A window rough opening is the framed opening in a wall, sized to receive a window unit with a small clearance for shimming and sealing. Get the dimensions wrong and the window won't fit; build the framing wrong and the window leaks or sags. The job involves cutting through existing wall framing (load-bearing or non-load-bearing), installing a header, jack studs, and sill — exactly per the window manufacturer's rough opening dimensions.

This guide covers framing a new rough opening in an existing wall (cutting in a window) for a typical 30-60 inch wide window. Larger spans and load-bearing walls follow the same principles but require engineered headers.

Before you start

Three things to settle:

1. Rough opening dimensions from the window spec. Window manufacturers spec rough opening (RO) sizes — typically the actual window dimension + 1/2 to 3/4 inch in each direction. Get the exact RO from the window manufacturer's installation instructions or website. Don't measure the existing window; the RO is different.

2. Load-bearing wall = engineered header. Exterior walls usually load-bearing. Header sizing per OBC Section 9.23 span tables for one-storey above; engineer for two storeys above or wider spans. Ottawa's 2.5 kPa snow load matters.

3. Permit for new windows in Ontario. Adding a window where there wasn't one (cutting through structure) requires a building permit. Replacing a window in the same opening usually doesn't. Bedroom egress windows have specific size requirements (OBC 9.9.10.1).

What you'll need

Tools

Reciprocating saw or oscillating multi-tool
Circular saw or mitre saw
Drill with bits and driver
Hammer or framing nailer
Tape measure
Level (4-foot)
Speed square
Stud finder
Pry bar
Safety glasses, hearing protection, dust mask

Materials

Header lumber (2× lumber sized per OBC — typically 2-2×8 or 2-2×10)
1/2 inch OSB or plywood spacer (for built-up header)
King studs and jack studs (existing 2× wall lumber)
Sill stud and cripple studs (2× lumber)
16d framing nails or 3-inch construction screws
Construction adhesive
Temporary shoring lumber (2×4s)

RO = window width + 1/2-3/4 inch; height + 1/2-3/4 inch

Get exact RO numbers from the window manufacturer's installation guide. Different brands use different clearances.

When sizing rough opening: build to RO dimensions, not window dimensions. The clearance allows shimming for plumb and level.

Step-by-step instructions

Get the exact rough opening size

Pull the window manufacturer's installation instructions — RO width and RO height are listed.

Common: window 30×48 has RO 30.5×48.5 inches.

Verify before any cutting.

Mark the opening on the wall

Snap chalk lines or mark with pencil on interior wall surface.

Mark the RO outline. Add: width + 3 inches (jack studs on each side) + 3 inches (king studs on each side). Add: top: header thickness (typically 7.5 inches for 2-2×8 with spacer). Add: bottom: sill stud (1.5 inches) + cripple studs.

These additional measurements give you the rough framing area.

Build temporary shoring (load-bearing walls only)

If wall is load-bearing: build temporary 2×4 wall ~4 ft from work area, floor to ceiling, with shims at top.

This carries the load while existing studs are cut.

Skip if wall is non-load-bearing (interior partition with no joists running perpendicular).

Cut and remove existing wall studs in opening

Use reciprocating saw to cut existing studs in the framing area.

Cut at marks where header will install (top) and where sill will install (bottom).

Remove cut studs and any drywall in the framing area.

Caution: locate and shut off any electrical before cutting.

Install king studs

Cut king studs to floor-to-ceiling length. Position at outside edges of framing area (3 inches outside RO).

Nail/screw to top plate and bottom plate. Plumb (vertical) before fastening fully.

Install jack studs and header

Cut jack studs: floor to (top of RO).

Position jack studs inside king studs, at outside edges of RO.

Build header: 2 pieces of header lumber + 1/2 inch spacer, glued and nailed/screwed every 12 inches.

Set header on jack studs. Nail/screw header to king studs through the side.

Install sill stud

Sill stud: 2× lumber spanning between jack studs at the bottom of the RO.

Cut to fit between jack studs.

Nail/screw to jack studs.

Install cripple studs

Cripple studs above header: from top of header to top plate, at 16 inches on-centre.

Cripple studs below sill: from bottom plate to underside of sill stud, at 16 inches on-centre.

Cut all cripples to length and install.

Verify RO dimensions before cutting exterior

Measure rough opening width and height inside the framing.

Should match the window's spec RO exactly (within 1/8 inch).

If RO is too small: re-cut and re-position studs. If too large: install filler 2× lumber to tighten.

Don't cut through exterior sheathing/siding until interior framing is verified.

Cut exterior sheathing and siding

From the inside, mark the RO outline through to the exterior.

Drill a hole at each corner of the RO from the inside, all the way through siding.

From outside, connect drill holes to mark the cut.

Cut sheathing and siding with reciprocating saw.

Remove temporary shoring

After framing is complete and load path is established (header → jack studs → king studs → bottom plate), remove temporary shoring.

Verify ceiling is still flat above the opening.

If sag visible: header is undersized; reshore and re-engineer.

Common mistakes that cost you

Building RO to window's actual dimensions, not RO spec. Window won't fit (too tight) or won't seal (too loose). Always use manufacturer's RO numbers.
No header in load-bearing wall. Wall sags above opening. Header required.
Skipping temporary shoring. Ceiling drops while framing is open. Always shore in load-bearing walls.
Cripple studs missing. Drywall above header has nothing to nail to. Cripple studs at 16 inches OC required.
Sill stud too thin. Sill bears the window's weight. 2× lumber minimum.

Frequently asked questions

Do I need a permit to cut in a new window?

Yes — new openings in load-bearing walls or exterior walls require Ontario building permit.

Replacement windows in same opening: usually no permit (verify with municipality).

Egress (bedroom) windows: must meet OBC 9.9.10.1 size requirements for emergency exit.

What's the egress window requirement?

Bedroom egress windows in Ontario must have minimum 0.35 sq m (3.77 sq ft) opening area.

Minimum dimension: 380 mm (15 in) in either width or height.

Sill height: maximum 1.5 m (59 in) above floor.

Opens directly to exterior with no obstructions.

Can I make an existing window opening larger?

Yes — but it's the same as cutting a new opening. Header sizing must accommodate the new wider span.

Always pull a permit and verify header size with engineer for major enlargements.

When to call us instead

Adding or changing windows is structural work that often requires permits. Our team handles design, permits, and installation as part of renovations.

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