Under stair room expansion

Question

We have used a small room under the interior stairs as a pantry for a while and would like to expand it. Removed the drywall today and found this small framed out structure. Is it structural to the stairs or is it there to support the drywall? Ideally I'd like to remove these 2x4s and push the wall back several feet. Am I able to remove the vertical supports? The horizontal header and footer too? If I do, is there another way to add support while keeping this space open?

For reference, the stairs rise 13 steps (about a 10 foot span.) The boards I'd like to remove are under the 9th and 10th stair. There are 3 stringers.

Answer 1

I can't say for sure.

Off-hand, I would think it is for the drywall. If it really was for support, I would expect the stud to be directly under the middle stringer to provide direct bearing weight, but that setup looks like it would be strong enough as well, so maybe.

To be 100% sure and covered, you'd need either the building plans (which you might be able to find from your local building department) or an engineer to approve the removal.

But, you could still have your pantry with that existing setup and not spend the time and/or money to gain a couple of feet. My suggestion is slide out drawers on that entire wall, pretty much filling the void. Or box it out with deep storage cubbies, but you don't get as much space with that method.

Answer 2

Stair stringer sizes are famously under-prescribed in the International Residential Code (IRC). There's no way for a DIYer to determine if that wall is structurally necessary (nor for a home-builder, not that this stops them). If pressed, I would probably hack the IRC's floor joist tables.

The following figure demonstrates the math process converting a stair stringer's live load of 40 psf to an equivalent live load applied on a horizontally oriented floor joist, where I simplify things by isolating a section of a stringer that would cast a 1 ft shadow at high noon:

First I get the vertical force by multiplying the 40 psf by the shadow width, 1 ft, and by 1 ft of loading width perpendicular to the screen to get a 40# force. This force isn't perpendicular like a floor joist's live load should be, so I break the loading down into an axial component along the stringer's long axis (which I'll ignore as inconsequential) and a component acting perpendicular to that long axis. Taking the stair slope as 11" run by 7-3/4" rise, I reduce the 40# to its perpendicular component with (40#)(cos(arctan 7.75/11)) = 33#.

Now I want to convert this back to psf, so I'm going to divide it by that same 1 ft perpendicular to the screen, but for the other dimension I want to use my isolated section's length along the stringer's long axis: 33# / [(1ft)(1ft/cos(arctan 7.75/11))] = 27 psf.

The IRC contains a joist table for 30 psf live loads (for bedrooms). Since the 27 psf live load computed above is less than this 30 psf, but still somewhat close, the 30 psf live load table will provide conservative stair stringer design lengths that aren't terribly overbuilt. Measure from a stair notch down to the bottom edge of the stair stringer (the shortest distance, not straight down) and round down to the nearest dimensional lumber size, e.g. 6-1/4" ↦ 5.5" implies 2X6. Now go to the 30 psf live load + 10 psf dead load floor joist table and look up 2X6 (for the 6-1/4" demonstration case, not necessarily your case) for your grade and species of stair stringer lumber. The table will give you a conservative maximum length (measured along the stringer's long axis) for your stair stringer.

Answer 3

The strength in a flight of stairs such as this comes from the stringers (those large pieces of wood on either side). There should be no need for any other uprights for support. They were probably there as a means of attaching the plasterboard. Had they been support for the stairs themselves, I'd have thought there may be bird's mouths joining then to the cross member at the top, for proper support.