Calculating Impervious Surface
An impervious surface is a surface that does not absorb water and includes
roads, driveways, parking lots, sidewalks, roofs, and other hard surfacing materials.
It does not generally include dirt, crushed stone, or gravel surfaces, although
compacted soil or a stone or gravel surface will absorb less water than will
looser, less  compacted
soil. The amount of impervious surface directly affects the quality of water
and the amount of erosion by changing the flow of storm-water over the landscape.
Plants and soil limit the amount of runoff from storms by absorbing some of
the storm-water into the ground water system and slowing the remaining flow
of water. As more surfaces are covered with non-absorbing (impervious) materials
(concrete, asphalt, bricks, etc.), more stormwater runs off at a much faster
rate. One way to control the amount of runoff and erosion is by limiting the
amount of land covered with impervious materials.
Each zoning district has specific lot requirements, including the maximum coverage
by buildings. For residential zones, the maximum coverages for impervious surfaces
(IS) and buildings are as follows:
| District |
Building and Impervious Surface |
Percentage of Impervious Surface |
| A-1 |
Buildings + IS |
20% |
| R-R |
Buildings/IS |
10/30% |
| R-1 |
Buildings/IS |
30/50% |
| R-2 |
Buildings/IS |
30/50% |
| R-3 |
Buildings/IS |
30/50% |
| RM-1 |
Buildings/IS |
40/60% |
To calculate the amount of impervious surface, you must first determine the
square footage of the structures on your property and any and all pavement,
including patios, driveways, and sidewalks. The square footage of the structures
is not based on the actual square footage in the house (esp. if it is a multi-story
house), but the square footage of the structure’s “footprint” (based on the
outer dimensions of the structure and the amount of roof). Add the square footage
of each surface to find the overall amount of impervious surface on your property.
Next, determine the total amount of square footage of the property. For example,
a half acre lot has 21,780 square feet. Finally, divide the amount of total
square footage of the lot into the total amount of impervious surface, which
will give you the percentage of the lot covered by non-absorbing materials.
Allowable Square
Footage by Percentage and Parcel Size
|
Parcel Size
|
10%
|
20%
|
30%
|
40%
|
50%
|
60%
|
| 10,000 sq ft (R-3) |
|
|
3,000
|
|
5,000
|
|
| 20,000 sq ft (R-1, R-2) |
|
|
6,000
|
|
10,000
|
|
| 1/2 acre (21,780 sq ft) |
|
|
6,534
|
|
10,890
|
|
| 1 acre (43,560 sq ft) |
|
8,712
|
13,068
|
17,424
|
21,780
|
26,136
|
| 1.5 acres (65,340 sq ft) |
6,534
|
13,068
|
19,602
|
26,136
|
32,670
|
39,204
|
| 2 acres (87,120 sq ft) |
8,712
|
17,424
|
26,136
|
34,848
|
43,560
|
52,272
|
| 5 acres (217,800 sq ft) |
21,780
|
43,560
|
65,340
|
87,120
|
108,900
|
130,680
|
|
|
