For most DIYers, fitting baseboard moldings on the interior corners of the room is best accomplished with miter joints—45-degree miter cuts to each adjoining piece of molding. When fit together, these corners make 90-degree angles. But room corners sometimes vary slightly from perfect 90-degree angles, and professional trim carpenters have a number of tricks they use to ensure that baseboard moldings fit perfectly. The technique described here, modified for DIYers, will also work for other trim moldings, such as door and window case moldings, or chair rail moldings.
Before You Begin
Cutting trim is best done with a power miter saw fitted with a good fine-tooth trim blade. General-purpose construction blades are too coarse to produce the very smooth cuts you want when fitting trim joints.
Power miter saws are now so affordable that any DIYer can own one, and so versatile that all DIYers should have one. But if you have a manual miter box and backsaw, it is entirely possible to cut miter joints with this setup.
Another exceedingly useful tool is a power brad nailer. These tools come in several types, including pneumatic nailers powered by small air compressors, corded electric nailers, and cordless nailers that operate with batteries. Like power miter saws, brad nailers are extremely versatile and will quickly pay back your investment.
Although it may feel a little awkward, it is best to operate the saw blade with your "opposite" hand when the orientation of the baseboard requires it. It is possible to cut the trim upside down in order to always use your favored hand, but this can often lead to cutting mistakes. The best method is to position the baseboard on the saw in the same direction as it will sit on the wall. And NEVER cut "cross-handed" by trying to operate the saw with your "good" hand while reaching across the saw to hold the baseboard with your other hand. This is a very dangerous practice.
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Equipment / Tools
- Tape measure
- Power miter saw
- Eye protection
- Stud finder
- Hearing protectors
- Power brad nailer
- Baseboard molding
- Painter's tape
- 1 1/2-inch brads for power nailer
Mark Direction of Cut on First Baseboard
Press the squared-off end of the first baseboard piece tight into the corner along the floor. It doesn't matter if you begin with the left or right side; in our example, the first piece is on the right side. Use a pencil to make a direction line on the top of the baseboard, marking at a roughly 45-degree angle from the inner corner outward toward the face of the baseboard. This will help you orient the piece of trim on the miter saw before you cut it. It's surprisingly easy to get the direction wrong when you are cutting many pieces of trim.
Set the Saw Angle
Move the miter saw's blade to an angle just slightly less than 45 degrees to the back fence. While it might seem as though 45 degrees is the perfect angle to achieve 90-degree corners, just a fraction of a hair less than 45 degrees (but no less than 44 degrees) produces a better fit when you fit the two adjoining pieces of baseboard. This is because room wall surfaces usually are slightly rounded off where they meet in the corners, falling short of being perfectly square.
Cut the First Piece
Position the piece of baseboard on the saw table so that the back of the trim is tight against the vertical fence of the saw. You may need to support the end of the baseboard so it rests flush against the table. Orient the baseboard in the same way as it will fit in the room—think of the table of the saw as the floor, and the fence of the saw as the wall against which the baseboard will fit.
Before starting the saw, momentarily pivot the blade down so it just touches the baseboard and make sure it is oriented in the same direction as the marked line drawn on the top of the baseboard.
Wear eye protection and hearing protectors when cutting with a power miter saw. Power up the saw, then lever the spinning blade down through the baseboard using a smooth motion. Make sure you are tightly gripping the baseboard with your free hand. Some saws have clamps you can use to secure the baseboard against the fence and table.
Clean up the Cut Edges
The saw blade may leave small splinters along the cut edge of the miter. And if you are using plastic baseboards, there may be burrs of melted plastic left along the edges of the cut.
After the baseboard cools down from the friction of the blade, snap off any splinters or plastic burrs that remain on the edges of the cut. Or, lightly scrape the edge with the blade of a utility knife.
Reverse the Saw Blade
Pivot the saw blade to the opposite direction, again setting it to an angle just slightly less than 45 degrees. This will allow you to cut the adjoining piece of baseboard.
Cut the Adjoining Baseboard
Mark and cut the adjoining piece of baseboard, using the same technique. Again, clean-up splinters or plastic burrs from the edges of the cut once the baseboard cools down.
Mark the Straight Cuts
The easiest way to mark the opposite ends of the baseboards for the straight cuts that will fit against door moldings is to position the baseboard in place and mark where it will fit against the case molding. Where this method is not practical, you can also use a tape measure to determine the cutting length.
Set the miter saw at a square 90 degrees, then cut the baseboard pieces off square where they will fit against case moldings or other obstructions.
Test-Fit the Baseboards
Position the baseboards in place to test the fit. The miter joint should fit tightly together with no appreciable gap between pieces.
If the angles prove to be wrong, you may be able to carefully retrim the ends of the board, making slight adjustments to the saw angle. But avoid the temptation to fill large gaps with wood filler or caulk, since these patches will eventually come apart due to seasonal expansion and contraction. It is generally better to recut new pieces of baseboard with adjusted miter angles to accommodate a corner that is badly out of square. Usually, you can get by cutting one adjusted piece rather than both baseboards.
Find and Mark Wall Studs
With a stud finder, locate the wall studs and mark their locations with small tabs of painter's tape. Studs usually run every 16 inches on-center. You won't need to locate studs in corners, as studs are always found there.
Nail the Baseboards in Place
Tack the baseboard into place by shooting brads at an angle into the wall where the studs are located. Between the nails driven into the studs, alternate with nails positioned near the bottom of the baseboard, so they drive into the wall's soleplate. After driving the first brad, carefully examine the head of the brad to make sure it is slightly recessed below the surface of the baseboard. If necessary, adjust the tool to change the penetration of the brads.