thicker pretty custom treads were safely stored away till us jackbooted thugs were done with the messy work.)

If I was OP, I'd change the design to either 2 'underneath' stringers inset from the end, or use the full-width treads with the grooves that drain water and resist splitting, or use a center stringer. If appearance is an issue, a dark stain on the center stringer before it is installed can make it visually blend into the background more when seen from a low angle. Same trick as painting the rocker panels on a car black to make it seem sleeker.

-- aem sends...

I struggle with this issue from an engineering perspective and I'm hoping another engineer can set me straight.

Deflection of a simply supported beam with a concentrated load is: y= (F L^3) /(48EI)

Using an L/480 stiffness criteria gives an allowable deflection of 36"/

A standard 2x6 has an I = 24.1 in^4 and E = 1.6 e6 psi

Therefore the allowable force on a SINGLE 2x6 would be: 2975 lbs ! That's a 10X safety factor for a 300 lb man !

Spread the load over two boards and the allowable load would double.

Given these calculations, why would a third stringer be even remotely necessary?

Well, for one thing, you're using the wrong axis for the moment of inertia...

a...- Hide quoted text -

You are absolutely correct.

I had looked it up in a table, which didn't specify the axis. Since I wasn't sure of the axis I did actually check it with bh^3, which coincidentally gives 24.1 in^4, which caused me to stop right there, forgetting of course that the actual formula is bh^3/12 !!!

So correcting my original post, the allowable load on a single 2x6 spanning 36" would be 247 lbs which might be a little close for comfort.

Thanks for the help.

quoted text -

You are absolutely correct.

I had looked it up in a table, which didn't specify the axis. Since I wasn't sure of the axis I did actually check it with bh^3, which coincidentally gives 24.1 in^4, which caused me to stop right there, forgetting of course that the actual formula is bh^3/12 !!!

So correcting my original post, the allowable load on a single 2x6 spanning 36" would be 247 lbs which might be a little close for comfort.

Thanks for the help.

The Sagulator sag calculator at

You would definitely be able to feel a step that bounced by almost a quarter of an inch. Whether you would care is something you'll have to decide for yourself.

replying to SteveBell, Scott J. wrote: If you have enough nose hanging out over the steps you could add 1 X 8 riser boards on the vertical face of the stairs. This will cut way down on the springiness on the outer part of the step. A lot easier than adding a stringer. I'm an Engineer but I'm also practical and lazy.

replying to Mikepier, Brad wrote: so you should have 4 stringers...

HomeMoaners strikes again!! Brad, I have some sad news for you. You're NINE YEARS too late with your 'helpful' post.

Nuts? Its pretty common to spread 2x6s on a 36 inch span on stair treads