Why Drainage Is the #1 Pavement Issue
Water is the single biggest destroyer of asphalt and concrete. Surface water that doesn’t drain off saturates the sub-grade, reduces bearing capacity, and creates conditions for sub-grade pumping. Sub-grade water freezes in winter, expands, and pushes pavement up from below. Done right, drainage extends pavement life by 10+ years. Done wrong, it cuts pavement life in half.
How Surface Drainage Works
A properly graded parking lot directs surface water away from buildings, toward catch basins, and out through the storm sewer system. Standard cross-slope: 1.5 to 2.5%. Standard longitudinal slope (along the lot): 0.5 to 1%. Slope must be steep enough to drain but not so steep that ADA accessible stalls and routes exceed code (ADA limits accessible stall slope to 2.08% maximum).
Catch Basins
Catch basins are the inlet structures that connect the parking lot surface to the storm sewer. They’re built of precast concrete or cast-in-place concrete, with a frame and grate at the surface. Failure modes include: wall deterioration from freeze-thaw and salt, frame settlement, pipe joint failure, grate damage, and clogging from debris. Maintenance includes annual cleaning and visual inspection.
Pavement Structure and Drainage
Beneath the asphalt is the aggregate base (typically CA-6 stone). Below that is the sub-grade. Each layer has a role: asphalt sheds water, base distributes load, sub-grade bears the load. If water gets through the asphalt and into the base, it can move horizontally to the sub-grade and cause failure. Sealcoating, crack sealing, and proper grading all keep water from getting in.
Sub-Grade Pumping
When water reaches a saturated sub-grade and heavy traffic loads it, water and fines get pumped up through the asphalt. You see this as persistent potholes that come back after every patch — water and fines reappear at the same spot. Surface patches won’t fix it. The fix is excavation, sub-grade repair, and full-depth reconstruction of the affected area.
Trench Drains
For long linear drainage problems (along curbs, building entrances, dock-door approaches), trench drains are channels with grates that collect water across distance. We install precast trench drains tied into the existing storm system. Used most often at industrial sites where dock-door runoff or fleet wash-down water needs to be collected.
Detention and Retention
Some commercial properties have detention basins (temporary storage that releases slowly) or retention basins (permanent storage that infiltrates). These are typically governed by municipal stormwater regulations and need to be sized based on the lot’s runoff. We coordinate with civil engineers when reconstruction affects detention/retention sizing.
Common Drainage Mistakes
Sealing over a wet pavement (sealer fails). Patching potholes without addressing the drainage causing them (potholes return). Allowing catch basins to clog (water can’t enter). Adding pavement (e.g., expansion or new outparcel) without updating drainage to handle additional runoff. Ignoring water staining at the base of curbs (signal that water isn’t draining).
How to Diagnose Drainage Problems
Walk the lot 24 hours after a rainstorm. Standing water = drainage problem. Visible water staining = drainage problem. Persistent potholes that come back = drainage problem. Catch basins overflowing during heavy rain = drainage problem. Ice in winter where it shouldn’t be = drainage problem. We do free drainage assessments as part of paving estimates.
Get a Free Commercial Paving Estimate
Need help with a commercial paving project? Every estimate is free, on-site, and itemized. Most on-site estimate within 48 hours, with on-site estimate within 48 hours. Use the form on this page or call request a free estimate.
Get a Free Commercial Paving Estimate
Need help with a commercial paving project? Every estimate is free, on-site, and itemized. Use the form on this page or call request a free estimate.