Architecture
Impervious Surface Calculator
Almost every county now caps impervious surface for stormwater compliance. Add it all up — building, drive, walk, patio — and check against your limit.
FAQ
What's impervious? +
Surfaces water can't soak through: asphalt, concrete, building footprint (with roof), traditional pavers in mortar, swimming pools. Gravel is generally NOT impervious. Permeable pavers usually get a 50% credit.
What's the runoff coefficient? +
It's how much rainfall runs off vs soaks in. A natural lawn is ~0.25; pavement is ~0.95. Total site C is weighted by area. Stormwater engineers use this for sizing detention ponds and pipes.
Watershed-protected zones? +
Critical Area / Chesapeake Bay / coastal jurisdictions often cap impervious at 15-25%. Always check your specific overlay.