Deck Stair Stringer Spacing?

Noted two different references on InterNACHI?

Course shows below: The image above depicts deck stair stringers. Stringers should be no more than 16 inches apart for wood decking and 12 inches for composite.

But Library shows: The image above depicts deck stair stringers. Stringers should be no more than 36 inches apart.

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Jim, send an email to

Thanks Larry, what is your opinion?

Seems like these stringer spacing rules should also apply to any open riser stairway (ie basment stairs with no risers) too? Not sure why they would say 36", then additional requirements for wood and composite? Are they tring to say metal is ok at 36"?

I’ll send then an email, thanks…

Probably a mistake…

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Thinking a typo on the 36" spacing.

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So most likly stringers should be no more than 16 inches apart for wood decking and 12 inches for composite, but should this rule also be applied even to any open riser stairway? Example - 36" wide wood basement set stairs with open risers, should it also have additional stringers, or is this just a concern cause of exterior conditions?

I’m assuming, a deck stairway with risers, does not require the extra stringers because of the riser support?

Basic Open-Tread Stair

probably got mixed up with 36" min width of a stair.


The stringer spacing depends on thickness and the material of the treads. Did you look it up in the DCA6?

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That would usually be the case but If one had 7"x11" hunks of red oak for the treads, what stringer spacing would one need for 2"x14" red oak stringers? :sunglasses:

Dear Larry, if you wish to build a custom deck that does not follow the conventional building standards as prescribed by the AWC and IRC, please contact an engineer to design the spacing specs for your application :slight_smile:
For your family’s safety, I will need to see the blueprints for the deck so I could ensure the contractor built the deck accordingly.


If the stair assembly looks weak, I may call it out. What I do call out is missing risers, either interior or a trip hazard, [safety issue].

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Dear Simon,

Good answer!

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Jim; 2 inch nominal lumber treads (2 x 6, etc) can span 24 inch. Bull nose 5/4 treads can span 16 inches. Same for composite material, 16 inch. That dictates your stringer spacing.

@msenty, that makes it easier…

But does this only apply to exterior stairs or just open riser type stairs, because I see many 30"+ wide basement (interior) stairways with risers that only have 2 stringers and never thought twice, they are strong?

Take the risers out and it’s another story…

Just trying to fully understand the references.

I don’t know that answer about the riser/tread question without looking it up in IRC code. However, when the riser and tread are connected together in a secure manner as illustrated in your picture, you obviously have much greater load capacity. Nothing wrong with that tread and stringer design in my opinion. Good thing to look up in the code book if so inclined. When dealing with old construction, one method I use is to walk up/down the stairs, do the “jump test” - does it pass?

Lots of old homes, especially cellar stairs, were built with 2 x treads, no risers, and two stringers built with 2 x 10’s. I’ll call that out with the caveat that building practices have changed since the time the stairs were built. Issues: safety for the buyer, liability and credability for me if I don’t mention it. It’s your call and opinion on how you address old construction practices.

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Here’s a statement I have at the beginning of all my reports that pertains to these issues of changed construction practices:

NOTE: Some items listed under Safety are recommended upgrades that typically were not a component of construction at the time this house was built. This report may include recommendations to the buyer for future safety upgrades to the home including garage firewall protection, garage fire door condition, bedroom egress, window opening limitation devices, stair and railing conditions, GFCI condition in the garage - exterior - basement, older ungrounded wiring systems, and smoke alarm/CO alarm placement.

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Looks good, Michael. I would put an etc. at the end of the statement though…just a thought.

According to TREX (the leading brand of composite decking) the stringer spacing should not exceed 9" for their “enhance” line or 12" for their premium “transcend” and “Select” lines.
I’m 150 lbs and I have found decks with composite stairs on 16" OC spacing to be too flexible and I call them out (with reference to manufacturer instructions below)

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I read that too, Bert.

^^^^^Good info!^^^^^

Thanks all, great info. That TREX design should last forever…