Trimming a window that is out of square

btheisMay 7, 2008

Hi Folks,

I want to trim a exterior garage window - essentially just "picture frame" it so it ties in more with the house. I plan using trim molding cut at 45 degree angles. Took all the height and width measurements and they are the same top to bottom and side to side. I'm not an experienced woodworker by any stretch of the imagination, but do know enough that in order to use 45 degree angles, the window that I want to frame must be square. It's not - off by 1/4". How do you account for this when planning the miter cuts? Or, do you just cut the pieces and take the difference into account when you nail up the trim pieces, filling the gaps in with caulk.

Any advice will be greatly appreciated.


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You wrote the side to side and top to bottom measurements were the same---and then said the window was 1/4" out of square.

That is confusing.

If the side to side is different from the top to bottom---that simply makes the window a rectangle instead of a square---and all miters will be 45 degrees. Two pieces will be 1/4" longer than the others---but the corners are still a total of 90 degrees.

    Bookmark   May 7, 2008 at 8:55PM
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Do experimental cuts with scrap to determine what the angles are, as it will be in fractional degrees. Write each different angle on the wall so you can keep them straight, or draw it out on a plan before cutting the "real" wood.
Miters are unstable joints anyhow; what is tight in July will open "inside" in January. And vice versa: what is tight in January will open up on the outside of the joint in July. What happens is that the angle changes whenever the wood swells or shrinks in width, and there is no way around this characteristic of woodwork. Miters need to be avoided on wide trim, especially if in an exposed situation.

    Bookmark   May 7, 2008 at 10:29PM
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You could use Fypon. Then you don't have to worry about wood swelling and contracting.

    Bookmark   May 8, 2008 at 9:33AM
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I never use miters on outside windows. Say your trim is 3 1/2" wide. Cut the top piece 7" longer than the window size, the sides 3 1/2" longer, and the bottom piece even with the window. Butt all the joints. This keeps out rain better Put a healthy bead of caulk behind the trim boards before nailing. caulk the outside joints again before painting

    Bookmark   May 8, 2008 at 11:30PM
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i could tell you except that i agree 110% with the last post.

    Bookmark   May 11, 2008 at 3:13PM
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@handymac: equal side / top measurements but out of square *does* make sense. It's a parallelogram. Side and top piece trim lengths will the same, but you won't be splitting a 90 degree angle with 45 deg. miters. Instead, two angles will be slightly acute and two will be obtuse.

    Bookmark   December 9, 2012 at 12:19PM
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This is why God made the reveal.

Cheat it.

    Bookmark   December 26, 2012 at 3:23PM
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Measure the angles and split them for mitering.

    Bookmark   January 4, 2013 at 5:18PM
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Hi I am a UK Carpenter and Joiner and the answer to your question is easy.

Forget about the angles being 45 Degree cuts, if the windo is out of square the cuts will not be 45 Degrees.

To obtain the correct cuts (angles)

Take a piece of trim material and cut it a couple of inches longer than one side of the window, include enough length for the two angle cuts (mitres) . Place it flat against the the window frame, (in the position it will be finally fixed) use a couple of small pins to tack it in place, making sure it is placed the correct distance from the edge of the window edge (top to bottom). Next take a pencil and mark along both inside and outside edges of the trim at the corners where it will intersect with the top trim (where it will cross the top trim) . When you have marked top and bottom corners take the trim off the window. You should have four vertical pencil lines, two in each corner.

Next Cut a piece of trim for the top of the window, tack it against the window frame as before, but this time it will be on the top of the window. Mark the inside and outside ends at the corners, as for the side, but this time you will have made two horizontal lines in the corner. Remove the trim.

You should now have four lines intersecting in the corner of the window. The angle you need at that corner will be the diagonal angle that intersects the straight lines. You need to mark all the corners in a similar way as each corner will be very slightly different.

If you have a bevel you can place it on the window frame and slide it to fit the diagonal, then transfer it to the trim for cutting. You need to adjust the bevel for each piece of trim (re set the bevel each cut). If you do not have a sliding bevel you can obtain the cut directly from the window, just re tack the trim on the edge of the window and transfer the diagonal lines directly from the window to the trim.

Make sure you use the same trim for marking and cutting on each edge of the window.

Hope this helps. I may do some videos and post them on utube in the future if you think it would help.

    Bookmark   January 12, 2013 at 3:19AM
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