Why Does Water Pool in a Parking Lot, and How Can the Drainage Be Fixed?

In the fall, any poorly drained platform reveals its flaws: water pools in the same spots after every rain. In the winter, those same spots turn to ice, and in the spring, the asphalt gives way right there. Puddling isn’t a surface defect—it’s a design flaw.

Slope Is Not Optional

A platform needs a minimum 1% cross slope for water to drain on its own; below this value, normal construction tolerances are enough to create flat areas. For large surfaces, composite slopes oriented toward the collection points are used, rather than a single slope across the entire platform.

Collection points must be determined before any concrete is poured. This includes concrete gutters running in the direction of drainage, drain outlets at low points, and a clear path to the collector or retention basin.

What Goes Under the Surface

• a foundation layer of ballast or crushed stone, compacted in layers and inspected
• longitudinal drainage where water flows in from adjacent land
• a separation geotextile, which prevents soil from rising into the stone
• Drain outlets with silt traps to prevent the collector from clogging

A properly constructed road system without drainage will not hold up: infiltrating water saturates the foundation, and under traffic, settlement and ruts occur.

Water from the platform cannot be discharged just anywhere

Parking platforms, freight terminals, and gas stations discharge water containing hydrocarbons. Direct discharge into storm sewers or receiving waters is not permitted without treatment: hydrocarbon separators sized for the design flow rate, equipped with a coalescing filter and a bypass, are required.

And the separator requires maintenance. A separator that hasn’t been cleaned for years no longer retains anything—it lets the water pass through just like an empty pipe, but with a certificate of compliance on file.

We construct both the aboveground and underground components together: slopes, ditches, drainage, and separators. They are part of the same project, even if they appear under different sections in the estimate.