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Calculating the Importance of Gutters and Drainage

How much do gutters and drainage matter to your home? We've run the numbers to calculate how much water Michigan homeowners face during a rainstorm.

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On average, homes in Michigan will have to manage about 30 to 40 inches of rain each year. That doesn’t sound excessive if it’s a slow drizzle that’s spread out evenly across 365 days. However, 40 inches of rain is quite a bit if you’re accumulating water in your basement or crawl space

Understanding the exact impact of gutters and draining requires math, soil science, and hydrology. Below, we look at how water moves around a property, how it can be controlled, and what can happen to your home without effective water management. 

How Much Rain Falls on an Average-Sized House?

Let’s begin our calculations by looking at the quantity of water per inch of rainfall. 

In a scenario with a 1,600-square-foot single-story home, one inch of rain falling across the entire surface area would accumulate to nearly 1,000 gallons of water. If you have gutters connected to four downspouts at each of the home’s four corners, that would be about 250 gallons of water per downspout. 

That’s a lot of water for just one inch of rain. If we look at a bigger storm, such as a hurricane with a foot of rain, you’d have nearly 12,000 gallons of water falling on your roof.

calculating rainfall accumulation on a single-family home

Rainfall on a 1,600-square-foot home

  • 1″ of Rain: 997 gallons of water
  • 5″ of Rain: 4,984 gallons of water
  • 12″ of Rain: 11,962 gallons of water

So far, we’ve calculated water accumulation by looking at square footage and the amount of precipitation. However, there’s one more factor to consider—the slope of your roof. 

The issue is that rain rarely falls straight down. As the wind blows the rain, a steeper roof will catch more precipitation than a flatter roof. For example, one inch of rainfall could actually total 1.25 inches of precipitation falling on your roof because of the wind and the roof pitch. 

How Does Rainfall Intensity Affect Water Drainage?

During a heavy rainstorm, an undersized gutter system can overwhelm your home’s drainage. Overflowing gutters can mean water is directed next to your home’s foundation. This increases the underground water pressure against the home, and rainwater can quickly become an indoor problem. 

When professionals are sizing your gutter needs, they’ll include in their calculations a home’s square footage, the roof pitch, and the maximum rainfall intensity of your location. The system will then be sized based on gutter dimensions, gutter slope, the shape of gutters (K-style or half-round), and the number of downspouts. 

How Does Ground Saturation Affect Drainage?

What happens above ground is only part of the picture. Belowground, soil saturation can build up around the home. Even when the property’s terrain is graded away from the house, a phenomenon known as the clay bowl effect can cause water below the surface to flow toward the house. 

This drainage issue is a result of the soil disruption that happens when the home’s foundation is built. The soil that was excavated and backfilled has different drainage properties. That’s why soil within the area immediately surrounding a home can catch and hold groundwater next to the house rather than allowing it to drain away from the home.

Even with the best gutter systems, if homeowners aren’t addressing water saturation and buildup belowground, homes can still flood after heavy rains. 

Components of a Home’s Water Management and Drainage System

Several different tools can be used in and around the home to manage water. Typically, a home will need a combination of strategies to successfully keep water out of the basement or crawl space. 

Outdoor Home Water Management 

  • Rain gutters installed along the edge of the roof
  • Downspouts to carry the water from the gutter to ground level
  • Extension pipes to carry water from the downspout away from your foundation
  • Property grading or exterior drainage to direct surface water away from your house

Barrier Between Outdoors and Indoors

  • Basement waterproofing to protect against hydrostatic pressure or water seepage
  • Crawl space encapsulation to seal foundation walls and floor against external moisture

Indoor Home Water Protection and Control

Preventing water issues can cost a fraction of what water damage costs. According to FEMA, just one inch of flooding can cause $25,000 in damage to your home. 

It’s not just the properties in flood zones that could be at risk. These calculations for water drainage show that any home without the right water management systems could face flooding or water problems. 

If you are concerned about your home and its waterproofing measures, contact FSM to schedule a free inspection and learn the best way to protect your home from water damage.

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