To frame a window opening, build a header, two jack studs, two king studs, a rough sill and cripple studs around the rough opening size given by the window manufacturer. The 2021 IRC (R602.7, Tables R602.7(1) and R602.7(2)) sizes the header, not the opening.
A rough opening is the framed hole in the wall that a window unit fits into, slightly larger than the unit so it can be shimmed plumb and level. This guide is about the opening itself: where its dimensions come from, how the framing around it is built, and which 2021 International Residential Code (IRC) rules apply. The full build sequence for the rest of the wall is in how to frame a wall.
Key facts
- The rough opening width and height are not given in the 2021 IRC. They come from the window manufacturer's instructions for the specific unit.
- Header spans for sawn lumber in bearing walls come from Tables R602.7(1) and R602.7(2) (2021 IRC Section R602.7).
- Each header end is supported by one or more jack studs or approved framing anchors, as the NJ column directs (2021 IRC Section R602.7.5).
- Studs must be continuous from sole plate to top plate, except jack, trimmer and cripple studs at openings that comply with Tables R602.7(1) and R602.7(2) (2021 IRC Section R602.3).
- A single flat 2x4 may be the header in a nonbearing wall for openings up to 8 feet wide, if the distance to the nailing surface above is not more than 24 inches (2021 IRC Section R602.7.4).
- An operable window with its clear opening bottom less than 24 inches above the floor and more than 72 inches above grade needs fall protection (2021 IRC Section R312.2.1).
What size is the rough opening for a window?
The rough opening size comes from the window manufacturer, not the code. Every manufacturer publishes a rough opening width and height for each model and size, and they differ between brands, between new construction and replacement units, and between window types.
Get the installation sheet for the exact unit before you mark the plates. It gives three framing dimensions you need: rough opening width, rough opening height, and the sill height you frame to. If the window is also an emergency escape opening, the net clear opening and the maximum sill height are separate code questions, covered in egress window requirements.
What are the parts of a framed window opening?
A window opening has six framing members, and all of them are sized from the rough opening and the header table.
| Member | Where it goes | What it does |
|---|---|---|
| King stud | Full height, beside each end of the opening | Braces the header end and carries the jack stud |
| Jack (trimmer) stud | Under each end of the header, beside the king | Carries the header load to the bottom plate |
| Header | Across the top of the opening | Carries load above the opening to the jacks |
| Rough sill | Across the bottom of the opening | Frames the bottom of the rough opening |
| Header cripples | Between the header and the top plate | Fill the wall above the opening |
| Sill cripples | Between the bottom plate and the rough sill | Support the rough sill |
R602.7.5 says the full-height stud next to each end of the header must be end nailed to the header in accordance with Table R602.3(1). Table R602.7.5 sets the minimum number of full-height studs at each end in exterior walls. The wall framing basics guide shows how these members sit in the whole wall.
How do you frame a window opening step by step?
Work from the manufacturer's sheet down to the stud marks. The order matters because the header height is measured from the floor and sets the length of every other short member.
- Fix the rough opening. Write down rough opening width, rough opening height and the height of the rough sill above the floor.
- Size the header. Use the header tables (next section) for a bearing wall.
- Mark the plates. From the rough opening width, mark the jack stud and king stud positions on both plates, then continue the common stud marks beyond them.
- Build the header. For a built-up header, Table R602.3(1) item 10 gives 16d common nails at 16 inches on center on each edge (2-inch to 2-inch header with a 1/2-inch spacer). A single-member header uses a flat 2-inch nominal member or plate top and bottom, face nailed with 10d box nails at 12 inches on center (R602.7.1).
- Set the jack studs and header. Nail each jack to its king stud, set the header on the jacks, and end nail through the king studs into the header ends (Table R602.3(1), item 12: 3-16d common nails is one listed option).
- Install the rough sill and cripples. Place the rough sill at the sill height, then add sill cripples below it and header cripples above the header. Cripples attach to the plates with the plate-to-stud rows of Table R602.3(1) (item 17, toe nail 4-8d box is one option).
- Measure before you sheathe. Check width, height, diagonals and sill level against the manufacturer's sheet.
What size header does a window need?
In a bearing wall, the header size comes from Table R602.7(1) for exterior walls and Table R602.7(2) for interior walls, using what the wall supports, building width and ground snow load. The tables cover No. 2 grade Douglas fir-larch, hem-fir, Southern pine and spruce-pine-fir only.
The snow load column changes the answer more than most people expect. This slice holds everything constant except snow load.
2021 IRC Table R602.7(1): exterior bearing wall supporting roof and ceiling only, 2-2x10 header, 24 ft building width, No. 2 grade DF-L, HF, SP, SPF. Spans in feet-inches; NJ = jack studs at each end.
| Ground snow load | Maximum span | NJ |
|---|---|---|
| 30 psf | 6'-10" | 2 |
| 50 psf | 5'-11" | 2 |
| 70 psf | 5'-3" | 2 |
For other row groups (one floor, two floors, clear-span or center-bearing floors), use the header size chart, which reads the table in detail. The same page covers king stud counts and garage-width openings.
Two footnotes affect window framing directly:
- Footnote d. Where NJ equals one, an approved framing anchor attached to the full-height stud and to the header may replace the jack stud.
- Footnote f. The spans assume the top of the header is laterally braced by perpendicular framing. Where it is not, for example with cripple studs bearing on the header, the 2x8, 2x10 and 2x12 spans must be multiplied by 0.70, or the header designed. A window with cripples above the header is exactly that case, so read the footnote before you rely on a tabulated span.
What if the window is in a nonbearing wall?
Load-bearing headers are not required in interior or exterior nonbearing walls (R602.7.4). A single flat 2x4 is permitted as the header for openings up to 8 feet wide when the vertical distance to the parallel nailing surface above is not more than 24 inches, and no cripples or blocking are needed above it. Confirm the wall is nonbearing first, because a wall that carries joists or a beam is not.
What code rules affect window framing besides the header?
Three other rules decide how you frame the opening, and each has its own page:
- Egress. A bedroom, basement or habitable attic window may need to meet net clear opening and sill height limits. See egress window requirements.
- Safety glazing. Windows near doors, tubs, stairs and floors may need tempered glass, which changes the unit you order. See tempered glass requirements.
- Fall protection. Under R312.2.1, where the bottom of the clear opening of an operable window is less than 24 inches above the finished floor and more than 72 inches above the finished grade or surface below, the window must not pass a 4-inch-diameter sphere in its largest opened position, or it must have a window opening control device or fall prevention device that complies with ASTM F2090. The rule applies to dwelling units.
Flashing, shimming and sealing the unit follow the window manufacturer's installation instructions. This article does not quote the code flashing section, because the source text for it carries Washington amendments.
How do you set and check the opening?
Check that the rough opening is the width and height the manufacturer lists, with the sill level and both sides plumb, before the sheathing goes on. Opening size errors are far cheaper to fix at this stage. Hold the header's bearing at each jack and confirm the king studs are nailed through to the header ends.
The tables here are prescriptive limits for the stated conditions only. The adopted code edition and local amendments govern, engineered headers use the manufacturer's tables, and the code is available from the International Code Council.
Summary
The rough opening size for a window comes from the manufacturer. The header comes from Table R602.7(1) or R602.7(2) in a bearing wall, with jack studs per the NJ column and full-height studs per Table R602.7.5. In a nonbearing wall, R602.7.4 allows a single flat 2x4 header up to 8 feet wide. Footnote f's 0.70 factor catches headers with cripples bearing on top.
Framing Plan is a preliminary drawing aid. It draws joist layouts, beams and openings and exports PNG, PDF and DXF, but it does not size headers or check code, and the editor works in millimetres. Open the framing studio to place the opening on your plan, then take the header size from the table or the manufacturer.