Structural Cracks

What are the causes and which cracks are problems?

Types of Cracks

A. Horizontal cracks in walls

B. Vertical cracks in walls

C. 45 degree cracks

D. Cracks in ceramic tile floors

E. Cracks in plaster ceilings

A basic rule of thumb for approximately 80% of the wall cracks seen in residential structures can be broken down into three types: horizontal, vertical and 45 degree cracks.

A. Horizontal cracks are typically structurally related, however, their significance depends on whether there is lateral movement or not. If a wall has moved 1/3 of its thickness out of plumb, it is in imminent danger of falling. If it has not moved or if the movement is less than 1/3 of its thickness, the wall can probably be reinforced.

B. Vertical or nearly vertical cracks are only structurally related when there is lateral movement. This assumes the crack is consistent in width from top to bottom. In most cases, vertical cracks are relatively narrow, even those running from top to bottom and caused by contraction. Expansion and contraction are normal for building materials. The more rigid the material is, the more likely it is that a crack will develop. Porous or less dense materials will be flexible and less likely to crack.

Vertical cracks in a wall with a vertical bow or when the wall planes shear are structurally related. Contraction cracks are hardly ever a structural concern.

C. 45 degree wall cracks are almost always structurally related. The cracks must break through the material to qualify. Step cracks in a block wall which only crack at the mortar joints and not through the block are not structurally related.

When the bearing or foundation under a wall is not adequate, the wall will settle or sink at the weak area. The adjacent wall structures which are adequately supported will resist this movement. This situation will cause the 45 degree cracking.

To determine the cause or the source of a 45 degree crack, draw a line perpendicular to the crack down from the approximate center of the crack. Once you determine the source of the crack, proper corrections will be relatively easy to determine.

D. Cracks in ceramic tile floors are generally due to a few things:

1. Materials of different density expand and contract at different rates (ie. ceramic tile vs. wood)
2. Floor systems not stiff enough to properly support a ceramic tile

If the strength of the floor is adequate, using an additive in the grout would allow the ceramic system to flex with the wood floor system and would eliminate the typical differential expansion and contraction of the materials.

E. Cracks in plaster ceilings

Cracks in plaster ceilings are generally due to one of two causes: gravity and poor floor structure.

Gravity over the years will cause plaster to crack. Generally cracks develop in larger ceilings in the direction of the longer dimension at approximately 30 to 40 years of age. Cracks develop perpendicular to the first cracks when the ceiling is approximately 50 to 60 years old. Sections of the ceilings loosen at approximately 70 to 90 years and repair or replacement is then needed.

When the floor structure above a ceiling is not stiff enough to keep the plaster from cracking, premature cracks are inevitable.

Excessive weight or bouncing can cause floors to flex, and cause plaster on the ceiling below to crack at any time.