Issue 57 January 12, 2002

Reading and Understanding Foundation Wall Cracks

Have you ever wished you could understand the cracks you observe in foundation walls? 
Have you ever wondered why some cracks the inspector almost ignores and others cause alarm. 

Believe it or not cracks have a language all their own and it is fairly easy to learn. I am not a structural engineer, but from years of crack observation and a few classes I think I have an understanding and will make an attempt, in layman's terms, to pass the language of foundation wall cracks on to you. First let it be clear that we are talking about building foundation walls not free standing retaining walls. As a practical matter, almost any crack in a free standing retaining wall should cause alarm. A foundation wall is a wall (hopefully) resting on a footing which has a floor or roof system resting on top of it. This wall is supported at the bottom and top by the footing and floor system where a free standing retaining wall must be able to support itself and all of the loads acting upon it. This language specifically relates to masonry walls, although concrete wall cracks are very similar. 

Here we go:

Any crack in a foundation wall should be read to determine what caused the crack and if it is dynamic (moving). 

The issues you need to observe are:

1. The size of the crack
2. The pattern of the crack
3. The number of cracks
4. The relation of cracks to each other.
5. The dynamics of the crack (is it moving)
6. The age of the crack.

Size: As a general rule small hair line cracks say less than 1/8" are not of concern unless they are dynamic (moving). Most of the smaller cracks you will observe, irrelevant of the pattern, are typical and are not cause for concern, but they should be monitored. If they move or grow larger then they are dynamic and need to be checked out by a structural engineer. These cracks are caused by various issues including curing, shrinking of the material, movement, soil pressure or settlement. For example: block which are stored unprotected on site before they are installed and become wet may shrink in the wall usually causing vertical hairline cracks. Cracks from 1/8" to 1/4" are of more concern but if they are not dynamic may not need repair. 1/4" or over should be inspected by a structural engineer dynamic or not. If a cracked has opened up 1/4" or more this is indicative of possible major failure at the footing. A crack with inconsistent width (one end wider than the other) is cause for concern. If a vertical crack is opening up in a V being wider at either the top or the bottom this is a sign of a major settlement problem. 

Pattern: As a general rule, vertical is of the least concern and horizontal the most. A stair stepping crack is of a little less concern than a straight slopped crack. A crack other than vertical which breaks across the block rather than at the joint is cause for concern. 

A dynamic horizontal crack below grade is the crack which should start red lights flashing and flags waving. This type of crack is caused by soil and hydrostatic (water) pressure pushing against the wall and will lead to ultimate collapse. If you see this type of crack and the inspector starts running toward the basement door, I suggest you leave with him as quickly as possible. I have been present on several occasion during or just after walls fell (blew out) from this type of crack. It is not a pretty site and you don't want to be nearby when it occurs. 

A single crack radiating from the top corner of an opening which extends up and away from the opening and does not appear to be related to any lower crack extending to the ground may not be of much concern. However a crack from an opening extending in any other direction and especially one or more which extend down toward the ground should be investigated further. 

Number & Relation: Settlement cracks will bracket (be on each side of) the problem and show up in pairs. They are often around the corner from each other. They may be slopped or stair stepping but they are seldom vertical or only horizontal unless there is a major problem. They will slope up and toward the problem. It should go without saying that the more cracks you have, the more concern there should be. If you observe a pattern of vertical hair line cracks in the block four foot apart all around the building don't be very alarmed. They are most likely shrinkage cracks and not cause for concern. If you observe horizontal, sloping or stair stepping cracks one on top of another call for help no matter what the size. 

Dynamics: If its moving in any direction, you need help. If the movement is differential, moving out instead of apart, be alarmed. If both are occurring, out and apart, be even more alarmed. 

Age: The younger the house the more concerned you should be. You may be witnessing a small thing which will become much worse. The older the house the less concerned you should be. If that cracked just showed up a few days ago, be alarmed. If the crack has been present for 53 years and has not moved in the last 45 years, you shouldn't be very alarmed. What you have now is most likely what will be present for a long time.

 
Thought for the week
 
Remember, amateurs built the ark.
Professionals built the Titanic.