Installing wood trim | Pro Construction Guide
Combination square as marking gauge

Installing wood trim

create layout of the room and cut list

Create a simple layout of the room and a cut list so you can cut all trim pieces at once. This works best with baseboard, chair rail or crown molding.

Trim carpentry can be one thing that distinguishes high-end work from average work.

Trim carpenters must often work with substrates and materials that are not always plumb, level or square, but their job is to make sure the doors, windows and cabinets work well and the whole house looks great. That is not always an easy task, but the following tips and tricks will help you attain the results that want when installing wood trim.

Map out a cut list

This works particularly well with baseboard and crown molding, but can be applied to other trim details as well. Before installing wood trim, map out the room, measuring all components at once and making a cut list. Begin at the door and work around the room (counterclockwise if you are right-handed, clockwise for lefties), recording the measurement for each piece and the type of joint required on each end (butt, cope or outside corner). Right-handers should try to keep most of the copes on the right side of the trim pieces if possible, and left-handed carpenters should try to keep copes on the left ends for easier cutting.

Measure to a block cut to exact length

When taking inside measurements, it can be quicker and more accurate to measure to a block cut to an exact length and do a little simple math.

Make each measurement as accurate as possible, but if some corners are severely out of square or other conditions arise where an exact measurement is too difficult, cut the piece a bit long and mark the cut in place. Take the cut list to your miter saw and cut all the pieces at one time. Then lay the pieces around the room where they will be attached. Generally, it makes sense to begin nailing the pieces where you started the measurements and work around the room in the same manner. An exception would be any very small trim pieces that may be split by nailing. Place them first but don’t nail them, and let intersecting trim hold them in place. It may also be easier to glue outside corner trim pieces together before nailing to the wall.


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Exact measurements made easy

Scrap with nice straight edge and measure to that.

Rounded corner bead can make it difficult to get a precise measurement on outside corners. Use a piece of scrap with a nice straight edge placed tightly to the adjoining wall and measure to that.

Rounded drywall corner bead and inside corners built up with joint compound can often make exact measurements tough to take. For accurate outside edge measurements, simply hold a piece of scrap wood with a straight edge tight against the adjoining wall and measure to that. On inside corners, cut a piece of scrap trim to an exact length that is easy to remember and add to it – 5 inches or so. Hold that piece tightly inside a corner and mark the end. Then measure from the other inside corner to the mark and add the length of your scrap.

When measuring between two inside corners that are less than 6 feet apart, use a rigid folding rule with a slide-out brass extension. Get the measurement, then bring the entire rule to the trim piece and transfer the measurement.


use scribe block and a sharp pencil or knife

To get tight-fitting joints between trim elements when things aren’t exactly plumb, level or square, use a scribe block and a sharp pencil or marking knife.

Fitting tightly when out of plumb

In a perfect world, all floors, walls, windows and doors would be perfectly plumb, level and square. Because that never happens, achieving a tight baseboard fit against a door casing, for example, requires a scribing block.

Make a block out of a flat, square piece of ¾-inch stock and cut a notch that is taller than the baseboard and just a bit narrower than the combined width of the casing and baseboard. Hold the block hard against the casing while pushing it and the baseboard tightly toward the wall at the same time. Scribe a line directly to the baseboard with a sharp pencil or marking knife.

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Short-point measurement

Combination square as marking gauge

Use a combination square as a marking gauge and lay out a consistent reveal around doors and windows.

Measuring from the short point of a mitered trim piece can be a recurring problem – there is nowhere to hook the tape to take a measurement. When you don’t have a helper ready to hold the tape on the short point, reach for a small square.

Line up the blade of your combo, tri or speed square with the short point and affix it to the trim with a spring clamp. Hook your tape and pull (but not too hard) a quick, accurate measurement.

Mark in place, cut, install

There are times when leaving your tape measure on the bench will give you quicker and more accurate trim results. When trimming doors for example, it makes sense to hold the pieces where they will go and mark the exact lengths in place.

Use reveal marks to measure door and window casing in place

Leave your tape measure alone and use those reveal marks to measure door and window casing in place.

First mark a consistent reveal around the jamb by using a combination square as a marking gauge. Just a few evenly spaced pencil marks along each length of the opening, plus intersecting marks at each corner are all you need – not a solid line. Install the head casing first by aligning a mitered end at one corner reveal and marking the point where the other ends meets that corner reveal.

Carefully cut, position and tack in place in case any repositioning is necessary. Then mark the sides upside down. Miter one end and flip the piece so the mitered end touches the floor. Mark the point where the bottom intersects the top of the head casing and make a square cut.

You may need to scribe a severely uneven floor’s profile on this cut. Glue and carefully nail the corners first, drilling pilot holes and using blunted 4d finish nails to fix each miter. Reposition head casing slightly if necessary.

You can then eyeball the reveal all the way down and fasten the casing to the jambs and wall framing. This same process can be applied to trimming windows, applying chair rail, building raised panel wainscoting, or just about any job of installing wood trim where a tape measure would only slow you down.

—By Rob Fanjoy

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