Fix cracks in plaster walls and ceilings

How lath and plaster holds on, and why it cracks

In houses built before drywall became common, plasterers nailed thin strips of wood lath across the studs and ceiling joists with small gaps between them. They pushed the first coat of plaster hard enough that some squeezed through the gaps and curled over the back of the lath. Those curls, called keys, harden and hold the plaster on the wall. One or two more coats go on top, ending with a thin, hard finish coat. A sound plaster wall feels solid and doesn't move when you press it.

Plaster is strong but not flexible. Wood framing and lath shrink and swell with the seasons, houses settle, doors slam and people walk on the floor above a ceiling. That movement cracks the plaster along its weakest lines, often from the corners of doors and windows. Over time, keys can also break, so a section of plaster loses its grip and starts to sag or sound hollow. That's why the repair has two parts: reattach anything loose, then fill and bridge the crack with something that bonds well and resists cracking again. Drywall methods alone often fail here, because plaster is thicker, harder and more absorbent than drywall paper.

Why plaster walls crack

Let's fix it

1. Diagnose the crackLook first, then press. Lay a flat hand next to the crack and push gently, working out about a foot on each side. Solid plaster doesn't move. If it flexes, crunches or you see the crack open and close, the plaster is loose; go to Step 3 before patching. Tap lightly with a knuckle; a dull, hollow sound often means loose plaster. Then check the shape. A thin crack that wanders is usually normal movement. A diagonal or stair-step crack from a door or window corner that grows, or doors that start sticking, can point to a structural issue. Mark the ends with pencil and the date. If it keeps getting longer or wider over a few weeks, stop and have a contractor or structural engineer look before you patch.
   Done

   Something went wrong?

   • The crack is wider than about 1/4 inch Wide cracks often come with loose plaster or movement. Do the press test carefully, and watch it for a few weeks before patching.

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2. Test for lead and protect the roomIf your home was built before 1978, test the paint around the crack before you scrape or sand. Hardware stores sell EPA-recognized lead test kits; follow the kit's directions, or hire a certified inspector. If the test is positive or you skip testing, treat it as lead. Lay plastic sheeting on the floor below and tape it to the baseboard. Close nearby vents. Mist the area with water before scraping so dust doesn't fly, and wear an N95 or better mask. Clean up with damp paper towels and a HEPA vacuum, not a dry sweep. Keep kids and pets out of the room until it's clean. Landlords and paid contractors working on pre-1978 homes generally must follow the EPA lead-safe (RRP) rules.
   Done

   Something went wrong?

   • The kit result is hard to read Treat the paint as lead and work wet, or have a certified inspector test it. Don't dry-sand to find out.

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3. Reattach loose plasterSkip this step if the plaster passed the press test. For a loose area, use plaster washers: thin, perforated metal discs about 1 inch across. Drill a pilot hole through the plaster about 2 to 3 inches from the crack, with a masonry bit sized for the screw. In a pre-1978 home, mist the spot first and hold a HEPA vacuum nozzle by the bit. Drive a 1-1/4 to 1-5/8 inch drywall screw through a washer into the lath or a stud. Tighten slowly until the plaster pulls snug, then stop; overtightening cracks it. Space washers about 4 to 6 inches apart around the loose zone. Some pros inject a plaster adhesive behind the area first for a stronger hold. If a large ceiling area sags, the plaster crumbles, or it moves more than a little, stop and call a plaster pro.
   Done

   Something went wrong?

   • The screw spins and won't grab You may be between lath strips. Move the washer up or down about 1/2 inch, or angle into the next strip.
   • The plaster cracks around the washer Back the screw off a turn. You can cover the washers with compound in the next steps.

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4. Open the crack into a VFill sticks better in a crack that's slightly wider at the surface than a hairline. Mist the crack first if you're working lead-safe. Drag the pointed tip of a can opener, a grout saw or a utility knife along the crack to make a shallow V, about 1/8 to 1/4 inch wide at the top. Angle the blade so the edges undercut a little; that helps lock the fill in. Remove any loose or crumbly bits, but don't dig into solid plaster. Brush or vacuum the dust out. Old plaster soaks up water fast and can pull moisture out of fresh compound. So dampen the groove with a sponge or brush on a bonding agent, following its label, right before filling.
   Done

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5. Fill and tape the crackUse setting-type joint compound, the powder you mix with water. It hardens by chemical reaction, shrinks less than premixed compound and holds up better in deep plaster cracks. A 20 or 45 minute product gives you time to work; mix only what you can use before it sets. Press it firmly into the V with a 4 or 6 inch putty knife and scrape it flush. For a crack that has come back before, add self-adhesive fiberglass mesh tape. Press the tape centered over the filled crack, then skim a thin coat of setting compound through it. Just cover the mesh; thick coats crack and mean more sanding. Let it harden fully before the next coat, as the bag directs.
   Done

   Something went wrong?

   • The compound hardens in the pan Toss it and mix a smaller batch. Adding water to setting compound that's already firming up doesn't save it.
   • The tape edges show as ridges Let it harden, scrape the ridge off with a knife, and cover it with a wider, thinner coat.

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6. Feather, sand and paintApply one or two more thin coats, each a few inches wider than the last, with a 10 or 12 inch knife. A wider, thinner patch blends in better than a narrow hump. You can use lightweight premixed compound for these top coats since it sands easier. Let each coat dry fully. Sand lightly with a fine sanding sponge, or wet-sand with a damp sponge to keep dust down. Wet-sand if lead may be present. Check the patch with a flashlight held flat to the wall; shadows show any ridges. Wipe off the dust, prime the patch, and paint. Primer seals the compound so the paint doesn't look dull over it. On older plaster, test any new paint in a small spot for adhesion.
   Done

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Watch it done

How to Repair Cracks in Plaster Walls | Ask This Old HouseBy This Old House on YouTube. Their method may vary a little from ours.

Watch on YouTube

If it comes back

A hairline that shows up again along a patch edge usually means the patch was narrow or skipped the mesh. Reopen it, tape it, and feather it wider. Doors that slam or a busy floor above can keep stressing the same spot.

If the crack grows longer, gets wider at one end, or comes with sticking doors and windows, the house may be moving. A contractor or structural engineer can tell you whether it's seasonal or something to fix. Plaster around a hole has its own fix; see how to patch a hole in a plaster wall.

Not sure your wall is plaster? Drywall cracks get a different repair; see how to fix a drywall crack. Hanging things on a repaired wall? See how to hang things on plaster.