Window Rough Opening Sizes: How Much Bigger Than the Window, Why Every Manufacturer Differs, Flange vs. Brickmould, Doors, ICF Bucks, and the Half Inch That Decides Whether It Fits

The numbers

What the gap is for

A window is set plumb, level and square with shims at the sill and jambs, then foamed. The gap is the room for those shims and that foam, and for the rough opening not being perfect, which it never is. Too little gap and the shims rack the frame; too much and the screws through the frame have nothing solid behind them at the shims. Half an inch total in each direction (a quarter each side) is the compromise most vinyl window makers design to. Wood-clad and fibreglass units often want more because their frames are stiffer and the installer needs room to work the shims. Brickmould units, where the exterior trim is part of the window, are sized so the brickmould laps the sheathing, and their rough opening is measured to the frame behind the brickmould, which trips people up.

How to size an opening, in order

1. Get the manufacturer’s rough opening sheet for the series you are buying. It is a one-page PDF with a table of nominal size to R.O. For custom sizes, the dealer gives you the R.O. on the order confirmation.
2. Frame to that R.O., not to the plan’s nominal size, not to “the usual”. Write the R.O. on the plan next to each ID. The schedule.
3. Check the opening after sheathing: width at top, middle and bottom; height both sides; diagonals equal within 1/4 in.; sill level. Fix now with a saw and a shim, not later with foam.
4. For doors, add the sill. A pre-hung exterior door’s R.O. height is from the subfloor (or the finished floor if the sill sits on it; the sheet says which), and the width includes the jambs and sidelights. Door R.O.s are the ones most often cut wrong because the floor finish was not decided.
5. For ICF, size the buck to the R.O. plus the buck. The concrete does not get trimmed afterwards. ICF openings.

Common sizes, as a sanity check

When it is wrong

Too tight (under 1/4 in. total): plane the jack stud if it is wood, or order the window 1/2 in. smaller if it is concrete or already bricked. Never force it; a frame bowed in by 1/8 in. is a sash that will not lock. Too big (over 1 in. total): pack the opening with solid lumber, screwed, so the shims have something behind them and the foam gap is back to normal. Foam is not structure. Out of square more than 1/4 in.: the window can be shimmed square inside the opening if there is room; if not, fix the opening. Sill out of level: shim the sill level across its whole length with solid shims, not just at the ends, or a casement sags and a slider rolls.

Where builders go wrong

1. Framing from the plan’s nominal size. The plan says 30 x 60 and means the window. The framer builds 30 x 60 and the window does not fit.
2. One rule for every maker. Switching from a vinyl maker to a fibreglass one mid-job without a new R.O. sheet.
3. Door heights from the wrong floor. 3/4 in. of hardwood later and the door is 3/4 in. too tall for the hole, or the sill is a tripping lip.
4. Trusting the forms on ICF. Check the buck after the pour, while fixing it is still possible.

Sources and further reading

• JELD-WEN Canada: windows and exterior doors; the rough opening sheets and warranty documents are on the site.
• Ontario’s Building Code: the official page, with the current regulation. Egress is 9.9.10, glazing and windows 9.6 and 9.7, energy is Supplementary Standard SB-12.