There are several factors in Tacoma and the western portion of Washington state that lead to regular foundation issues.
Much of Washington is known for its regular rainfall. While Tacoma only gets slightly more precipitation in inches than the countrywide average, the city experiences some kind of rainfall over 150 days of the year. This means that the soil rarely has time to dry before the rain begins to fall again.
Water from runoff is the leading underlying cause of foundation damage for two reasons. First, any rain that isn’t redirected by your gutters and downspouts makes the soil around your home heavy as it soaks into the ground. The dirt puts additional pressure on your concrete foundation walls and can eventually cause them to bow inward and even crack to relieve the stress.
Second, when water gets held against concrete by the wet soil, it tends to make its way inside. It can wick through porous foundation walls and seep in through foundation cracks. When it gets into the area under your home, it can damage other building materials, like insulation and floor joists. It can evaporate once inside and increase the air’s moisture content, leading to additional indoor humidity issues.
Landslides are common in Tacoma, in large part due to the amount of rainfall and how wet the ground stays nearly year-round.
Landslides create ground movement that can be catastrophic for your home. Since your home’s foundation relies on the surrounding soil for structure and support, a landslide can create structural instability. This can occur immediately or over time as dirt moves toward or away from your home during a landslide.
Tacoma is situated near several geological fault lines, including the Tacoma fault, the Seattle fault, and the Whidbey Island fault zone. Not all of these faults are active regularly or produce large enough earthquakes to cause severe property damage.
However, cities like Tacoma, Matvey, Kent, Puyallup, and Spanaway experience a relatively high degree of earthquakes and shaking and are considered to be at moderately high risk for movement.
Even small earthquakes can put stress on your house foundation and cause damage. Since concrete doesn’t flex readily, it can crack easily in response to movement in the surrounding soil.
Much of the real estate in western Washington, including Tacoma, is constructed on expansive soil. The ground has a high concentration of clay, which readily absorbs water and grows in volume as it gets wet.
Expansive soil is detrimental to foundation walls for three reasons. The first is that the expanding soil puts more pressure than normal on your concrete block walls and slab as runoff gets absorbed. The stress can quickly cause your concrete to bow inward until it cannot flex any further. At this point, a rift can form in your concrete to relieve the pressure. The result is often a horizontally oriented foundation crack.
The second reason expansive soil is problematic for foundation health is that it becomes less voluminous when it dries. As the soil around your home loses moisture, it can shrink away from your foundation and leave sections of it unsupported. Depending on where the loss of support is located, you may see areas of your foundation cracking severely and sinking into the ground.
Lastly, expansive soil holds moisture and presses it against your concrete. This leaves your basement or crawlspace open to water intrusion more than dirt with less clay would.
Lastly, high humidity is detrimental to most areas of your home, but it’s especially problematic under your home, where moisture is naturally an issue. The average relative humidity in Tacoma is between 75 and 80%, which is relatively high. To make matters worse, the expansive soil and the frequent rainfall leave your under-home area regularly exposed to moisture and water intrusion.
Moist air under your home leaves construction material prone to damage, including rot. It can warp your floor joists and framing over time, destroy insulation, and make the space uncomfortably warm in the summer and cold in the winter.
The air under your home typically moves up into your living space in a process called the stack effect. Humidity in your basement or crawlspace can cause a similar uncomfortable temperature in your first and second floors.
Additionally, humidity under your home creates the perfect environment for mold and mildew growth.