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Here's How: Do Your Own Finish Carpentry to Lower Costs

James Dulley on

Dear James: I had a room added to my house. I want to do the trim-finish carpentry myself to save money, but things are not fitting properly. What are the proper techniques for a good-looking job? -- Debi K.

Dear Debi: The quality of the trim-finish carpentry in a home can make all the difference in how the new room looks. But keep in mind, as you are attempting to fit everything properly, only you know where the errors are. This knowledge makes them much more apparent to you than to guests. If you are a perfectionist, it will drive you nuts until you get it right.

Don't feel bad if you are having trouble making the trim on the windows or doors fit properly. This is often more of an art than a science, and it can take a carpenter a long time to master it. Wood, even high-quality trim lumber, is not always square and flat, so some effort is involved in creating acceptable, not perfect, joints.

One of the first tips that beginning carpenters learn is to not measure the lengths and widths of pieces in numbers (feet and inches). When using a tape measure, you will first have to determine the exact measurement. This may be difficult if the door opening is not perfectly square or the edge is not smooth and flat.

At best, every measurement is just an approximation with a degree of error. You will likely have the same degree of error when you lay out the same measurement on the piece of trim lumber you are planning to cut. If you are lucky, the errors will negate each other. If you are unlucky, they will compound and result in a poor fit.

Whenever possible, place the piece of trim lumber up against the wall opening, and mark the locations of the cuts without measuring. This way, you know it is the proper length. Take the width of the saw blade into account, and make your first cut. If you are mitering a corner, which is common around windows and doors, use a high-quality miter saw set at the proper angle for that specific opening.

Before making any cuts, check the squareness of the wall openings. This can be done by stretching a string across one diagonal and then the other. If the diagonals are equal, the opening is square, and the miter cuts should be 45 degrees.


If the diagonals are not equal, this generally means the corners of the opening are not 90 degrees. One is larger, and one is smaller. This requires miter cuts at slightly less than 45 degrees for one side and slightly more than 45 degrees for the other to get perfect corner fits. Either get out your high school geometry book to calculate the angles based on the diagonal difference, or use a carpenter's protractor to measure them.

Once you make the miter cuts on the side pieces, tack them in place on the opening. Again, instead of measuring the distances with a tape measure, place the top piece against the wall and mark the cut points. For an inside measurement, use a high-quality folding rule with an extending brass end. Pull out the brass end for a snug fit between the pieces. Just place it on the trim lumber, and transfer the length with marks.


Send your questions to Here's How, 6906 Royalgreen Dr., Cincinnati, OH 45244 or visit To find out more about James Dulley and read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at

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