US Inspect US Inspect

Your home’s foundation cannot be overlooked. It consists of the soil, gravel and a concrete slab. But there is more than meets the eye. Close attention needs to be paid to your foundation over the years to ensure there are no signs of shifting or cracking. In some cases, these issues can arise from poor building materials, improper installation or setup of the concrete. Get to know more elements of your home’s foundation.

Inspection

The three common foundation types found in the United States are basements, crawlspaces and slab-on grade.

Basement: an area below the first floor with a minimum height of 6 foot 8 inches. Basement foundations have foundation walls placed on footings.

Crawlspace: a shallow and uninhabitable area, between the soil and the first floor of the home. A crawlspace usually extends below the frost line or to a stable substrate. Crawlspaces are generally constructed with foundation walls and footings, however, piers may be used when the crawlspace is above grade.

Slab-on-grade: a concrete floor (slab) that is poured directly at grade (ground level) and acts as the first floor sub-surface. The slab is usually supported by a continuous (spread) footing, piers or piles and grade beams

Back To Top

Walls

Foundation walls are those vertical components that extend below grade and rest on a footing. The foundation wall must be able to transfer the weight (load) of the exterior walls above grade downward to the footing as well as withstand the lateral forces applied by the exterior soil. With modern construction, foundation walls are usually 8 to 10-inch-thick, poured reinforced concrete or 8 to 12-inch-wide concrete masonry units (CMUs or concrete blocks). The thickness of the wall is determined by the weight (vertical load), depth below grade (lateral load) and the material used. Other materials used include brick, stone, cinder block, clay tiles and wood. Each has its own advantages and disadvantages.

Back To Top

Cracking

Is Your Home Cracking Up?
Most of us have seen cracks in our homes.  Some of us may have even seen cracks appearing on the foundation of the home.  But do you know how to tell if a crack is structurally deficient?

There are many variations of cracks, but there are three primary cracks that are commonly found--vertical cracks, horizontal cracks and diagonal crack.  The following overview of the basic types of cracks should provide you with the information you need to help determine if the cracking you see is structurally related.

Vertical Cracks
Vertical cracking is not structurally related or deficient unless there is lateral movement, displacement, bowing, or if the crack is uneven from top to bottom.  If the plane of the wall is the same on both sides of the crack, then it is a contraction crack, and not a structural issue.

Since everything expands and contracts, it is common in commercial work to provide control joints in areas where expansion and contraction is expected.  However, control joints are not typically installed in residential construction.  Ridged materials, such as poured-in-place concrete, tend to crack from expansion and contraction much easier than more flexible materials.

Horizontal Cracks
Horizontal cracking in block construction is usually structurally related because the bond between the block is broken.  However, this does not mean that the wall will collapse.  The amount of movement and the cause of movement are important issues to consider.  Frost, backfilling, or a significant unbalanced load may cause cracking.

A frost line crack is not likely to have serious repercussions unless conditions, such as negative grading, are not corrected.  Cracking due to poor backfilling practices, or heavy equipment close to the walls, is usually a one-time occurrence.  Movement caused by an excessive unbalanced load presents the most serious situation.  A significant unbalanced load may be destined for collapse.

Diagonal Cracks
Diagonal cracking is almost always structurally related.  Diagonal cracking is defined as a crack that tears through the material, not a step crack that follows the mortar joints.  If you are looking at a foundation wall with diagonal cracking, you should be concerned, however, determining the source is fairly easy.

To determine the source or cause of a diagonal crack, draw an imaginary line perpendicular to the center of the crack, downward towards the ground.  There you should find the source of the cracking.

Being able to determine the cause or source of a crack can serve as a guide to the action or solution needed.  If a design solution may be required, a structural engineer should be consulted.

Back To Top

Columns and Posts

Piers are columns that are designed to support a specific point of contact on a support beam or girder.  In modern construction, they are usually made with poured concrete or concrete blocks (CMU) and supported by a pad footing

Piers are also constructed with stone, brick and wood.

Piers (CMUs) are commonly used in  crawlspaces as a supplementary support to prevent the over-spanning of beams and girders.  Concrete piers are commonly used to support porches and decks. Also, concrete piers are used as an inexpensive primary support system for small additions to the home, in place of a continuous foundation wall and footing.

Back To Top

Footings

Spread Footings
Spread footings (or footer) provide a stable base or platform that prevents the house from settling into the ground.

The wide base (width) helps create a large area to transfer the weight of the structure to the ground and prevent the structure from sinking.  The thickness of the footer provides the footer with the strength needed to support the weight of the structure. In modern construction, a footer is usually 16 to 24 inches wide and 6 to 16 inches thick and made with poured concrete that is rated to withstand 2,000 to 5,000 pounds per square inch (psi) of compression pressure. The dimensions of the footer may vary according to the soil conditions under the structure, the weight (or load) placed on the footing and construction style of the home. Other footing materials used are wood, crushed stone, blocks (granite) and field stones. 

A continuous spread (or strip) footing is usually found around the entire perimeter of the structure to support the weight (load) from the exterior or foundation walls. In areas subject to seasonal frost, a footing must be placed below the frost line to prevent frost heaving that may lift and damage the footing and structure.
 
Pad Footing
A pad footing is like a spread footing but it is usually used to support a single point of contact, such as under a pier or post.  In modern construction, a pad footing is usually a 2-foot by 2-foot square pad, 10 to 12 inches thick and is made with poured concrete that is rated to withstand 3,000 to 5,000 pounds per square inch (psi) of compression pressure.

Alternatives to Footings
Piles are wood, concrete or sometimes metal columns that are driven into the ground, used to support the structure and prevent it from sinking into the ground. Piles are either driven down until they rest on a solid substrate, such as bedrock, or to a depth where the soil friction against the side of the pile is sufficient to prevent any further downward movement. A continuous grade beam is placed across the top of the piles, forming the platform on which the structure is constructed. Piles are used in areas where footings are not feasible or desirable, such as with poor soil quality or a high water table near a beach. In general, piles are more expensive to install than spread footings.

Back To Top

Structural Inspection

How can you be certain that a crack is structurally deficient?
There are generally three types of cracks—vertical, horizontal and diagonal. However, this forum does not allow for every variation to be discussed. If you understand the three basic crack patterns outlined below, you will have a reasonable understanding of approximately 85% of all of the cracks that you will see. Let’s cover them one at a time.

Vertical Cracking
Vertical cracking is not structurally related or deficient unless there is lateral movement, displacement, bowing, or if the crack is uneven from top to bottom. If the plane of the wall is the same on both sides of the crack, it is typically a contraction crack and not a structural issue.

Since everything expands and contracts, it is common in commercial work to provide control joints in areas where expansion and contraction is expected. However, control joints are not typically installed in residential construction. Ridged materials, such as poured-in-place concrete, tend to crack from expansion and contraction much easier than more flexible materials.

If all loads were evenly distributed on this type of wall, were a consistent thickness and were the same height for the entire length, the vertical cracks would most likely occur at or close to the center of the wall.

If there is a window or door in the wall, the crack will occur at this opening, as a window or door creates a void or weakness in the wall. The crack would typically occur in the center of the wall if there were no such opening. The vertical cracking in this situation may be 5–10 degrees out of plumb (vertical), however, it is still considered a vertical crack. Such cracks are most common in poured-in-place concrete and concrete block construction.

The rule of thumb is to expect vertical cracking if a wall made of ridged material is 32' or longer and forms a straight line without pilasters, jogs or other structures, such as a masonry chimney. First look in the center of the wall, unless there is a window. The crack will almost always occur at the window in this scenario.

Horizontal Cracking
Horizontal cracking in block construction is structurally related because the bond between the block is broken, however, this does not mean that the wall will collapse. The amount of movement and the cause of movement are important issues to consider. Frost, backfilling, or a significant unbalanced load may cause cracking. 

While no cracking is desirable, it is important that you understand the differences. A frost line crack is not likely to have serious repercussions unless the conditions, such as negative grades, are not corrected.

Cracking due to poor backfilling practices, or heavy equipment close to the walls, is usually a one-time occurrence. Movement caused by an excessive unbalanced load presents the most serious situation, and failure is progressive. A significant unbalanced load may be destined for collapse. Example: A 4' unbalanced load against an 8'' block wall is the current limit. It was not unusual to build homes with 8'' block and 6' or 7' of unbalanced load in the 1940s and 1950s in many areas of the country.

The engineering community suggests a rule of thumb that if a wall has moved 1/3 of its thickness, then the wall is considered to be in imminent danger of collapse. This may also suggest that if there is only 1/2'' of movement, it is not likely that the wall will collapse.

Diagonal Cracking
Diagonal cracking is almost always structurally related. Diagonal cracking is defined in this example as a crack that tears through the material, not a step crack that follows the mortar joints. To understand the source or cause of a diagonal crack, draw an imaginary line perpendicular to the center of the crack, down. This should point to the source or reason for the cracking.

Example: Assume that a frame floor system is carrying enough weight to cause it to sag, and the subject wall is parallel to the joist system. The sag would typically be farthest from the bearing points (at mid-span), where there is likely to be diagonal cracking. Draw an imaginary line perpendicular to the crack, from the center of the crack, downwards toward the floor. This should point to mid-span of the floor joist.

In the above example, the cracking would typically occur 1'–4' from the corner of the settled wall, because the wall that is perpendicular to the settled wall is likely to be supported differently and resist movement. A wall that is on a foundation, beam or is perpendicular to the floor joists, will be supported better than a wall that is parallel to the floor joists. 

You are looking at a foundation wall with diagonal cracking, you should be concerned, however, determining the source is fairly easy—again, draw an imaginary line perpendicular to the diagonal crack and follow to the floor. This does not mean straight down to the floor, or 90 degrees to the floor. It means that you follow the line on the angle that is perpendicular to the crack. 

Being able to determine the cause or the source of the cracking problem will be a guide to the action or solution needed. By understanding a number of different crack patterns and their significance, causes and sources, the solutions are relatively straightforward.

If a design solution may be required, a structural engineer should be consulted for an opinion, options and an recommended solution.  Learn more about the structure of a home.

Back To Top