Replacement Windows

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Speaking of a fit, that's what the manager of our local building-supply house would have had if I'd tried to return my first mismeasured replacement windows—eight double-hung units that turned out to be an inch too wide.

I learned the hard way that you can't measure what you can't see. We were planning to replace the exterior siding and gut the interior of a dilapidated 1880s farm house. I figured I could adjust trim around the windows. And, based on my experience as a house carpenter, I ordered windows to fit what I thought the old house's rough window openings would measure under the trim. My "guestimates" were wrong.

In older structures, windows that look identical—and may even measure the same on the inside—may not be the same size around the outside where they mate with the rough opening in the house frame. Our windows were old factory made stock units alright, but their frames had been planed and trimmed to fit varying-sized rough openings in the framing. A quarter inch here, a half inch there might not seem like much, but it forced me to modify all eight roughs before I was done.

I also had to contend with studs (the vertical boards—2 x 4s usually—that form the wall and partitions in a frame house) that had bowed and warped out of true over the years. Some were canted in the up/down dimension, some in the left/right, some were twisted out of sync with the plane of the wall, and some were warped all out of shape in all dimensions.

In such a situation, you should measure both height and width at ends and middle of the window rough opening and choose the smaller of the measurements. Check corners for square, sides for plumb (perfectly vertical), and the top and bottom for level. As the illustration "An inch out of square..." shows, a rough opening an inch out of square can hold a window an inch narrower than the width measurement. The same is true of window height if the horizontal frame members are not level.

Copy the dimensions and angles of each rough and figure the largest true rectangle that will fit the space... or determine to square and plumb the rough opening to match window dimensions.

Now determine how deep your jambs should be. Measure the thickness of the wall surrounding each window. Precisely measure each layer: inside plaster and lath, rough framing, exterior sheathing, and existing exterior trim. The window's jamb should extend from the outside of the exterior sheathing (not including siding and trim) to the inner surface of the inner wall plaster or other finished surface (but not including interior trim.) See the drawing: "Measuring the Jamb:"

Ordering Windows

Take your list of measurements to the building-supply center. Ask to look through their window catalogs. Inspect and measure windows in stock and compare with catalog dimensions. You will find that catalogs list rough dimensions a little bigger than the windows—giving you about 3/4 inch of leeway in width and a 1/2 inch in height to allow for level/plumb adjustments on the job. A clerk should be willing to help you interpret the catalogs and help assure that your windows will fit.

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