Nothing is more disconcerting to the buyer of a well maintained home, new or old, than foundation or wall cracks. Generally, these conditions provide some information about the house (or its owners!) but rarely are they evidence of the presence of a significant structural problem.
Cracks are common in masonry because masonry products such as plaster and concrete are brittle and rigid, but are supported on materials that may not be equally rigid. Plaster walls are attached to flexible wood structures, and foundations rest upon compressible soil. In addition, house components expand and contract with temperature changes and relative humidity. Wood and other construction materials shrink when they lose moisture in dry weather and expand in humid weather. If you inspect your house in the early spring, but buy in June, you may find that not all the doors close in July.
Most homes built prior to the 1920’s have stone foundations, often with stone below grade and brick above. Loose stones and cracks in mortar joints are common, particularly at exterior corners. The most common cause of corner cracks and eroded mortar is roof water. When water runs over the mortar in masonry, the mortar either spalls (due to frost) or slowly washes out. Eventually the masonry loosens and pieces may even fall out. Buyers frequently get very upset when they can pull out loose bricks from a foundation wall, but all residential brick walls on top of stone foundations that I have seen contain at least a double brick wall; if the outer bricks are loose, and the house is still standing, the weight of the house is on the inner brick wall. The outer wall is easily repaired.
Roof water from poor drainage or dispersal at the corner of a building can cause symmetrical, diagonal settling cracks in the foundation. Over a long period of time, the soil under the corner of the foundation shifts away because of the excess moisture, causing the corner to settle relative to the adjacent walls. Generally this condition is not serious, and installation of downspout elbows and splash blocks along with grading improvements will stop the corner settlement. Then the cracks can be mortared.
Starting in the 1930’s, most homes were built with poured concrete foundations.
Shrinkage cracks are normal and quite common in concrete foundation walls and floor slabs. This type of crack is either vertical or diagonal and radiates from wall openings such as windows or between floor penetrations such as beam support columns. Cracks in concrete may widen in dry weather and become narrower when it’s humid. More serious cracks will increase in size over time.
You can purchase a crack monitor (Avongard, 8002447241) and quantitatively monitor what happens over time; keeping a clear written record to show a potential buyer that changes have not occurred might also help you sell your house.
The age of a crack cannot be determined with accuracy, but what’s in a crack can give you some ideas. Often, fresh cracks are clean, whereas older cracks have dirt and insects in them. Surfaces that have been painted may give you a clue as to the age of a crack. For example, a hairline crack in a yellow wall with no yellow paint in the crack suggests a recent crack. In a freshly patched and painted wall, the appearance of cracks can give cause for concern if they contain none of the new paint. If a surface has been painted many times and there are different colors of paint in the crack, chances are the crack is old.
Sometimes, new cracks in a house can mean that some changes have occurred.
Increased street traffic, nearby construction or energetic children can cause ceiling cracks. Ceiling cracks in buildings with plaster lath ceilings are quite typical. The cracks usually form because the floor joists have sagged; building vibrations help to loosen the plaster bond to the lath as well as tightness of the nails. The pattern in the cracks often is a reflection of the ceiling structure. Long cracks appear under the strapping where joists have deflected. Shorter cracks form in the plaster between the wood lath strips, usually perpendicular to cracks under the strapping.
For the purpose of painting, plaster cracks cannot be eliminated by just applying joint compound. A thin layer of joint compound over a 3/8 inch deep hairline crack does not fill the crack; the faces of the crack in the underlying plaster are still disconnected; vibrations or movements due to changes in humidity will cause the joint compound to split and the crack to reappear.
For a permanent repair to a hairline crack in plaster on lath, count on widening the crack to about 3/8 inch back down to the lath, cleaning it out and wetting it with Elmer’s glue diluted 50% with water. If sections of plaster are disconnected from the lath, they can be secured with “plaster buttons” prior to plastering the crack. Press fresh patching plaster (not joint compound!) into the crack so that it is forced into the spaces between the lath.
When plaster lath ceilings are full of cracks, apply drywall directly over the plaster and don’t bother patching; it’s just not worth the effort. When installing drywall, be sure that the it is screwed into either the strapping or the joists, and not just the lath.
Buildings built between about 1930 and 1950 may have plaster on metal lath. Ceilings and walls with this type of plastering rarely crack, but nailing up pictures is difficult because the plaster is so hard. Sometimes you can distinguish between plaster on wood lath and on metal lath by hitting the wall: metal lath walls tend to be much stiffer. Loose plaster lath often makes a crunching noise when pushed.
If you have concerns about cracks in the home you are purchasing, don’t be afraid to consult a structural engineer if the home inspector recommends it. Structural repairs can be very costly; when purchasing, it’s better to be a safe buyer than a sorry owner.