Introduction

Homeowners love turning that dead triangle under the stairs into storage. You’ve probably framed a few and thought, “It’s simple—box it in and hang a door.” Then inspection flags the missing fire lining, the closet smells musty a month later, or the client finds the electrical panel hidden behind shelves. This guide lays out what to watch: fire protection, moisture, utility access, and hardware that won’t sag. We’ll walk the problem, show you the fix, and give ready-to-use proposal language so you avoid callbacks—and get paid smoothly when it’s signed off.

Table of Contents

• Key Takeaways
• Code And Fire Protection That Pass
• Moisture And Ventilation That Prevent Musty Closets
• Utilities, Clearances, And Serviceability
• Layout, Framing, And Hardware That Last
• Estimating And Proposal Clarity (No Surprises)
• Frequently Asked Questions
• Conclusion

Key Takeaways

• Under-stair enclosures typically require 1/2-inch gypsum on the enclosed side for fire protection; skipping it is a common fail.
• Keep electrical panel working space clear: generally 30 inches wide by 36 inches deep—don’t shelf over it.
• Target less than 16 percent wood moisture content before closing; aim for 30–50 percent interior RH to avoid musty odors.
• Doors and shelves fail when spans and hinges are undersized; use 3 hinges on tall doors and keep heavy-load shelf spans conservative.
• Document access, finishes, and exclusions in your proposal to prevent scope creep and speed approval.

Code And Fire Protection That Pass

Many contractors get tripped up here—not because the work is bad, but because the details don’t match what the inspector expects.

The Problem

• Missed fire-lining under treads and stringers.
• No fire-blocking at concealed cavities.
• Doors to attached garages not meeting separation rules when the space opens that way.

The Solution

• Line the enclosed side (walls, soffits, underside of stairs) with 1/2-inch gypsum board as commonly required by the International Residential Code for under-stair protection.
• Fire-block concealed spaces at floor and ceiling intersections, and at 10-foot intervals in stud bays.
• If the closet opens to a garage, follow local separation rules—commonly a self-closing, solid-core or 20-minute-rated door and 1/2-inch gypsum on the garage side. Confirm with your AHJ.

Technical Details

• Gypsum fasteners: 1-1/4 inch coarse-thread screws for 1/2-inch board to ensure about 5/8 to 3/4 inch embedment into framing.
• Joints: tape and mud, or use Type X where your AHJ asks for enhanced protection.

Real-World Example

A small carpentry shop kept failing final because the underside of the stair soffit was left with MDF. They swapped to 1/2-inch gypsum, added simple fire-blocks at the stud tops, and passed reinspection the same day. The change was under two hours of work and avoided a full re-visit later.

Moisture And Ventilation That Prevent Musty Closets

Closets under stairs can become stale fast—especially on exterior walls or over cold slabs.

The Problem

• Musty odor, peeling paint, or cupped MDF shelves after a season.
• Clients storing linens or cardboard that wick moisture.

The Solution

• Check framing moisture with a pin meter before you close: contractors commonly aim for less than 16 percent MC on wood to avoid trapped moisture.
• Manage interior conditions: homes typically perform well between 30 and 50 percent RH. If you’re in a humid region or basement, plan for ventilation.
• Provide a door undercut around 1/2 inch or a small louver to allow air exchange if the space is fully sealed.
• If the closet backs a foundation wall, decouple with treated bottom plate, a thin insulated stud wall or rigid foam where permitted, and a capillary break under plates.

Technical Details

• On slab: lift finishes off concrete with a 1-inch plinth or PVC base to keep contents away from any incidental moisture.
• Use mold-resistant gypsum in damp-prone areas if your AHJ allows it for the fire-lining layer.

Real-World Example

A contractor in a coastal market added a 1/2-inch door undercut and specified satin urethane on 3/4-inch plywood shelves. With RH hovering around 45 percent indoors post-occupancy, the recurring odor complaints stopped entirely.

Utilities, Clearances, And Serviceability

Under-stair spaces often become utility closets by accident. That’s risky.

The Problem

• Shelves built over cleanouts or main shutoffs.
• Electrical panels buried behind storage.
• Low-voltage hubs inaccessible for service calls.

The Solution

• Electrical panel working space is commonly 30 inches wide by 36 inches deep clear floor space (and about 6.5 feet high). Do not intrude with shelving.
• Keep plumbing cleanouts visible and reachable; use a labeled, tool-free access panel if you must cover them.
• Plan removable sections for low-voltage racks: thumb-screw face frames work well.
• Never reduce combustion air around any fuel-burning equipment. If equipment is present, consult the mechanical code and your HVAC partner before enclosing.

Technical Details

• Label critical shutoffs inside the door. Many contractors add a simple schematic sticker showing valve locations.
• Use 1-5/8 inch metal stud track to frame slim, removable service panels without bulky wood framing.

Real-World Example

A remodeler built a neat pull-out “ladder shelf,” but it blocked a 3-inch cleanout. They retrofitted a 12 by 12 inch magnetic access panel and notched the shelf upright, preserving both storage and serviceability—and avoiding future emergency call-backs.

Layout, Framing, And Hardware That Last

The geometry is odd. If you don’t respect spans and hinge loads, doors sag and shelves bow.

The Problem

• Tall triangular doors racking and rubbing after a season.
• Bowed shelves from mixed loads (paint cans, tools).

The Solution

• Doors: use at least 3 hinges on tall doors; place the middle hinge slightly above center. Consider a 3/4-inch plywood or solid-core door blank skinned to match trim for stability.
• Shelves: keep heavy-load spans conservative. Many carpenters limit 3/4-inch plywood shelves to about 24–30 inches between supports for household loads. Add a 1x2 solid nosing to stiffen edges.
• Framing: tie new walls to the stringer with screws, not nails, to reduce squeaks. Shim plumb and scribe to irregular stringers.
• Thresholds: if the stair is above a slab with slope, plane or pack the threshold so the door bottom has a consistent 1/2-inch clearance.

Technical Details

• Fasteners: GRK or similar structural screws hold better in old stringers than smooth nails.
• Edge banding: a 3/4 by 1-1/2 inch face frame stiffens shelf fronts and hides ply edges.

Real-World Example

A team switched from two to three hinges on 78-inch triangular closet doors and added a 1x2 maple nosing to all shelves. Callbacks for rubbing doors went to zero across the next six builds.

Estimating And Proposal Clarity (No Surprises)

Most disputes on these jobs come from fuzzy expectations: paint included or not, lighting added, what happens if you uncover a plumbing cleanout?

The Problem

• Scope creep on finishes and extras.
• Delays waiting for client decisions on hardware and paint.

The Solution

• Break the scope into clear components: framing and fire-lining; doors and hardware; shelving; paint and caulk; optional lighting; utility access solutions.
• List assumptions plainly: “No electrical relocation included,” “Closet interior receives one coat primer plus one coat finish,” “All existing utilities remain accessible with labeled panels.”
• Add site photos and a quick sketch in your proposal so clients see where shelves start and stop.

Practical Workflow With Donizo

• Capture the space with voice notes and photos while you’re in the home. Donizo’s Voice to Proposal turns that into a clean, branded PDF in minutes.
• Send for approval and get a legally binding yes with Donizo’s e-signature. If you’re on Ascension or Autopilot, convert the accepted proposal to an invoice in one click and track payments.

Real-World Example

Many contractors report that including 3–5 photos of the under-stair area—with arrows showing utility clearances—cuts back-and-forth questions by half. Using Donizo to narrate those photo notes on-site and send the proposal the same day often gets next-day approval on small interiors like this.

Frequently Asked Questions

Do I always need 1/2-inch gypsum under the stairs?

Generally yes on the enclosed side of under-stair spaces in dwellings, per common interpretations of the IRC. Some jurisdictions ask for Type X or specific assemblies. Always verify with your AHJ before you build.

Can I put shelves in front of an electrical panel under the stairs?

No. Most codes require clear working space—commonly 30 inches wide by 36 inches deep—in front of panels. Treat that zone as a no-storage area and show it on your drawing and proposal.

How do I prevent a musty smell in an under-stair closet?

Start with dry materials (many pros target less than 16 percent MC on framing), keep indoor RH around 30–50 percent, provide a 1/2-inch door undercut or louver for air movement, and avoid direct contact between shelves and exterior masonry.

What hinge setup stops tall triangular doors from sagging?

Use 3 quality hinges (not 2), place the middle hinge slightly above the centerline, and fasten into solid framing. A solid-core or stiffened plywood door blank resists racking better than a hollow-core leaf.

Can I enclose a gas appliance under the stairs?

Only if the space meets combustion air and service clearances from your mechanical code and manufacturer instructions, which is uncommon in tight closets. In most small residential contexts, it’s safer to keep fuel-burning equipment out of enclosed under-stair storage.

Conclusion

Under-stair storage is small work that shows your professionalism. Line it right, keep it dry, protect service access, and build doors and shelves that stay true. Capture what you see on-site with voice, text, and photos, turn it into a clear proposal, and get a signed yes without the ping-pong. When you’re ready to send, e-sign, and—if you’re on a paid plan—convert accepted work to an invoice, do it in one flow with Donizo. That’s less admin for you and a cleaner experience for your client.