Repairing hairline cracks in drywall, plaster, concrete, brick
February 24, 2022
It’s a good time to fix cracks on the inside walls of your house. Fortunately, you can repair a cracked wall in a weekend. Here is our basic guide to fixing small cracks in drywall, plaster, concrete, and brick. You can do it!
Cracks on walls can be a charming irregularity that adds character or an eyesore that really bothers you or a sign to a potential seller that something is wrong with your house even if this is untrue. Generally, small or hairline cracks are simply caused by the natural settling of a house over time. They’re easy to patch up. Simple tools and materials from your local home improvement store will have you on your way to a smooth wall.
Note: If you find the cracking or surface distortion is severe, this may be a cause of structural problems or even signs of possible collapse. If you are concerned, have your home inspected by a qualified building professional before repairing the drywall.
Minor cracks in drywall or plaster
To fix minor cracks, you’ll need to buy: patching plaster or drywall compound.
Patching plaster comes as a dry powder and has to be mixed with water to the consistency of soft-serve ice cream.
Drywall compound is best purchased premixed in several sizes. The standard five-gallon plastic bucket of drywall compound (or “mud,” as it’s called in the trade) contains much more than you may need and won’t keep for more than a few months, but is inexpensive and will give you plenty of material to practice with.
Before you can patch you’ll need to widen narrow cracks enough so that the patching material can fill the crack, rather than simply covering it and leaving an unsightly bulge.
You can use the corner of a putty knife or any other improvised tool, but one of the best is an old-fashioned “church key” of sort once used for opening cans of beer. Just drag the point along small cracks to make them into narrow, V-shaped grooves.
Brush the dust out, and mist plaster (but not drywall) cracks with water to prevent the dry plaster from sucking all of the moisture from the patching material and causing it to become too stiff to work.
Use a broad-bladed putty knife to fill the prepared cracks with patching material or joint compound.
Non-expanding cracks in poured concrete
Use a hammer and cold chisel (a chisel with a wedge-sharpened point, called a cape chisel, works best) to widen the crack enough to admit the patching material.
Undercut the edges to provide a good bonding surface. Several types of patching materials are available. To save money, mix up a batch of portland cement mortar, which comes in 50-pound bags, and press it into the prepared cracks with a steel trowel.
Don’t use your hands; cement is extremely caustic, and even relatively brief exposure can cause severe skin burns.
Cracks in brick or concrete block
These can be repaired in the same way as cracks in poured concrete, although you should be careful not to chisel deeper than 3/4-inch or so into a block, or you risk punching the chisel into the block’s hollow core.
Chisel out the cracked joints to a depth of about 1/2 inch, and fill with portland cement mortar.
Once the mortar has set for two or three hours, fashion smooth concave joints by running a jointing tool (a short length of 1/2-inch copper tubing bent into a gentle S shape works well) over the partially set mortar.
How to tell if a crack in concrete is growing
Find a scrap of thin glass and glue it across the crack with five-minute epoxy or super glue. (If the masonry is damp, dry the area you’ll be gluing with a flame from a propane torch first.)
At the same time, mark a pencil line perpendicular to the crack at that point, and measure its width to the nearest 64th of an inch.
Wait a few months and have another look. If the glass has broken, you’ll know that the crack has opened further, and by taking another measurement you’ll know by how much it has widened.