How to Frame a Stair Opening in a Floor: Sizing, Headers, and Code Checks
A stair opening that is too short creates a head-bump hazard and a failed inspection. Here is how to size the opening, frame it with doubled headers and trimmer joists per IRC R502.10, and verify the 6 ft 8 in headroom before you close anything up.
Every staircase needs a hole in the floor above it, and that hole is the part most often framed wrong. The stair gets measured carefully, but the opening, or stairwell, is cut by guesswork: too short, and headroom drops below code where the stair ducks under the upper floor; too narrow, and the stringers will not fit; framed weakly, and the cut joists have nothing solid to hang from. This guide walks through sizing the opening from your stair dimensions, framing it with header and trimmer joists per IRC R502.10, and checking headroom before anything gets covered up.
What the code requires
Headroom (IRC R311.7.2): at least 6 feet 8 inches of clear vertical space everywhere on the stairway, measured from the sloped line connecting the tread nosings to the ceiling or the underside of the floor above. The critical point is the edge of the opening, where the stair passes under the upper floor; that edge decides whether the opening is long enough.
Stair width (IRC R311.7.1): residential stairs must be at least 36 inches wide clear. The opening must be at least that wide, plus working room for the stringers.
Floor framing around openings (IRC R502.10): cut joists must be supported by header and trimmer joists. When the header span exceeds 4 feet, both must be doubled; single members are only allowed for very short headers within 3 feet of the trimmer bearing.
Step 1: Calculate the required opening length
The opening must start at the top of the stair and extend far enough that the stair has already dropped low enough for 80 inches of headroom at the opening's edge. The math uses your stair's slope. Take your riser height (R) and tread depth (T): because the stair drops R inches for every T inches of horizontal run, the distance D from the top of the flight where the stair has dropped a full 80 inches is D = 80 x (T/R).
With a 7.5-inch riser and 10-inch tread: D = 80 x (10/7.5) = 106.7 inches, or about 8 feet 11 inches. That is the minimum distance from the top edge of the stair to the far edge of the opening. Measure from the finished upper floor edge, and measure headroom to the underside of the finished ceiling, not the top of the joists: a thick ceiling finish steals inches that matter here. Add at least 2 inches of margin beyond the calculated minimum, because real framing is never as perfect as the drawing. The opening can be longer than this number when convenient, but never shorter.
Step 2: Set the opening width
The opening width must clear the stair plus its stringers. For a 36-inch-wide stair, frame the opening 38 to 40 inches wide so the stringers and any skirt boards fit without being forced. Keep the opening centered on the stair run, with parallel sides, and mark it from the confirmed stair layout, not from memory.
Step 3: Plan the joist cuts and temporary support
Before cutting anything, identify every joist the opening will interrupt and confirm which way the joists run. The opening's long sides run parallel to the joists (trimmer joists go here), and the short ends run across them (header joists go here). If the opening lands under a load-bearing wall, or the floor uses engineered I-joists or trusses, stop and get guidance: solid-sawn joists can be cut and headed off with standard framing, but I-joists and trusses must not be cut without the manufacturer's written approval. Support the joists on both sides of the planned cuts with temporary walls or shoring before you cut.
Step 4: Install trimmer joists
Trimmer joists are the full-length joists running alongside the opening on both long sides. They carry the header joists and, through them, the cut joists, and per IRC R502.10 they must be doubled whenever the header span exceeds 4 feet, which is the case for nearly every stair opening. Sister the second trimmer tightly to the existing joist with nails or structural screws in a proper nailing pattern, full length, so the two members act as one.
Step 5: Install doubled header joists
Header joists close the short ends of the opening and pick up the cut joist ends. Cut the interrupted joists back to the opening edge, then install two header joists (same depth as the floor joists, doubled) spanning between the trimmer joists at each end. Each cut joist end connects to the header with a joist hanger sized for the joist, installed with the hanger manufacturer's specified nails. The header-to-trimmer connection also gets hangers. Do not toenail these connections: the entire cut section of floor hangs from these hangers.
Step 6: Verify headroom before closing
With the framing in place and the stair layout marked, stretch a string along the nosing line of the planned stair and measure vertically to the underside of the finished ceiling at the opening edge. You need 80 inches clear at every point, and the tightest reading is the one that counts. This is the last cheap moment to fix a short opening: lengthening it now costs a few more cuts, while discovering the problem after drywall goes up costs a remodel.
Common mistakes to avoid
Sizing the opening from the stair's total run instead of the headroom math. The run tells you how long the stair is; only the headroom calculation tells you how long the opening must be. These are different numbers.
Measuring headroom to the top of the joists. Headroom is measured to the finished ceiling surface. Forgetting the drywall, and any flooring above, shaves inches off the real clearance.
Single headers and trimmers on a long opening. Any header span over 4 feet needs doubled headers and doubled trimmers per IRC R502.10. This is the most common framing defect inspectors flag on stair openings.
Cutting engineered I-joists or trusses like solid lumber. These members lose their rating the moment they are cut. Manufacturer approval and an engineered detail are required, not optional.
Skipping temporary support before cutting. Cut joists under load shift and drop. Shore both sides first, every time.
Framing the opening before the stair layout is final. If the riser height changes after the opening is cut, the headroom math changes with it. Finalize the stair dimensions first.
FAQs
How long should a stair opening be? Long enough that headroom at the opening's edge is at least 6 feet 8 inches. Calculate it as 80 x (tread depth / riser height): with 7.5-inch risers and 10-inch treads, that is about 8 feet 11 inches from the top of the stair, plus a couple of inches of margin. Measure to the underside of the finished ceiling.
Can I cut floor joists to make a stair opening? Yes for solid-sawn joists, provided the cut ends are properly supported by doubled header joists hung between doubled trimmer joists with joist hangers, per IRC R502.10. Do not cut engineered I-joists or floor trusses without the manufacturer's written approval.
Do stair opening headers need to be doubled? Yes, whenever the header span exceeds 4 feet, which covers virtually all stair openings. The trimmer joists must be doubled as well.
What is the minimum width for a stair opening? The stair itself must be at least 36 inches wide per IRC R311.7.1, so frame the opening 38 to 40 inches wide to clear the stringers and skirt boards with a little working room.
What if my headroom measures less than 6 feet 8 inches? Extend the opening further from the top of the stair so the stair drops lower before passing under the floor. Check whether a thick ceiling finish is stealing inches, and confirm the measurement is truly vertical.
Do I need an engineer to frame a stair opening? For a straightforward opening in solid-sawn joist framing with no load-bearing wall above, the IRC prescriptive rules in R502.10 cover it. Get an engineer involved if the opening is under a bearing wall or the floor uses I-joists or trusses that need cutting.
