Understanding and Calculating Impervious Areas

Impervious surfaces are a critical component of our built environment, significantly impacting local hydrology, water quality, and urban ecosystems. From rooftops to driveways, understanding and accurately calculating these areas is essential for homeowners, developers, and municipalities alike. This guide will walk you through what impervious areas are, why their calculation matters, and how to perform these calculations effectively.

What Are Impervious Surfaces?

Impervious surfaces are materials that prevent water from infiltrating into the soil. Instead, they cause rainwater to run off, often at increased speeds and volumes. Common examples include:

• Roofs: Asphalt shingles, metal, tile, or flat roofs.
• Driveways and Parking Lots: Asphalt, concrete, pavers, or even heavily compacted gravel.
• Patios: Concrete, stone, or brick patios.
• Sidewalks and Walkways: Paved paths around a property.
• Roads: Streets and highways.
• Compacted Soils: Areas where soil has been so heavily compacted by construction or traffic that it acts like an impervious surface.

In contrast, pervious surfaces like lawns, gardens, and natural landscapes allow water to soak into the ground, replenishing groundwater and filtering pollutants.

Why Is Impervious Area Calculation Important?

Calculating impervious areas serves several crucial purposes:

1. Stormwater Management and Regulations

Increased imperviousness leads to higher volumes of stormwater runoff. This runoff can overwhelm drainage systems, leading to:

• Flooding: Both localized and downstream.
• Erosion: Of streambanks and natural landscapes.
• Pollution: Runoff picks up pollutants like oil, grease, chemicals, and sediment, carrying them into rivers, lakes, and oceans.

Many municipalities implement stormwater management regulations, requiring property owners to manage runoff on-site, especially for new developments or significant redevelopments. Accurate impervious area calculation is often the first step in complying with these regulations and determining stormwater utility fees.

2. Environmental Impact Assessment

Understanding impervious cover helps assess a property's ecological footprint. High imperviousness contributes to:

• Reduced Groundwater Recharge: Less water infiltrates the ground, impacting local water tables.
• Heat Island Effect: Dark impervious surfaces absorb and retain heat, increasing urban temperatures.
• Habitat Degradation: Altered hydrology impacts aquatic and terrestrial ecosystems.

3. Property Valuation and Development Planning

For real estate and development, impervious area data can influence property value, potential for expansion, and compliance with zoning ordinances. It's a key metric for site planning and designing sustainable landscapes.

How to Calculate Impervious Areas

The most common method for calculating impervious areas is by breaking down your property into simple geometric shapes, primarily rectangles and squares. Here's a step-by-step approach:

1. Gather Your Tools

• Measuring tape or laser distance measurer
• Pen and paper (or a digital drawing tool)
• Calculator (or use the one above!)
• Property survey or aerial imagery (e.g., Google Maps, local GIS portal) for larger or complex properties.

2. Identify and Segment Impervious Surfaces

Walk around your property and identify all impervious surfaces. For each surface, try to divide it into simple rectangles or squares. For example:

• Roof: Measure the length and width of each distinct roof section.
• Driveway: Often a long rectangle, but might have a flared entrance or parking pad. Segment these.
• Patios: Typically rectangular or square.
• Walkways: Measure length and width.
• Sheds/Outbuildings: Measure their footprints.

For irregularly shaped areas, approximate by drawing a rectangle that closely encompasses the area, or divide it into multiple smaller rectangles and triangles (though for simplicity, most calculations focus on rectangles).

3. Measure Dimensions

For each segment, carefully measure its length and width in feet. Be as accurate as possible.

4. Calculate Area for Each Segment

Use the simple formula: Area = Length × Width.

For example:

• Roof Section 1: 40 ft × 25 ft = 1000 sq ft
• Driveway Section 1: 50 ft × 12 ft = 600 sq ft
• Patio: 20 ft × 15 ft = 300 sq ft

5. Sum All Segment Areas

Add up the areas of all the individual impervious segments you've measured. The total will be your property's total impervious area in square feet.

Total Impervious Area = Σ (Length × Width) for all segments

Advanced Methods

For larger properties or professional applications, Geographic Information Systems (GIS) software can be used with aerial imagery to precisely delineate and calculate impervious areas. Property surveys often include building footprints and paved areas, which can also be used for calculation.

Strategies for Reducing Imperviousness

Once you know your impervious area, you might consider strategies to reduce it and mitigate its impacts:

• Permeable Pavements: Use permeable pavers, porous asphalt, or pervious concrete for driveways and patios, allowing water to filter through.
• Rain Gardens: Depressed landscape areas planted with native vegetation that collect and absorb stormwater runoff.
• Green Roofs: Vegetated roof systems that absorb rainfall and reduce runoff.
• Rain Barrels/Cisterns: Collect rainwater from rooftops for later use in irrigation.
• Minimize Paved Areas: Only pave what is necessary; consider narrower driveways or shared access.

Conclusion

Calculating impervious area is more than just a numerical exercise; it's a fundamental step towards responsible land management and environmental stewardship. By accurately assessing the non-permeable surfaces on a property, individuals and communities can make informed decisions that reduce stormwater runoff, protect water quality, and foster healthier, more resilient environments. Use the calculator above to get started with understanding your property's impervious footprint today!