Article 2: Types of Cracks – Part 1

In previous issue, Article 1: Finding and Monitoring Foundation Cracks, the early signs of foundation problems were discussed. Some basic inspection methods were also discussed that would help home owners to identify potential problem areas. Knowing some basic information about the type and location of cracks in the foundation can help homeowners determine if there is a serious problem and when they should call in a specialist.

In this article, the different types of cracks, as well as the likely cause and repair method, will be discussed.

Cracks in the basement walls can indicate a potential problem with the foundation of a home. The masonry material and concrete that are used in the construction of foundation walls are strong in compression but not as strong in tension. Therefore, any kind of movement or force that creates tension stresses in the foundation walls can cause cracks.

This type of crack usually occurs in the basement on the mortar joints between concrete blocks, on concrete walls and floors, and also on drywall with either steel or wood studs. The cause of these cracks is usually the movement caused by the expansion and contraction of the construction materials. This type of crack is common as this slight movement is normal and is not related to the quality of the construction of the building.

These cracks are not usually a concern and are easy to repair. In order to repair hairline cracks on drywall, speckling paste or drywall mud that is available at any home hardware store can be used. Repairing cracks on mortar joints and concrete surfaces is slightly different and will be discussed further below.

This type of crack forms where the foundation wall meets the grade level. The most likely cause of this type of crack is concrete shrinkage. The telltale sign is that the crack follows an irregular path and narrows to a hairline crack.

Concrete is comprised of water, cement and aggregate. As the concrete cures, it will shrink and the curing process is highly sensitive to temperature. For example, a freezing cold night at the time the foundation concrete is poured will make the concrete cure too slowly and conversely, a hot and humid sunny day will make the concrete cure too quickly.  If the curing process is interrupted by a sudden change in temperature cracks are likely to form.

These cracks are not usually a sign of problems in the basement and foundation walls. However, it’s highly recommended that these cracks be repaired in order to prevent moisture and bugs from getting into the house. In order to repair and seal the crack, first remove any excess cement or mortar by lightly chipping it away using a screwdriver. Second, clean the crack with a vacuum or brush. Then fill the crack with concrete repair caulk, smoothing for a uniform finish.

The main reason for the formation of the stair-step cracks is uneven settlement of the house footing. Most stair-step cracks are caused by the normal settlement of a building over time. However, uneven settlement can be the result of something like moisture behind the foundation walls. For example, a plugged gutter could cause water to be directed to the area around the foundation walls rather than away from the house and that water will exert hydrostatic pressure on the foundation walls.

Stair-step cracks have diagonal zig zag pattern which follows the mortar joints between foundation concrete blocks. If moisture behind the foundation walls is the cause, the crack path usually follows the line where the moisture is present. If the crack is wider than ¼ inch or the basement wall is bulging, immediate attention is required.

Before attempting to repair these cracks, the propagation (spread) of the cracks needs to be stopped. In order to confirm that the crack is active, monitoring the crack is suggested. Stair-step cracks due to normal settlement will usually reach a steady state (no further propagation) within a short period of time; monitoring the cracks  for 2 to 4 months should ensure the cracks are no longer active. Since these cracks do not pose a serious risk to the structural stability of the house, there is time available to perform the monitoring.

In order to monitor a crack, mark both ends of the crack with pen and measure the width. This information and the time and date of the marking and measurement needs to be recorded. Repeat the process every month for 2 to 4 months. This record will help to identify the progress of the crack if it is active or if it is at a steady state.