Dec 17, 2025 | 1-800-Striper Bellevue
If you’re resurfacing (or building) a concrete sport court in Western Washington, basketball, pickleball, tennis, or multi-use there’s one topic that comes up occasionally and rarely gets explained well: moisture coming up through the slab.
Quick summary: Some court coating failures aren’t caused by rain on top. They’re caused by moisture vapor migrating up from below the concrete. A vapor barrier under the slab helps control that.
This guide is written for homeowners, HOAs, schools, churches, and facility managers in Seattle, Bellevue, Kirkland, Redmond, Sammamish, Issaquah, Mercer Island, and nearby areas.
Concrete looks solid, but it’s porous. Moisture can move through it over time. When a slab is poured on grade, moisture can migrate from the soil into the slab as water vapor.
A vapor barrier (also called a vapor retarder in many specs) is a sheet membrane installed under a slab to reduce how much moisture vapor can travel upward. It’s not “extra” for the sake of being fancy, it’s there to protect surfaces above the slab, including coatings.
Why courts care: Sport court coatings can be less forgiving than typical exterior paint. If vapor transmission is high, it can shorten coating life or cause adhesion issues.
In the Greater Seattle area, our soils stay wet for long stretches (especially fall through spring). That doesn’t automatically mean your slab is “bad”, but it does mean vapor pressure can be persistent.
The practical takeaway is simple: two concrete courts that look identical on day one can age very differently depending on what’s below the slab (and how the site drains).
If you want to see how seasonal weather patterns shift in our region, NOAA’s local climate pages are a good reference point: NOAA NWS Seattle climate data.
Moisture vapor issues don’t always show up immediately. Sometimes the court looks great at first, then problems appear months later. Here are common signs that can point to vapor transmission or sub-slab moisture issues:
Important: These symptoms can also come from surface water, drainage issues, sprinklers, shade, or poor prep. Vapor transmission is one possible cause, not the only one.
If you’re planning a coating system (especially anything more “sealed” or higher-performance), testing matters.
There are a few common test methods used in the industry to evaluate concrete moisture conditions before coatings or flooring go down.
If you want to go deeper on the “under-slab” side of the conversation, the industry commonly references membranes meeting ASTM E1745 for vapor retarders under slabs on grade.
If your court is already poured, you can’t magically add a vapor barrier underneath it. But that doesn’t mean you’re stuck. It just means your approach should be more thoughtful.
Before blaming “vapor,” check for the obvious moisture sources:
Some coating systems and primers are more tolerant of moisture conditions than others. The right product depends on the slab condition, the intended use (basketball vs pickleball vs multi-use), and the performance expectations.
If a slab has ongoing vapor transmission and poor drainage, the most honest expectation might be: you can improve the court significantly, but resurfacing intervals may be shorter than a best-case site.
Planning a sport court resurface in the Seattle area?
If you want a realistic assessment (not salesy, not dramatic), we can look at surface condition, drainage, shade exposure, and whether moisture testing makes sense for your specific court.
If you’re building a new concrete sport court in Western Washington, your biggest advantage is that you can get the “below the slab” details right. A few basics make a big difference:
Specs often reference vapor retarders that comply with ASTM E1745. That standard covers performance properties like puncture resistance and vapor permeance.
Subgrade compaction, capillary break (granular base), and drainage all matter. A vapor barrier is helpful, but it’s not a substitute for good site prep.
Courts fail faster when water is allowed to sit at edges or run under slabs. In our climate, keeping water moving away from the court is a big deal.
For a straightforward technical overview of vapor retarders under slabs, the concrete industry has good summaries like NRMCA’s CIP guidance: NRMCA CIP 29 (Vapor Retarders Under Slabs on Grade).
I’m not a fan of using “vapor barrier talk” as a scare tactic. Most customers don’t need a chemistry lecture, they need clarity.
The point is simply this: moisture can come from above and below. And when you’re installing coatings on concrete, that can affect adhesion and long-term performance. In Western Washington, it’s worth at least knowing the concept so you can make smarter decisions (and avoid surprise failures).
If you’re pouring a new slab on grade and you plan to apply coatings, a vapor retarder is commonly recommended in many specs and is often cheap insurance compared to the cost of future coating problems. The exact “need” depends on the design, drainage, and what’s going on under the slab.
Most homeowners don’t. Unless you have original plans/specs, it’s often unknown. That’s why site observation and (when appropriate) moisture testing can help guide coating decisions.
Not automatically. Bubbling can come from surface contamination, poor prep, incompatible coatings, trapped moisture from rain events, or vapor transmission. The fix starts with diagnosis, not guesses.
Testing is a tool for risk management, not a crystal ball. It helps you choose the right products and set realistic expectations based on the slab’s condition.
This conversation shows up more with concrete because concrete is porous and sport court coatings tend to “seal” the surface. Asphalt behaves differently and usually fails in different ways.
We see concrete sport courts and multi-use backyard pads throughout the region, including: Seattle, Bellevue, Kirkland, Redmond, Sammamish, Issaquah, Mercer Island, Renton, Woodinville and nearby communities.
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