Have you seen cracks along the walls inside your house? If you live along the Rocky Mountain Front Range, the answer is typically: yes. Most people associate drywall cracks to foundation movement but that is often incorrect. There are various reasons why those cracks develop. To best determine what is causing a drywall finished wall to crack, it is important to first understand how the house was built. A typical brand new home in the Front Range area is built with a concrete foundation that supports a wood-framed floor, walls and roof. The interior, including the walls and ceiling, are finished with drywall boards that are nailed or screwed onto the wood framing and finished with a fine layer of textured compound. For this specific method of building, the three most common reasons for drywall cracks are:
- Basement floor slab movement,
- Basement foundation wall deflections, and
- Differential vertical foundation movement
One of the most common reasons for the development of drywall cracks on a finished wall is basement floor slab movement. The majority of homes in the Front Range are built over a basement type foundation. Before the 1950s the basement was an unfinished portion of the house that was used as storage and would mainly house the furnace equipment. Today, the basement has evolved to become a fully finished part of the home. The floor of a basement almost always consists of a “floating” concrete slab, which is an independent unit of concrete that simply rests over the ground; unattached from the house. It is labeled “floating” because it is expected to experience some upward/downward movement. The expected movement is due to seasonal soil moisture fluctuations in the region that occur naturally throughout the year. During wet seasons (spring and summer), a slab that rests on clay material could move up as the clay becomes moist and swells.
As the season shifts to a dry condition in the winter, the slab can experience downward movement as the clay dries and shrinks. To deal with slab movement, modern construction has incorporated the “voided” wall system, more commonly known as floating walls. This system requires the basement partition walls to be hung from the main level floor framing or basement ceiling, with a 1 ½” to 3” separation between the slab surface and the bottom of the walls. This separation allows the slab to move up and down without affecting the partition walls. The separation is concealed from finished space when the trim is installed along the bottom of the walls, but can sometimes be observed from an unfinished utility room. This framing technique requires the drywall boards to be cut flush with the bottom of the hung wall. In many cases, this is missed and the drywall is installed the full-height of the wall and extends down to the sill plate or slab. When this occurs, upward slab movement is transmitted into the drywall, generating compression forces that result in vertical or directional cracks on walls, especially at the top of openings. Additionally, if door jambs and vertical trim around openings are not installed with a separation from the slab, these can also be pushed up and cause the door frames to become out-of-square. When this occurs, diagonal cracks tend to develop at the top corners of the door openings.
Another common reason for drywall cracks to develop is basement wall deflections. The perimeter basement walls are foundation walls that support the house structure and also hold the soil back from the surrounding excavation. When the pressure behind the basement walls exceeds the anticipated soil pressures used for design, the walls can develop inward bows or inclinations that will damage the drywall finish at the interior. Some of the reasons why the anticipated soil pressure behind the basement walls increases are excess moisture and presence of clay material. Excessive moisture can occur when water-demanding vegetation is placed too close to the foundation. Watering the vegetation will introduce excessive moisture to the subsurface soils surrounding the foundation and, as it collects within the foundation perimeter, the moisture will create an additional hydrostatic pressure behind the wall. Poor drainage can also allow water to collect around the foundation and create the same problem. If the moisture collects along the entire height of the basement wall, the hydrostatic pressure could be up to 20,000 pounds for a 10 ft. wall segment of an 8 ft. tall basement. This pressure will ultimately result in basement wall deflections. Basement wall deflections can also occur when the soil behind the basement walls consists of clay material. If the soil moisture content of the clay material is significantly increased, the clays will swell and deflect the basement walls inward.
In some less frequent cases, drywall cracks can be associated with differential vertical foundation movement. This type of foundation movement is the most damaging of the three. The term “vertical” refers to upwards/downwards movement, while “differential” indicates that the movement varies at different locations throughout the foundation footprint, instead of uniformly. When this type of foundation movement occurs, diagonal cracks on drywall finished walls tend to form at the top and bottom of wall openings. Additionally, the floors become out-of-level, and in some cases, the perimeter house walls develop inclinations. This type of foundation movement, if significant, can also lead to diagonal cracks at the top and bottom of wall openings at the exterior of the house. This movement occurs when the soils underneath the foundation experience volumetric changes. In the Rocky Mountain Front Range, the soils are predominantly clay and significant foundation movement is typically a result of significant moisture changes within the clay material.
Although some reasons that lead to drywall cracks have been presented here, there are other factors that can contribute to the development of cracks. Drywall cracks should always be accurately diagnosed by an experienced professional, since an inaccurate evaluation could result in very expensive and unnecessary repairs. When drywall cracks develop, the wisest decision a homeowner can make is to hire a professional structural engineer to perform a structural evaluation. RMG has over 30 years of experience performing structural evaluations along the Front Range and has saved thousands of dollars for homeowners in many cases.