When you lay baseboard against a wall and see gaps between the top of the trim and the drywall, it's usually because of a misaligned stud or a built-up ridge of taping compound over a joint. Both will create bulges that cause gaps. You'll also find the problem at outside and inside corners, again caused by built-up taping compound. While you can scrape off small lumps with a putty knife, you can't move studs or sand off the bulge of compound without creating a huge mess. It's almost always easier to adjust the trim rather than to attempt any wall fixes.
Solution 1: Most standard baseboards these days are 3-1/4 in. high and 3/8 in. thick, which makes them pretty doggone flexible. First hold the trim against the wall after you cut it to length, and look for gaps. If you see some, cut a few braces from 4- to 6-in. lengths of scrap baseboard and put 45-degree angles on the ends. Then apply construction adhesive at the top and bottom. Nail the trim to the studs and then tack the braces to the trim with 1-in.brads. Force the gaps closed with the scrap and tack the blocks to the trim. Leave it overnight to let the adhesive set, and then pry the blocks free. Fill the little holes when you fill the rest of the nailheads.
Solution 2: Thick (3/4-in.) board trim isn't flexible, so it's unlikely you'll be able to flex it against the wall as we show (see Solution 1), nor will glue hold it. The best solution is to simply fill any gaps with caulk. Select a premium acrylic latex caulk ($3-plus per tube) that says “paintable” on the label. Snip a hole in the spout no larger than 1/8 in. for the neatest job. Force the caulk into the gap and fill it a little higher than the top of the trim. Immediately wipe off the excess with a damp rag. The next day, paint the caulk to match the wall color. You'll be surprised how well even large gaps nearly disappear.
Baseboards, especially narrow ones, will frequently tilt toward the wall at the bottom. That's because the lower nub on thin trim can fall into the space between the drywall edge and the floor. In addition, the baseboard follows the tapered drywall edge near the floor. Taller baseboards usually aren't affected because they're high enough to rest against the flat drywall surface above. Don't worry about small tilts if they're noticeable only at inside corners (see “Tilted Inside Corners,”). Push the base firmly against the wall with your hand to determine how severe the tilts will be. If gaps appear between the top of the trim and the wall, you'll need to use the solutions we show.
Solution 1: Carpeting and pad together are at least 1 in. thick, meaning that it covers at least the lower 3/4 in. of baseboard, so there's no reason to rest base right on the floor. You can fix a tilted baseboard by raising it above the drywall gap. Cut a few 3/8- to 1/2-in. thick spacers (6-in. scraps of standard base will do). Lay them flat on the floor every few feet, and then cut the base to fit, rest it on the spacers and nail it into place. This technique also leaves more of that handsome baseboard showing above the carpet. One more thing: Carpet installers prefer to cut the carpet a little long and tuck the excess into the gap for a neater job.
Solution 2: When you're installing baseboards over vinyl, wood or plastic laminate floors, the best way to handle gaps between the drywall and the flooring is to shim the gap with narrow blocks of 1/2-in.wood. Any wood will work, but chunks of 1/2-in. scrap plywood are ideal. Cut the blocks and tuck them into the gap every few feet. There's no need to nail or glue them into place; just install the baseboard right over them, tight against the floor. It's best to keep the nails at the bottom of the base above the gap so they go through drywall, not air.
Trim at inside corners often won't meet evenly despite a perfectly cut cope. That's generally because corner tape joints don't always get filled or sanded all the way to the floor. (I know a few drywall finishers who don't like to bend over.) The tapered drywall edge can also cause tipping.
Solution: The key to a clean inside corner is to use test pieces to help you prepare the corner for the permanent base. Conduct this test before you nail any base permanently to the wall, because you'll just have to remove it if the corner joint is bad. Cut a perfect cope on the end of a foot-long chunk of baseboard, then use that and another short piece of base with a square end to check the corner. Be sure to push the pieces tight against the wall (especially at the bottom) to simulate the pressure the nails will exert. If the joint looks good, go ahead with the standard installation. But if there's a gap at the bottom of the joint, set aside the test pieces and drive a 2-in. screw about 1/2 in. above the floor and an inch or two away from the corner behind the square-ended piece. Sink the screw until the head is protruding slightly beyond the drywall, then check the joint again with the test pieces. Adjust the screw in or out and continue adjusting and testing until the joint is perfect. Then go ahead and install the baseboard.
Only about half the time can you cut 45-degree miters on baseboards and have them meet perfectly on outside corners. Built-up drywall compound and imperfect corner bead installation or framing generally leave you with something greater or less than an exact 90-degree angle, and standard 45-degree miters won't work. Don't even bother trying to measure the angle with an angle finder. It's easier to find the miter angle simply by testing with scraps.
Solution: Start by cutting 45-degree miters on the ends of 10-in.-long base scraps. Put them together at the corner to check the fit. Recut both pieces at angles slightly more or less than 45 degrees as required. There's no magic to it—after a few cuts and test fits, you'll find the right angle. After you find the angle, save the miter saw setting and cut the finished base miters. And don't think you have to limit yourself to whole numbers. More often than not, the right angle will fall between the marked degrees on your saw. It's surprising how big a difference even a half-degree adjustment will make. This method also works great for finding miters for corners other than 90 degrees. Just guess an angle to start with, and with a little practice, you'll find the perfect miter angle for any corner in moments.
While it's best to run a single length of base on a wall, at some point you'll be faced with splicing two pieces in the middle of a wall. When splicing, make the joint over a stud to keep the ends tight. But a simple butt joint is a poor technique because the glue won't hold the ends together, and it's tricky to line up the profiles perfectly when nailing. And to nail into the stud you have to nail close to the end of the trim, where the wood is prone to split.
Solution: The key to a clean splice is a “scarf” joint. Cut the first trim board about 1 in. short of a stud with a 30-degree bevel facing the room. Nail it into all the studs, then cut the second board with the same 30-degree bevel but in the opposite direction. Assemble the joint (no glue) to check the fit and recut a slightly greater or lesser angle as needed for a perfect joint. Smear wood glue on both bevels. Push the joint together, and then nail the overlapping piece into the stud near the joint. The overlapping bevel will clamp the one behind it against the wall. Wipe off the glue squeeze-out with a damp rag right away. If you're installing clear finished baseboards with a prominent grain pattern, select two pieces that match as closely as possible.
Power Nailers Really Pay Off
You can hand-nail trim—carpenters have been doing it for centuries—but there's no reason to anymore. The advantages of using a gun over nailing by hand are numerous. You'll be able to get trim placed exactly where you want it with one hand while you fasten it with the other. Pieces don't move around while you're nailing on them, and of course, no more hammer marks or nail setting. And with some trim gun models selling for under $100, it's worth getting one of your own. Of course you'll need a compressor, too, but kits that have hoses, fittings, a gun and a compressor are available at any home center for about $200. If you’re not ready for the investment, we strongly recommend renting the tools (about $50 per day). Nail guns for trim come in three sizes for shooting 15-,16- and 18-gauge nails, with 15-gauge nails being the thickest. If you only buy one gun, get an 18-gauge unit that shoots nails ranging between 5/8 in. and 2-1/4 in. long. Although the thin nails won't be hefty enough for anchoring door or window jambs, they'll work fine for most modern trim. When you need extra strength, pull out the old hammer and hand-nail.