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Q: My 50-year-old townhouse has a problem: recurring nail pops that resist repair on either side of a central wall on the top floor. The nail pops are roughly at the house's centerline, front-to-rear. I've had them repaired twice in two years, but both attempts by contractors have failed. I think the workers are just pushing nails back into place, filling the void with joint compound, drying it with a heat gun and repainting. I'd like a more permanent fix. What guidance can you offer?

A: You’re describing a problem that often occurs in older homes with roofs supported by trusses, rather than rafters. Truss-framed roofs are less expensive to build, because they create a strong roof using much skinnier wood, often 2-by-4s, to get strength that would otherwise require lumber eight, 10 or even 12 inches wide.

Trusses work because the pieces are nailed together to create triangles, which brace the wood and let several pieces share the pressure when a heavy load, such as snow, presses down on the roof. But because the bottom of the trusses are typically covered in insulation while the top pieces, next to the roof, are not, the long pieces at the bottom lift up in winter and move back down in summer. This phenomenon, called truss uplift, occurs because wood shrinks and swells with changes in humidity, and the wood directly under the roof absorbs and releases moisture in the air at a different rate than the wood tucked under insulation.

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When truss uplift occurs, the ceiling drywall nailed to the bottoms of the trusses has to move up and down with the wood. Some of the nails hold; some get loose and show up as nail pops. In some houses, cracks also appear at the tops of walls.

Today, builders avoid truss uplift consequences by attaching metal truss clips to the tops of interior wall framing, then fastening the edges of the ceiling drywall to the trusses through slots in the clips. But there is no way to add the clips once the drywall is in place.

In situations where big cracks have appeared, one solution is to install crown molding fastened only to the ceiling, so it covers the cracks and floats up and down, as needed, when the truss wood shrinks and swells with changes in the weather.

You could install crown molding as a permanent fix to hide nail pops close to the walls, but there is an easier solution when there are just a few nail pops, as shown in the picture you sent. Simply pull out the nails and fill the holes with drywall joint compound. The ceiling won’t cave in, because, even 50 years ago, builders installed ceiling drywall before they covered the walls. The upper edges of the wall drywall support the outside edges of the ceiling drywall.

If a nail pop appears farther into the room, first screw 1¼ -inch drywall screws into the ceiling framing about six inches on either side. (If you were fixing a nail pop on a wall, you’d need only one screw, about 1½ inches away from the popped nail.)

You can determine where the ceiling framing is with a stud finder or by tapping to see where hollow sounds become thuds. Roof framing is always perpendicular to walls, but if you’re not sure whether the pieces run side to side or front to back in your house, go outside and look at the roof slope. Roof framing is always perpendicular to the peak.

It’s important to drive the screws so the heads press into the paper covering and dimple the drywall slightly, allowing the head to be slightly recessed in comparison to the surrounding drywall. But the screw can’t rip through the paper or crumble the gypsum core of the drywall. It’s tricky to set the screw at just the right depth, so even if you need to drive just a few screws, it’s worth getting a drywall dimpler bit, which consists of a Phillips driver surrounded by a rim that recesses the screw head by the perfect amount. (One example is the DeWalt Drywall Screw Setter Bit Tip, $2.25 on Amazon.)

Once the screws are in place, pull the nail or nails that have popped. Take care to pry against a board or wide putty knife, so you don’t punch through the drywall. Cover the holes and the new screw heads with drywall joint compound.

Whether you’ve pulled nails close to the walls or added screws farther out into the room, let the drywall joint compound dry, then smooth over the surface with sandpaper or a damp sponge. When the surface is dry, brush on primer paint, let that dry, then top with paint.

Paint made especially for use on ceilings is very matte, because shiny paint tends to make irregular surfaces show up more. The light reflects differently across the humps and valleys. If your ceilings are matte, you may be able to repaint just the areas you patched without seeing streaks where the new paint overlaps the old paint. If your ceilings are shinier, you might need to repaint the whole surface to get a uniform appearance.

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