Adding a Window

A new window can transform a dark room, and in an old house it’s often the most satisfying carpentry you’ll do. It’s also a structural change to the building, so it pays to plan it carefully and do it in the right order.

Before you start

• Permits. Wisconsin’s Uniform Dwelling Code covers houses begun after June 1, 1980, and alterations to them; it doesn’t apply to older houses unless your municipality has adopted it ¹. Your city or village may still require a permit for a structural change like a new window opening, so call the building department first.
• Bedrooms. If the window will serve a bedroom, consider making it large enough for emergency escape. The UDC’s egress standard for newer houses is a net clear opening of at least 20 by 24 inches with the sill no more than 60 inches above the floor ²; it’s a sensible target for any bedroom.
• Lead paint and plaster. You’ll be cutting through painted siding and plaster; work lead-safe (see the Residential Construction course).
• What’s in the wall. Look for wiring, plumbing, and ductwork. If the wall is finished, remove the plaster carefully to see ³. Anything in the way has to be rerouted ^{3 4}.

Hold the house up

Most exterior walls carry load: the floor joists above, the ceiling, or the rafters ⁴. A wall is likely load-bearing if it runs perpendicular to the joists, sits over a foundation wall or beam, stacks through floors, or holds up rafters; joists usually run at right angles to the roof ridge ⁴. This Old House’s Tom Silva advises confirming with a structural engineer or experienced contractor ⁴.

If the wall bears load, you must hold up the ceiling and everything above it while the opening is cut and the header goes in ³. Fine Homebuilding’s method: tack two 2x6 plates flat to the underside of the joists about 3 feet back from the wall, set screw jacks under them on the floor (or the basement slab), about 4 feet apart, with the plates reaching at least one joist past each end of the opening ³. A temporary stud wall works too; Silva cuts the temporary studs an eighth to three-sixteenths of an inch long and drives them in tight so they truly take the load ⁴.

Open the wall

Lay out the opening, then remove the siding outside, saving it to reuse ³, and the plaster inside. Mark the rough opening from the window maker’s specifications, then cut the studs.

Frame the opening

A framed opening has the same parts in an old house as a new one:

• King studs: full-height studs on each side of the opening.
• Jack studs: shorter studs nailed to the kings, holding up the header.
• Header: a beam across the top that carries the load from above to the jacks. Its size depends on the span and what it carries; Wisconsin’s code tables, or an engineer, give sizes ².
• Cripples: short studs above the header and below the rough sill.

Balloon frames are different. In a balloon frame the exterior studs run continuously from the sill up past the second floor, and the second-floor joists hang on a ribbon let into them ⁵. When you cut studs for a first-floor window, the pieces above the opening still run up the wall, carrying the second floor’s joists, so they now stand on your header. Make sure the header, the jacks, and whatever is under the jacks can carry that load down to the foundation. In a first-floor wall of a balloon frame, there’s no plate between floors to stop at; you’re cutting a stud that is part of a two-story column.

Flash it like a roof

New windows leak at the sill. A fully flashed opening starts at the bottom: a back dam along the inside edge of the rough sill and a flexible self-adhering sill flashing pressed in and turned up the jambs, so any water that gets past the window drains outward instead of into the wall ⁶. Then set the window, tape its side flanges over the sill flashing, add head flashing, and lap the building paper or housewrap down over the head flashing so every layer sheds water onto the one below. The Residential Construction course walks through this sequence in detail.

Finally, re-side around the window with the boards you saved, seal and insulate the gap between the window and the framing, and patch the plaster inside.

Try it

Pick the wall where you’d want a window. From the attic or basement, find which way the joists run and whether the wall sits under the ends of joists. Measure where the studs are, and look outside for vents, pipes, or the electric meter on that wall. That’s the information an engineer or your building department will ask for first.