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Why Western Washington’s Wet Season Puts Extra Stress on Sinking Concrete
Quick summary: Western Washington typically gets the bulk of its yearly rainfall between November and February, with November alone averaging around 6 inches of rain over roughly 18 to 20 rainy days. That much sustained moisture saturates soil, especially in the clay and glacial till common throughout the Puget Sound, and saturated soil is far more prone to erosion, compaction, and washout under a slab. These months don't cause every case of sinking concrete, but they do create conditions that can contribute to or accelerate settlement in driveways, sidewalks, and basement floors.
If you've noticed your driveway dipping a little more this time of year, or a sidewalk slab that seemed fine in September suddenly has a lip on it in December, you're not imagining things. We've been raising concrete around the Puget Sound for over 20 years, and without fail, our phones start ringing more the moment the fall rains settle in. There's a real reason for that, and it comes down to how much water our soil has to deal with in a short window every year.
The numbers behind our wettest months
Seattle and the surrounding area average close to 40 inches of rain a year, and a disproportionate amount of it falls between November and February (happy football season Seahawks!). November is typically our wettest month by a wide margin, often averaging somewhere around 6 inches of rain spread across 18 to 20 rainy days. December isn't far behind, and January and February keep the soil saturated even as the daily totals ease up a bit. By the time spring arrives, the ground in a lot of neighborhoods has been holding onto water, on and off, for nearly four straight months.
That's a very different situation than a single heavy storm. One big rain event can absolutely cause runoff and erosion, but it's the sustained saturation over weeks and months that tends to work soil loose from underneath a slab. Water finds the path of least resistance, and around a home, that's often right along a foundation, under a driveway edge, or beneath a walkway where grading sends runoff instead of away from it.
Why saturated soil can accelerate settling
Concrete itself doesn't really care about rain. What it cares about is what's underneath it. Most residential slabs in this region are poured over compacted native soil or fill, and that soil is what's actually holding the slab up. When that soil stays wet for weeks at a time, a few things can start happening:
The soil particles lose cohesion and start to shift or wash away, especially in sloped or poorly graded areas.
Repeated saturate-and-drain cycles can compact the soil unevenly, so some sections settle more than others.
Water traveling along a slab edge or foundation line can slowly carry fine soil particles out with it, leaving a void the concrete eventually drops into.
We've talked about this before on our site when we covered how seasonal cycles in the Puget Sound region affect concrete slabs, but it's worth repeating here because this stretch of the year is when those conditions really stack up. By February, a fair amount of the settling we get called out for has been building quietly since October. Homeowners often don't notice it until the slab has dropped enough to trip on or until water is pooling somewhere it shouldn't.
“This time of year, we're not usually looking at a slab that sank overnight. More often it's weeks or months of saturated soil finally catching up with the slab. A homeowner might say they just noticed it last week, but based on the surrounding conditions, you can often tell the erosion or soil movement has been developing gradually since the rain started.”
-Israel Craig, Lead Field Advisor at 1-866-SLABJACK
Which areas tend to show signs first
A few areas tend to show it earlier than others. Driveways near the street, where water runoff naturally collects, are often the first thing homeowners notice. Sidewalks and walkways that run close to downspouts or along a slope tend to follow close behind. Basements and garage floors take a little longer to show visible settling since they're more protected, but they're not immune, especially if there's a grading issue pushing water toward the foundation. You can read more about that specific situation on our basement floor page if that's an area you're keeping an eye on.
Why late fall and early winter is actually a smart time to get ahead of it
We get it, nobody's excited to schedule an outdoor concrete project in the rain. But this is genuinely one of the better windows to get a free inspection done, for a couple reasons. First, active settling is often easier to spot right now than it will be once things dry out in the spring, because saturated soil tends to show its problems more clearly. Second, polyurethane foam handles wet and saturated ground conditions well, which is part of why it's become the standard over old-school mudjacking in a climate like ours. We go into that in more detail on our poly vs. mudjacking page if you want the technical side of it.
Homeowners who wait until spring to deal with a slab that's been settling all winter often end up with a bigger repair than if they'd called in December or January. Catching it early, while it's still a small settlement instead of a major drop, is usually the cheaper and easier fix.
The good news: Our dedicated team at 1-866-SLABJACK works rain or shine!
What to remember about concrete in the PNW area
November through February typically brings the majority of our region's annual rainfall, with November usually the wettest month.
Sustained soil saturation over weeks or months, more than any single storm, is what tends to drive settlement during this stretch.
Driveways, walkways, and slabs near downspouts or grading issues often show signs first.
Getting an inspection during the wet season, rather than waiting until spring, usually catches problems while they're smaller and less expensive to fix.
FAQ
Does concrete really sink more in the winter around Seattle?
Concrete itself isn't affected by rain directly. What happens is the soil underneath it can stay saturated for extended periods during our wettest months, and that saturation can contribute to or accelerate erosion, compaction, and voids forming under the slab. That can lead to visible settling, though not every slab is affected the same way.
Should I wait until spring to fix a sunken slab?
We generally don't recommend it. Waiting often means the underlying soil conditions have more time to worsen over a few more months of rain, which can turn a smaller fix into a bigger repair. Getting ahead of it during the wet season tends to be more cost effective.
Can slabjacking be done in the rain or during winter months?
Yes. Polyurethane foam cures quickly and handles wet or saturated soil conditions well, which is one of the reasons it's become the preferred method over traditional mudjacking in a climate like ours.
Is it normal for the same area of concrete to sink again each winter?
It can happen if there's an ongoing drainage issue feeding water to that spot every year. If you're seeing a repeat pattern, it's worth having both the settling and the surrounding drainage looked at together.
If you've noticed a driveway, sidewalk, or basement floor settling more than usual this season, don't wait for it to dry out to do something about it. Schedule a free inspection and we'll tell you honestly what's going on underneath, and whether now or spring makes more sense for your situation.
