How to Repair Cracks in Plaster Walls
Types of Plaster Cracks and What to Do About Them
Homeowners and homebuyers often value homes with plaster walls in prime condition. They are beautiful, solid, and especially good for soundproofing. However, it is normal for plaster walls to respond to inevitable foundation shifts and climatic changes by forming cracks. Everyday life, with its bumps and bangs, also takes its toll on plaster walls. Nearly all homes with plaster walls will eventually develop window and door cracks.
Though they may look ominous, your cracked plaster walls are not beyond repair. To quickly fix a plaster wall with spider or narrow cracks, you'll need some drywall tape, joint compound to fill the gaps, and sandpaper to get it looking even again. You do not even need a special plaster repair kit. Here's what you need to know about the causes of cracks, when to worry about them, and the instructions for repairing them.
What Causes Cracks in Plaster Walls?
Plaster cracks are common and to be expected, whether your home is five years old or 100 years old. Hairline and minor cracks are common as a home settles in its first 10 years. Drastic weather changes or homes without climate control that experience extreme temperatures are more prone to plaster cracks. Most cracks are cosmetic and can be easily fixed. However, large cracks and breaking plaster can be signs of serious issues to worry about, such as water damage or structural problems. These are common causes of plaster cracks:
- House settling on its foundation
- Climate control and weather fluctuations
- Water damage and leaks (stains and bubbling)
- Poor installation or dried-out plaster
- Structural damage (termites, foundation issues)
Types of Plaster Cracks
The way plaster cracks can tell you several things: the potential cause and the severity of the issue. In many cases, the smaller it is, the smaller the issue. However, a crack is not just a crack and becomes a damaged wall when the gap is 1 inch or larger.
A damaged wall with a large gap can signify subsidence. Subsidence occurs when the house and its foundation begin to sink because the ground beneath it shifts and sinks. This major problem requires foundation work, likely underpinning, to ensure the home remains stable and does not collapse.
- Hairline or spiderweb cracks: These cracks are benign; they are so small that you can hardly fit a fingernail in them. There is no need to bother with them unless you want to clean them up cosmetically. Hairline cracks are usually caused by plaster drying, expanding, and contracting.
- Bulging: Bulges in plaster create small cracks emanating from them. Bulging is usually caused by a simple problem like the outer layer separated from the inner layer of plaster, called delamination. The outer layer may only need removal and replastering.
- Bubbling or discoloration of plaster: If you notice cracks that include bubbling, wetness, discoloration, or water stains, you likely have water seeping into the wall. It could be from outside water getting in or a pipe in the walls that has sprung a leak. This is a sign of a bigger problem that should be remedied immediately.
- Diagonal, horizontal, or vertical cracks: These streak-like cracks are structural cracks caused by foundation movement, moisture expansion, or plaster drying and shrinking. They are most common in the first 10 years of plaster installation.
- Wide cracks: A crack of 1/4 inch to 1/2 inch in width is problematic; it's a sign of something going on that is more serious than a house settling or climate control issues.
Plaster vs. Drywall Repairs
If you have ever repaired drywall, you may know that it is often easier to replace entire portions with new pieces of drywall—it's possible to remove just the section that needs fixing and a few inches beyond without the whole wall collapsing. Because drywall is one layer with no backing, once you cut through drywall, there is nothing behind it except for studs and insulation.
Plaster walls, by contrast, are constructed of two layers: the outer plaster and the inner wooden or metal lath. With plaster, your best bet is to preserve and fix the existing plaster, rather than tearing it out. Ripping out chunks of plaster often becomes a seemingly endless process, with one chunk leading to another chunk. Successful plaster crack repair requires taking it slow and being patient enough to add multiple layers of drywall joint compound to the wall.
What You'll Need
Equipment / Tools
- Utility knife or 5-in-1 tool
- Shop vacuum
- 6-inch drywall knife
- 12-inch drywall knife (optional)
- Drywall joint compound
- Paper drywall tape
- 150-grit sanding sponge
Score the Crack
Use a utility knife or painter's 5-in-1 tool to cut along the edges of the crack, opening up the crack slightly and removing loose material. While opening the crack may seem counterintuitive, you need to increase the area for the joint compound to stick. Do not vigorously scrape into the crack; be gentle and go slow. Use a shop vacuum to remove all crumbs and dust from the crack.
Cut the Paper Drywall Tape
Measure and cut one or more lengths of paper drywall tape to fit the crack. Drywall compound begins to dry quickly, so cutting the tape in advance makes this work go faster and prevents accidentally creating creases, folds, or bumps. Use scissors or a utility knife to cut the tape, as tearing the tape by hand will produce a ragged edge.
Spread the Joint Compound
Mix the drywall joint compound, as needed. Scoop up a small portion with a 6-inch drywall knife, and smooth a thin layer of joint compound over the crack, making a path that's a little wider than the paper tape. Move immediately to the next step, as the joint compound dries rapidly.
Apply Tape to the Crack
Lay the tape onto the mud by hand, then smooth it with the 6-inch knife with one or two passes. The goal is to flatten the tape and ensure full contact with the compound, with no wrinkles or air pockets. Be careful not to overwork the tape and tear it. Let the joint compound dry completely.
Mud Over the Tape
Add a layer of joint compound over the taped area so that the compound extends past the tape's edges, using the 6-inch knife. Smooth the compound so it is flush with the surrounding surfaces. Let the joint compound dry completely.
Sand the Repair
Lightly sand the compound with a 150-grit sanding sponge or sandpaper to smooth prominent bumps or ridges. Do not sand so hard that you expose the tape.
Apply an Additional Mud Coat (optional)
Add a third layer of joint compound, this time using the 12-inch knife, to extend the edges even farther. This is an optional step that helps blend the repair into a very smooth wall surface. If the surface has some texture, it's usually better to take the opposite approach and minimize the width of the repair, since it's hard to texture the new compound.
An additional coat should take the joint compound out as far as 12 inches. Allow the joint compound to dry completely, then sand lightly to smooth out the repair area. Again, take care not to sand down into the paper tape—a very light sanding is all that is required.
Clean the Area
Clean the patch and surrounding area with a shop vacuum and wipe it with a dry cloth.
Why are my walls suddenly cracking?
If you notice sudden cracks all over the home, it can signify that the foundation is gently shifting. If the cracks are minor, this shifting is normal and expected to occur. However, larger cracks (1/4 inch or wider) can signify a major structural issue.
How can you tell if a crack is structural?
Minor or hairline cracks are usually not a sign of structural issues. Symptoms of structural problems include stair-step cracks (cracks that look like a staircase) or big cracks with gaps more than 1/4-inch wide.
What are the first signs of subsidence?
The most significant signs of subsidence are large cracks (more than 1/4-inch wide) radiating from windows, doorways, and corners. Also, windows and doors become harder to open and close as the frames become unaligned.