What Is Spandrel Void Optimization? (Stair Riser Drawers)
Spandrel void optimization means using the triangular space beneath a stair for storage without changing the stair’s structure, required dimensions, or fire protection. Riser drawers are one option, but they create moving openings in a walking route. Plan the closed and extended drawer positions, verify local code requirements, and do not cut structural stair parts without approved engineering.
A common misconception is that the triangular space beneath a stair is “wasted,” so any box that fits there is a safe storage solution. Fit is only one test. A drawer can interfere with a landing, alter a riser face, or expose a fall hazard when open. The goal is to use the spandrel void while preserving the approved stair design.
The code values below come from the 2021 International Residential Code (IRC). Your local authority may use another edition, amendments, or rules for a specific stair type. Check the adopted requirements before building.
Planning the Spandrel Void and Confirming Code Basis
Spandrel void optimization is the careful use of space under a stair flight for storage. The design must respect the stair’s load path, geometry, clearances, and any required fire protection. Start with the built stair and local rules, not a preferred drawer size.
First, identify the stair type and how it is supported. Stringers are the sloped members that carry the treads and risers. Some stairs use different framing or support details, so do not assume the visible side tells the whole story. Find plans if available, and inspect for utilities, framing, and signs of prior changes.
Then confirm the adopted code edition with your building department. Under the 2021 IRC:
- R311.7.5.1 limits riser height to 7¾ inches, with no more than ⅜ inch variation within a flight.
- R311.7.5.2 requires at least 10 inches of tread depth, with no more than ⅜ inch variation within a flight.
- R311.7.2 requires at least 6 feet 8 inches of headroom, measured vertically from the line connecting tread nosings and landings.
- R302.7 requires the enclosed side of an accessible space beneath stairs to be protected with ½-inch gypsum board. Ask the local authority how this applies to a drawer cavity, its opening, and its finish.
These dimensions are not permission to rebuild a stair to the limits. Measure the finished stair as it exists, and keep the approved profile intact. A concealed storage plan is not a substitute for permits or review when required.
Access, Stair Geometry, and Drawer Clearances
A riser drawer is a storage box that moves through or behind a stair riser face. Its closed position must preserve the stair profile, while its open position must not create a dangerous obstruction or opening. There is no universal code clearance for drawer slides; use the selected hardware maker’s instructions.
Measure each riser and tread across the flight, including floor and finish layers. Record the stair width, the sloped underside, nearby walls, and the space available at the landing. Mark utilities and framing before setting a drawer location. A single average measurement can hide a tight spot or an out-of-square stair.
Draw two outlines for every proposed drawer: the closed envelope and the fully extended envelope. The second outline matters just as much. Check whether it reaches a landing, door, hall, or walking path. Treat the stair as unavailable while a drawer is open. A warning label alone is not a strong safeguard if someone can still step onto the stair.
| Check | 2021 IRC reference point | Design response |
|---|---|---|
| Riser height | Maximum 7¾ in.; flight variation ≤⅜ in. | Keep the finished riser profile unchanged |
| Tread depth | Minimum 10 in.; flight variation ≤⅜ in. | Do not add a projecting front that reduces effective depth |
| Headroom | Minimum 6 ft 8 in. | Measure vertically from the nosing and landing line |
| Open-riser gap | Must not pass a 4-in.-diameter sphere | Do not leave a drawer opening that creates an unsafe gap |
| Under-stair protection | ½-in. gypsum board on enclosed side | Confirm the required assembly with the local authority |
A drawer front that sits proud of the riser can catch a foot or change the usable geometry. Aim for a flush finish, but do not treat “flush” as a code approval by itself. Confirm the face does not create a lip, reduce tread depth, or leave an opening that violates the rules for the stair design.
Framing, Hardware, and Finish Execution
A separate supported carcass is the box or frame that carries the drawer and its hardware. It should transfer storage loads to suitable framing without relying on a cut stringer or weakened tread. Hardware gaps, fasteners, and load ratings must follow the specific product instructions and the strength of the material receiving them.
Size the drawer from the actual cavity
Measure the available space at the drawer’s full height and depth. The sloped underside often makes the usable volume smaller than the outside triangle suggests. Draw the drawer box, slide locations, front panel, and access path to scale. For each dimension, subtract only the clearances required by the chosen hardware and construction method.
Do not choose a generic slide gap from an online rule of thumb. Slide makers specify side clearance and mounting conditions for particular products. Follow those instructions, including the required fasteners and minimum material thickness. A slide’s advertised rating does not prove the installed assembly can carry that load.
Build without changing the stair’s load path
Fit the carcass within the available void and support it independently where practical. Do not cut, notch, drill, or remove stringers, treads, risers, or supports to gain drawer space unless a qualified designer has approved the change and the building official accepts it. Even a small alteration can affect how the stair carries loads.
Fasten the carcass to sound framing with fasteners suited to the substrate. Check that screws do not hit wiring or plumbing, and do not rely on finish panels as structural support. If the framing or load path is unclear, stop and get an on-site assessment before opening the stair assembly.
For a flush front, use a measured reveal that suits the selected hardware and finish. Keep the face aligned with the surrounding risers, and avoid a handle or pull that projects into the stair path. A push latch or recessed pull may help keep the face flat, but it does not remove the need for safe operation and reliable retention.
Restore required gypsum protection and finish continuity after fitting the drawer assembly. The applicable fire-protection details depend on the local code and how the under-stair space is enclosed. Do not assume that a thin decorative panel or drawer front replaces the required gypsum layer.
Verification, Loading, and Safe Operation
Verification means checking the complete installation after finishing, not just confirming that an empty drawer slides. Test alignment, retention, clearances, stair dimensions, and the loaded assembly. Make the stair unavailable whenever a drawer is open, because the open position can obstruct a tread or create a hazardous gap.
Before use, confirm that:
- Each drawer opens and closes without binding, scraping, or shifting the surrounding finish.
- Closed fronts sit flush and do not alter the tread or riser profile.
- Open drawers do not block a required route or create an opening that can admit a 4-inch sphere where that limit applies.
- Stops or retention features prevent unintended movement, while still allowing safe access.
- The installed hardware and its fasteners suit the drawer’s total weight, including the box and contents.
- Under-stair protection and finishes meet the approved local requirements.
Load testing should follow the hardware maker’s instructions. Add weight in a controlled way, keep people off the stair during the test, and watch for sag, loose fasteners, or a change in slide alignment. Do not exceed the rating for the actual hardware and installation. If the maker does not provide a suitable test method, ask the supplier or a qualified installer.
Worked example: one stair, two drawer positions
Consider an illustrative retrofit with a measured cavity that narrows under the stair. I would map the full cavity first, then draw a shallow upper drawer and a deeper lower drawer, each with its own extension path. The box dimensions would come from those measurements and the selected slide specifications, not from a standard “stair drawer” size.
The upper drawer might fit the available height but still be unsuitable if its front changes a riser face or opens into the stair path. The lower drawer might offer more volume, yet need a shorter extension to keep clear of a landing. This is a planning example, not a code-approved design. The authority having jurisdiction and any required designer review decide what is acceptable for a specific home.
| Design choice | Best suited to | Main constraint |
|---|---|---|
| Shallow riser drawer | Small items and limited cavity depth | Must preserve the riser face and avoid a hazardous opening |
| Deeper drawer under lower steps | Larger items where the cavity allows | Extension can obstruct a landing or circulation route |
| Fixed access panel or cabinet door | Storage accessed when the stair is not in use | Must still meet required under-stair protection |
| No drawer in a given bay | Tight framing, utilities, or unsafe extension path | Less storage, but avoids compromising structure or access |
The useful result is not the largest possible drawer. It is the largest drawer that fits the verified cavity, works with its rated hardware, and leaves the stair compliant and safe.
FAQ: Spandrel Void Optimization and Stair Riser Drawers
What does “spandrel void” mean under a stair?
It is the space beneath the sloped stair flight, often bounded by the stair, floor, and an adjacent wall. Its shape depends on the stair framing and finishes.
Are stair riser drawers allowed by the IRC?
The 2021 IRC provisions cited here do not establish a universal approval for riser drawers. Local officials must assess the stair design, openings, fire protection, and applicable adopted code.
Can I cut a stringer to make a drawer deeper?
Do not cut or notch a stringer without engineered approval and any required building approval. Stringers are structural stair members.
How much clearance do stair drawer slides need?
There is no universal clearance. Use the slide manufacturer’s stated side gaps, mounting details, fasteners, and load limits for that exact product.
Can a drawer front project slightly from the riser?
A projection can catch a foot or change the effective stair profile. Keep the face flush where possible and verify that it does not alter required tread or riser geometry.
Can someone use the stair while a riser drawer is open?
Treat the stair as unavailable while any drawer is open. An extended drawer can obstruct a tread or landing and expose a hazardous opening.
What is the under-stair gypsum requirement?
Under 2021 IRC R302.7, an enclosed, accessible space beneath stairs requires ½-inch gypsum board on the enclosed side. Confirm the local interpretation for the proposed drawer assembly.
How do I calculate the safe drawer load?
Include the drawer’s own mass and its contents, then follow the hardware maker’s rating and installation instructions. Verify that the supporting framing and fasteners can carry the load as installed.
Does a drawer need a permit?
Permit rules vary by location and scope. Ask the local building department, especially if the work changes stair framing, fire protection, or an approved stair assembly.
What should I measure before designing the drawer?
Record each tread and riser, stair width, underside slope, cavity depth, framing, utilities, headroom, and the full extension path. Then check both closed and open positions against the stair and nearby circulation.
(This article was written by one of our staff writers, Garrett Vance. Visit our Meet the Team page.)
