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I am finishing a room in my basement, and a wall will butt up against where the stairs are. I'm planning on making a small door to access the volume below the stairs for a small storage area.

The building inspector said I needed to put drywall below the stairs for fire protection.

The stairs have a middle string. If I place the drywall over the stringer, that will significantly reduce the available volume.

Can I fulfill this requirement by putting two tightly fitting drywall panels in the spaces between the stringers, leaving the middle stringer exposed?

Text of relevant ICC code:

R302.7 Under-Stair Protection

Enclosed accessible space under stairs shall have walls, under-stair surface and any soffits protected on the enclosed side with 1/2-inch (12.7 mm) gypsum board.

Otherwise, simply "coating" the middle stringer would be a compliant alternative to maximize accessible volume, would it not?

Here are some pictures from under the stairs. The difference between the plane of the bottom of the stringers vs the bottoms of the treads and risers is a significant amount of space to store stuff.

First view Second view

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No, you can't leave the center stringer exposed. That would leave a path for fire through the drywall. You must create a continuous break, with all seams taped.

I can't see an inspector flagging either of your other ideas, but you should probably just ask first. You don't want to do the job twice.

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    Key terms are "surface" and "protected" - he would be in compliance if he cut several pieces and properly attached/beaded/taped/finished every single one. Silly, but I've seen stranger - kind of a waste of effort for such an insignificant space IMHO. I had a basement from 1994-2009; you put stuff under the stairs that you plan to never see again, maybe get out once a year, and want to increase the chances that it get invaded by critters, mold, and mother nature. 3 pieces minimum and done. This is a hoarders quest...
    – tahwos
    Commented Oct 27, 2019 at 14:33
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One possible solution--not ideal-- may be to put the drywall on the bottom of the treads and backs of the risers. Would this be approved by the inspector? (The spec is not clear). However, it is not a good solution because the flex in the stairs will cause any drywall mud to crack over time--but perhaps you could just use corner-mold to cover the drywall seams. You'd still lose some space, but not as much as covering the entire bottom of the stairs...and you wouldn't have to spackle anything.

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If you noticed, Section 302.7 is a sub-section (and only applies to) Section 302: Fire Resistant Construction.

Therefore, you first need to determine if the stair needs to be “fire resistant”. It only needs to be “fire resistant “ and have the 1/2” gypsum board, if 1) the stair is exterior AND falls within the fire separation distance between buildings, or 2) required egress located in garages next to the residence, or 3) required egress from townhouse unit.

I doubt if your stair meets these requirements and needs to be “fire resistant” with 1/2” gypsum board.

There is also an exception for all three conditions if the house has fire sprinklers.

Also, if the Building Official determines that you meet one of the requirements, you do not need to cover the stringers if they are solid sawn and a minimum size of 4x6’s. (This is called “Heavy Timber construction”. See Table 602.1)

In summary: You may enclose the underside of the stair without 1/2” gypsum board, unless it needs to be fire resistant construction.

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  • Perhaps there is a different relevant section than R302.7 than I cited, but the building inspector specifically mentioned I have to do the fire protection. I've tried to ask him what to do but he's on vacation now. The exceptions you mentioned don't apply unfortunately.
    – whatsisname
    Commented Oct 26, 2019 at 4:38
  • @whatsisname There is only one required fire resistive construction system required in a “single family residence”, on the INTERIOR of the house: and that is the wall between the garage and interior living spaces. First, remember the Building Official wants to get everything correct too. Second, there is a clause in the code that allows the Building Official to require anything. He can require you spread peanut butter on the walls. There is an appeal process, and he’d loose that argument...and Building Officials don’t like to be appealed. Third, perhaps he sees that there is only...
    – Lee Sam
    Commented Oct 26, 2019 at 8:00
  • there is only one way in-and-out of the basement (the stairs) because you do not have an exit door (a sliding glass door is not considered an exit door) AND there are no bedroom egress windows. So, in his mind he thinks someone could store matches and firecrackers under the stair leading to a hazardous and dangerous situation. (Don’t scoff...I’ve heard worse.) If however, you have another emergency exit, I’d gently ask him to cite a reference for the 1/2” gypsum board. Now, the reason you’re asking for the reference is so you can get it right. (Don’t be a wise guy.) The reason you need to...
    – Lee Sam
    Commented Oct 26, 2019 at 8:09
  • The reason you need to know is so you can get the nailing correct. Ask what the size AND spacing of nail is required into the bottom of the treads and risers. (There is none specified in the Code...or anywhere.) Act dumb and let him know you want to nail it correctly into the treads and risers. (Oops...Edit: a slider can be used as an emergency exit out of a basement.) Finally, the Code allows “Alternative Design”. If he still wants the gypsum board, suggest you provide a smoke detector at the top of the stair instead. This is considered “early warning” which is better than “fire protection”.
    – Lee Sam
    Commented Oct 26, 2019 at 8:27
  • Getting into an argument with the inspector and having to go through an approval process is not even remotely worth the hassle, even if I was guaranteed to prevail. The area will have flammable materials in it, I don't think it's unreasonable to meet the requirement, I just want to try to optimize available space.
    – whatsisname
    Commented Oct 26, 2019 at 17:41

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