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Tips & Techniques

Foolproof Trim Gauge

This gauge is basically a block of hardwood with a rabbet cut around the perimeter.

By Grant Litts Issue 316 - July 2023
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I install a lot of window trim and have always made some kind of a gauge block to quickly mark the reveal on the extension jambs when installing casing. A sliding square works, but it is not very fast or convenient and can mar the extension jamb surface; plus, I don’t like having a foot of metal sticking out when trying to maneuver the square.

This is my favorite gauge block of all the models I’ve made. It’s basically a block of hardwood with a rabbet cut around the perimeter. The width of the rabbet cut indicates the amount of desired setback. You simply set the inside edges of the block into the corner where the extension jambs meet and mark the casing location using the outside edge of the block. This gauge is fast and easy to make and to use, and it takes up very little space in my apron.

using a trim gauge

I used to make two different setbacks on the same block, but I’ve found that I like using a separate block for each reveal measurement. I like to cut the blocks to about 3-1 ⁄ 4 in. square, and I cut the rabbets on a tablesaw. A dado blade works best, but any good blade will do. I then bore a 1-1 ⁄ 8-in. hole in the centers and round over the edges on both sides of the hole with a 1 ⁄ 4-in.-radius router bit. This hole makes it easy to grasp the blocks while I’m using them and to hang them up when I’m not.

—Grant Litts, Norway, Mich.

Edited and illustrated by Charles Miller.

From Fine Homebuilding #316

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  1. bobbomax | May 31, 2023 09:47pm | #1

    I use 2 identical small blocks that are offset both directions by the reveal distance. Almost anything will do- square, rectangle, any convenient thickness- as long as they're the same shape & have square corners. I draw an arrow pointing to the functional corner.

    I nail them together with short trim nails, pounded over if they're too long. The important thing is being careful to maintain the offsets at the reveal distance. Using rabbets as suggested can be more accurate, but it's fussier & more time consuming.

    If you want 2 reveals in one tool, you can do the math or you can offset for the larger reveal, say 5/16, then make 2 trim cuts at 1/16th to change one corner to 1/4" reveal, being sure to use an arrow and to mark each side clearly for its reveal. It's good to make several so you can loan them out without fretting about getting them back.

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