Filler for cracks in walls: what to use and how to apply it

The right filler for cracks in walls depends on the crack and the wall. For a hairline crack or small nail pop in drywall, a lightweight spackle is enough. For a longer crack, or one that has already come back once, use joint compound over fiberglass mesh tape. Where a wall meets trim, a door casing or a tub, use a paintable flexible caulk instead. This page explains how to pick the filler, how to apply it so the crack stays hidden, and how to spot the cracks that point to the foundation, which are covered in more depth on our foundation crack repair page.

What is the best filler for cracks in walls?

The best filler is the one matched to the crack. Lightweight spackle suits pinholes and short hairline cracks because it goes on in one pass and sands easily. Joint compound, bridged with tape, is the better choice for any crack longer than a hand span or one that has reopened. Flexible caulk belongs only where two different materials meet, because those joints move every season.

Here is how the common fillers compare on an interior wall:

How do you fill a crack in a drywall wall?

To fill a drywall crack so it stays hidden, open it slightly, clean out the dust, press compound into it, bed mesh tape over it, then build two or three thin coats that feather out past the tape. Sand lightly between coats, prime the patch, and paint the whole wall section. A single thick coat over an untaped crack is the most common reason the line reappears.

1. Protect the room. Drop cloth on the floor, plastic over anything below the crack.
2. Open the crack into a shallow V with the hook of a painter's multi-tool or a utility knife, so the filler has something to grip.
3. Brush out the dust. This Old House points out that loose gypsum crumbs trapped under a patch can lead to the crack returning.
4. Press compound into the crack with a 6-inch taping knife.
5. Bed fiberglass mesh tape over the crack while the compound is still wet, then trim the ends.
6. Apply thin coats over the tape, each a little wider than the last. Let each coat dry as the product label says, then sand lightly.
7. Prime, then paint. Unprimed compound soaks up paint and shows as a dull stripe.

How do you fill cracks in plaster walls in older Houston homes?

Plaster cracks need more care than drywall cracks because the plaster may be loose from the wood lath behind it. Press gently on each side of the crack first. If it feels solid, rake the crack open, dampen it lightly, and fill it with setting-type compound before taping and finishing. If the plaster flexes or sounds hollow, it needs to be reattached before any filler goes on.

The City of Houston's design guidelines for Houston Heights describe its houses as mostly wood siding on pier-and-beam foundations, with Craftsman bungalows popular from 1905 to 1925. On a raised floor like that, plaster walls crack as the floor beams sag or the piers shift, so a crack that keeps reopening above a doorway often starts in the floor.

For reattaching and patching plaster, see plastering cracks in walls.

When should you use caulk instead of wall filler?

Use caulk where the wall meets something made of a different material: baseboards, crown molding, door and window casings, and the edge of a tub or shower. Those joints open and close with heat and humidity, and a hard filler there will crack again within months. Use spackle or joint compound for cracks across the flat face of the wall.

Caulk is a flexible sealant, not a surface filler, and on a flat wall it leaves a soft line that cannot be sanded. In a bathroom it helps keep moisture out of the wall cavity, but it is not waterproofing. If water is already getting into the wall, find the leak before you fill anything. For more on the choice between products, see caulking cracks in walls.

Why does a filled wall crack keep coming back?

A crack that reopens after a careful repair is usually moving. Filler only covers the surface. If the framing, the floor or the slab underneath keeps shifting, the crack opens again along the same line, often within a season. In the Houston area, that movement is commonly the clay soil swelling and shrinking under the house as it gets wet and dries out.

The USDA describes Lake Charles clay, common across southeast Texas, as 45 to 60% clay that cracks 1/2 to 2 inches wide at the surface when dry. Walls often show that movement first, as diagonal cracks from the corners of doors and windows.

A simple test before you fill again: mark each end of the crack in pencil with the date, take a photo beside a ruler, and check it every couple of weeks. A crack that grows past the marks is active, and filling it will only hide it for a while. If doors in the same area have also started to rub, our page on doors sticking explains what that combination usually means.

Which wall cracks should you not just fill?

Do not simply fill a crack that is wider than about 1/4 inch, has one edge sitting proud of the other, runs horizontally across a wall, or keeps returning after a proper taped repair. The same goes for any crack that appears alongside sticking doors, sloping floors or gaps where the wall meets the ceiling. These are signs the house may be moving, which is a serious problem filler cannot fix.

This Old House notes that several symptoms together are more telling than one crack on its own.

Houston homes almost never have a basement, so horizontal cracks from soil pressing on a below-grade wall are rare here. When a horizontal crack shows up on a Houston interior wall, it is more often a seam in the drywall or a sign of framing movement, and worth having someone look at. Our guide on when foundation cracks are worth worrying about walks through the warning signs.

If the cause turns out to be the foundation, the repair is not DIY work. In the City of Houston, foundation repair needs a building permit, and the city's weekly permit reports list dozens of them. A foundation inspection is the usual first step before anyone prices the work.

Is it safe to sand old paint when filling wall cracks?

It can be unsafe if the house was built before 1978. The EPA says homes built before then are more likely to have lead-based paint, and it advises against dry sanding paint that may contain lead. For small areas, the EPA advises fixing damaged paint without sanding where you can, and wetting the surface first if you must sand.

Keep the repair small, wet-sand only around the crack, and clean up with a damp cloth. For a larger job, the EPA recommends a contractor trained in lead-safe work practices.