Cracks in various materials and in various locations around the house (inside and outside) are a frequent topic of conversation. Since most homes have cracks, people are tolerant of their presence until they appear to get larger, or are joined by others.
Learning to judge the seriousness of cracks from a written article will be difficult, but the comments that follow should help you to expand your knowledge.
There are a multitude of shapes that cracks take, such as narrow, wide, short, long, vertical, horizontal, diagonal, stepped, jagged, smooth, regular, irregular, shallow, deep, displaced, and combinations of the above.
This article on foundation wall cracks is the first in a series devoted to exploring their origins and processes of development, structural and cosmetic implications, and methods of treatment. Other articles to follow will discuss cracking in retaining walls, interior and exterior finishes, and paved surfaces.
Foundation wall cracks, as usually seen from inside the basement, are among the most bothersome. Variously shaped cracks of "hairline" size (up to 1/16" wide) are often seen in new homes. They usually aren't a problem now, nor a warning of future difficult problems. There are usually tow causes: one is the shrinkage of materials which are still drying out from the construction period, and the other is unequal (or differential) rates of settlement of different sections of the foundation walls. Shrinkage is manifested in short, straight cracks in both vertical and horizontal directions.
Settlement will show both the horizontal and vertical lines just mentioned, plus some step cracks which follow the joints in brick and block masonry in a diagonal direction. Settlement may also display some vertical shear cracks which cut through individual bricks and blocks rather than following the joints between individual units Shearing indicates more powerful forces are causing the action.
To further check the condition of the walls one should stand close to each one and sight along its face. What you're looking for now are signs of bending in the vertical plane, such as an archer's bow, or bending in the horizontal plane, such as a curve in a fence. (Two other names for this phenomenon are "belly" and "heave"). Cracks may not accompany this bending but if there are any, the severity of the problem is judged by their widths. Horizontal cracks are usually more serious than verticaable person if its out-of-plumbeness exceeds two inches iable person if its out-of -plumbness exceeds two inches in a vertical height of eight feet, or if the cracks are growing in size, as determined by regular and accurate measurements.
Let me briefly describe the usual development of this problem. A foundation wall is stronger in the vertical plane than in the horizontal plane because it's usually braced at both ends: at the bottom by a concrete floor slab, and at the top by a floor framing system upon which are distributed wall and roof loads. So what then could cause an apparently strong wall to yield?
Most often, the principal and direct cause of a bend and crack is the excessive weight of the earth backfill placed against the wall when (a) it was still "green" (i.e., had not reached its full mortared strength), or (b) had insufficient weight bearing down from the superstructure above because the wall was backfilled too early, or (c) because the soil became too wet (and perhaps froze) before it reached sufficient compaction; or (d) any combination of the foregoing. (In fancy words, the hydrostatic and hydrodynamic pressures and forces behind the wall have overcome some of the resistive strength of the wall!)
Vertical cracks are usually not as serious. This is because a long, frequently unsupported or unbraced wall is susceptible to normal, minor cracking. However, if the wall is approaching 30 feet in length without some thickening such as a pilaster carrying a beam, or some reinforcing such as the end of a partition bracing it, cracking and heaving could be serious.
Along with the vertical cracks are sometimes seen diagonal cracks, often at or near corners. These are usually caused by movement or settlement of the building at different time rates (differential settlement). The probable cause is the soil upon which the wall footing is resting has significant variations in its weight carrying capacity. (Cracks in floors near corners may be seen in conjunction with these wall cracks, and will be discussed separately.)
So much for our analysis of foundation wall cracks; how about their treatment?
If the cracks are truly hairline, "caulking" with a resilient material should improve their appearance. Various products are available. Wider, more complex cracks (over 1/8" wide) should be patched or repaired by "pointing" which is the replacement of mortar joints, depending upon the condition of the remaining mortar. New mortar can be bought in bags to which only water need be added.
The previous two treatments are easy, do-it-yourself actions. But if there are displacements in the blockwork with some individual units or sections having moved, or if bending is significant as previously described in this article, you should consider seeking professional advice.