A stair opening is framed with header joists across the ends of the hole and trimmer joists along its sides, with the cut tail joists hung from the headers. Under 2021 IRC R502.10, once the header joist span exceeds 4 feet, the headers and trimmers must be doubled and sized to carry the joists framing into them.
A stairwell is usually the biggest hole in a house floor, and it lands in the most crowded part of the second floor framing plan. Getting the opening size right, and putting the load from the cut joists somewhere sensible, is most of the job. For the drawing as a whole, see the floor framing plan guide.
The parts of a framed floor opening
- Trimmer joists run the full length of the floor bay along each side of the opening. They are joists that were not cut, and they carry the headers.
- Header joists run perpendicular to the joists across each end of the opening. They catch the cut joists.
- Tail joists are the cut joists, running from a header to the far bearing wall or beam.
On a typical stair opening running parallel to the joists, there are two headers (one at each end) and two trimmers. If the stair runs perpendicular to the joists, the opening cuts more joists and the headers get longer, which is often the deciding factor in which way the stair should run.
What the 2021 IRC says: R502.10 framing of openings
The full prescriptive text, quoted from the 2021 IRC:
R502.10 Framing of openings. Openings in floor framing shall be framed with header and trimmer joists. Where the header joist span does not exceed 4 feet (1219 mm), the header joist shall be a single member the same size as the floor joist. Single trimmer joists shall be used to carry a single header joist that is located within 3 feet (914 mm) of the trimmer joist bearing. Where the header joist span exceeds 4 feet (1219 mm), the trimmer joists and the header joist shall be doubled and of sufficient cross section to support the floor joists framing into the header.
| Condition | 2021 IRC R502.10 requirement |
|---|---|
| Header joist span not over 4 ft | Single header, same size as the floor joist |
| Single header within 3 ft of the trimmer bearing | Single trimmer joists may carry it |
| Header joist span over 4 ft | Header and trimmer joists doubled, and of sufficient cross section for the joists framing into the header |
Caption: 2021 IRC Section R502.10, framing of openings in wood floor framing. Prescriptive conditions only; the section does not give header or trimmer sizes beyond the 4 ft case.
Two things follow from that text. A typical stair opening, with a header spanning the stair width, will often exceed 4 feet, so expect doubled headers and trimmers. And "of sufficient cross section" is not a size: the code leaves it to a table, calculation or engineered product to show the doubled members can carry the load. R502.5 points to Tables R602.7(1), R602.7(2) and R602.7(3) for girder and header spans of dimension lumber; check whether those tables fit your loading before relying on them, and use manufacturer tables for LVL or other engineered headers.
The trimmers deserve the most attention. Each trimmer carries its own share of floor plus a concentrated reaction from the header, often near midspan. That is very different from a uniformly loaded joist, so the floor joist span table does not size a trimmer by itself.
Header joists, tail joists, and hangers
The 2021 R502.10 text above does not set a span threshold for hangers at headers or tail joists. The relevant 2021 rules are elsewhere in R502:
- R502.6 Bearing requires the ends of each joist, beam or girder to have not less than 1-1/2 inches of bearing on wood or metal, not less than 3 inches on masonry or concrete, "or be supported by approved joist hangers."
- R502.6.2 Joist framing says joists framing into the side of a wood girder shall be supported by approved framing anchors or on ledger strips not less than nominal 2 inches by 2 inches.
A tail joist framing into the side of a header has no bearing on top of anything, so in practice it is hung with a hanger, and a doubled header framing into the side of a trimmer is hung the same way. Specify the hanger for the actual member sizes and ply count, and nail the plies of doubled members per Table R602.3(1). Your local building department may also enforce an older or amended version of R502.10 with its own thresholds, so check what has been adopted.
Stairwell opening size
The width is the easy part: the stair width plus wall finishes and any skirt boards, with trimmers set so drywall lands where the plans say.
The length is set by headroom. As someone walks down the stair, the floor framing at the top of the opening (the header and the floor assembly over it) must clear their head by the headroom dimension in IRC Section R311.7, measured vertically from the sloped line of the tread nosings. So the opening has to run far enough along the stair that, at the header, the nosing line has dropped by at least the headroom plus the full depth of the floor assembly. That length depends on:
- the riser height and tread depth you actually use, which must meet R311.7;
- the depth of the floor joists, subfloor and ceiling finish at the header;
- whether there is a landing, winders, or a turn inside the opening.
Do the arithmetic from your own stair geometry and check every value against R311.7 as adopted in your jurisdiction. Do not take a rule-of-thumb opening length from another project; a deeper floor system or a steeper stair changes it.
Placing the opening in the joist layout
Some practical steps before drawing members:
- Align the opening with the layout. If a trimmer can fall on a regular joist location, you avoid an extra short joist. Shifting the opening an inch or two on the architectural plan is often cheaper than adding framing.
- Keep the header near a support if you can. R502.10 allows single trimmers to carry a single header located within 3 feet of the trimmer bearing, so an opening tucked against a bearing wall needs less trimmer.
- Check the joists that land on the headers. Long tail joists put more load on the header; short ones put less. That load then goes to the trimmers and down to their bearings.
- Follow the load down. Trimmer ends become point loads on the walls or beams below. Mark them on the plan below so the load path is not lost.
- Mind the rim and blocking. Where an opening is near an outside wall, the rim joist still needs to restrain the joist ends. Solid blocking and bridging rules carry on around the opening; see joist blocking and bridging.
Stair opening framing detail on the plan
A framing plan shows the opening in plan and points to a detail for the connections. A good stair opening framing detail and plan annotation includes:
- the rough opening dimensions, located from a grid line or a wall;
- each header and trimmer tagged, with ply count and size, for example
(2) 2x10 HDRand(2) 2x10 TRIMMER, or a reference to the schedule; - tail joists drawn at the joist spacing and stopping at the header;
- hanger or connector notes at each header and tail connection;
- the stair direction arrow and a reference to the stair section;
- point loads from trimmer ends carried to the drawing below.
Conventions differ between offices; the framing plan symbols page covers the common ones.
Drawing it in Framing Plan
Framing Plan draws the joist field, beams and bearing walls, has stair and opening symbols in its palette for marking the stairwell, and exports to PNG, PDF and DXF. The joist field is not cut around the stair automatically, so draw the headers, trimmers and tail joists as beams and annotations. It is a preliminary drawing aid. Its bundled sample profile uses demo span figures and metric member sizes, and the editor works in millimeters, so it does not check headers, trimmers, hangers or stair geometry against the code. Treat what you draw as a layout to hand to whoever designs and approves the floor, and confirm the members against the adopted code or the engineered product's tables.