There are several factors at play in Seattle that lends themselves to foundation leaking and damage.
Seattle is perhaps best known for its excessive rain. While the accumulation of precipitation is right around the average for the United States, rainfall frequency is much higher than most areas throughout the country.
Runoff from precipitation is one of the leading causes of foundation problems. Rainwater soaks into the ground around your home and gets held against the concrete. Liquid water can easily find its way into your basement or crawlspace through cracks that form during settling or as a result of ground movement.
Even if your foundation isn’t cracked, water can wick through porous concrete and drip or evaporate into the area under your home.
With rain nearly every other day in Seattle, the dirt is wet or at least moist for much of the year. This means that homeowners in the city are at a high risk of basement water problems.
Large amounts of water can be devastating to your home’s interior, but even seemingly insignificant volumes can cause significant damages. Water vapor coming in through your foundation can lead to mold growth, crumbling foundation walls, or cracks in your concrete if it freezes inside. Over time, the water intrusion that Seattle homes are exposed to due to constant rainfall can even lead to very costly and dangerous structural issues.
Unfortunately, regular precipitation in Washington state also has the potential to cause ground instability. To make matters worse, Seattle is full of hills and already unsteady ground due to its geology. As such, the city is prone to frequent landslides. Even minor landslides can cause additional problems with water intrusion. More importantly, they can cause structural damage and instability in your home.
Your foundation walls rely partially on the surrounding soil’s weight to remain upright, but additional stress from a landslide can create more pressure than the concrete can withstand. This often results in a horizontal crack forming on your basement or crawlspace wall. Horizontal cracking is usually a clear sign of structural instability, and restabilization can be very costly and involved.
Landslides can also cause differential settling of the ground around your home. When the soil moves at different rates beneath or around your concrete, it can leave sections unsupported. These portions can crack or separate due to the added stress and sink into the ground. This type of instability is commonly corrected with expensive underpinning or piling.
Earthquakes are relatively common in Seattle and the Puget Sound’s surrounding areas, including Bellevue, Vashon, Tacoma, and Everett. Although they’re typically not strong enough to cause severe property damage, even relatively minor movement of the ground can compromise your foundation.
Since concrete doesn’t readily flex or move within the earth, displacement of soil around your home from earthquakes can cause immediate structural instability or cracking that can lead to worsening problems over time.
Many areas throughout Washington have soils that are far less than ideal for sturdy construction. Eastern parts of the state have compaction issues as a result of peat or, more commonly, loessial silt. However, much of Seattle and Western Washington – including Kirkland, Olympia, and Renton – is built on what is referred to as expansive clay soil.
Expansive soil has a high concentration of clay, which means it holds moisture more readily than other types of dirt and expands when it gets wet. Soil expansion presents a major threat to foundations.
As your soil absorbs water from the nearly-constant precipitation in Seattle, it grows in volume and weight, putting more and more pressure on the exterior of your foundation. This stress is called hydrostatic pressure, and it presents a two-fold problem for Seattle homeowners.
First, expansive soil readily holds water against your concrete, which can force liquid water through cracks or weaknesses, and it can allow water vapor to wick through your foundation and add to the humidity of your under-home area.
Second, the increased hydrostatic pressure makes the threat of horizontal cracking from rainwater more prevalent. Expansive soil combined with the wet conditions in Seattle can quickly cause foundation instability.
Lastly, Seattle experiences high humidity, in large part due to the frequency of rainfall. Humidity creates another threat to basements, which are naturally prone to humid conditions. If your crawlspace or basement isn’t vented correctly or conditioned with a dehumidifier, heating, and cooling, moisture will very likely build up under your home.
Humidity presents an issue for basements in Seattle because it leaves them prone to mold growth. Mold and mildew can damage the building material on which they grow, including concrete in your foundation, and they can also cause health problems for you and your family.