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Calculating the Importance of Gutters and Drainage

October 23, 2020

To calculate the importance of gutters, drainage, and water management around the home, we need to look at how water moves around a property, how it can be controlled, and what can happen if left unchecked. 

Using a combination of math, soil science, and hydrology, it's possible to calculate the exact importance of your home's water management plan, including gutters and drainage systems.

Is Rain the Only Water Issue to Consider? 

To protect against flooding, homeowners must manage water in its different stages.

  1. There's falling rainwater that can quickly accumulate during a downpour. 
  2. Surface water or runoff flows along the top of the ground. 
  3. There's also groundwater, which is the underground soil saturation around your home. 
  4. Lastly, there's the hydrostatic pressure of water, which is the buildup of force against your home's foundation or basement. 

What Home Systems Help You Manage Water and Drainage?

To manage water, your home needs a combination of a few different things:

  • Gutters installed along the roof edges
  • Downspouts to carry the water to ground level
  • Extension pipes to direct water away from the foundation
  • Property grading to keep all surface water away from your house
  • Basement or crawl space waterproofing to provide a secure barrier against water
  • Interior drains to catch leaking water 
  • Sump pumps to remove any water from your home
  • Dehumidifiers to remove moisture buildup

How Much Rain Falls on an Average-Sized House?

Let's start our calculations by looking at rain and the quantity of water that can accumulate. 

A single-story single-family home with 1,600 square feet will accumulate nearly 1,000 gallons of water after just one inch of rain. If there's a hurricane with a foot of rain, that would be 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

Wind also affects accumulation. The steeper a roof is pitched, the more rain that will be collected on your roof. For example, one inch of rainfall could translate to 1.25 inches of rain on your roof because of how the wind blew the water while it was falling. 

How Does Rainfall Intensity Affect Water Drainage?

In addition to considering water volume, the speed and intensity of a rainstorm affect water management. If one inch of rain falls slowly throughout a day, the water is easier to manage than if one inch of rain fell in an hour-long downpour. 

To control where rainwater goes and prevent it from flooding your basement or crawl space, your gutters and exterior drainage system need to be sized appropriately for your home and location. Undersized systems can mean that gutters will overflow, and you could have hundreds or thousands of gallons of water flowing toward your foundation or into your basement or crawl space.

What Drainage Capacity Do You Need For Heavy Downpours?

Drainage and gutter sizing takes into account home construction, weather patterns, and materials choices. 

Gutter sizing must consider:

  • Home square footage
  • Roof pitch
  • Maximum rainfall intensity of your location
  • Shape of gutters (K-style or half-round)
  • Gutter dimensions
  • Gutter slope
  • Number of downspouts

Sample drainage calculations: 

  • A two-story house in Washington, D.C., has a footprint of 800 square feet. 
  • The roof pitch is 8’x12’, which adds an extra 10 percent to rain accumulation on the home. 
  • That equates to an 880-square-foot watershed. 
  • Washington, D.C., has a 6.6-inch per hour maximum rainfall intensity. 
  • Multiplying 6.6 by 880 gives the adjusted square footage of 5,808.
    This is the measurement you'll use when sizing your gutters. 
  • A six-inch K-style gutter has a drainage capacity of 7,960 square feet. Since the capacity is larger than the need, this would provide adequate drainage. 
  • A smaller five-inch gutter has a lower capacity of 5,520 square feet. This capacity is lower than the maximum need. However, the homeowner can improve an undersized gutter's performance by adding additional downspouts or having a stronger tilt to the gutter. 

A construction professional can help you properly install and size your drainage system for your situation. Remember that cleaning your gutters prevents damage to your roof and maximizes your ability to catch rainwater and direct it away from your house. 

How Does Ground Saturation Affect Drainage?

What you do above ground can help you keep thousands of gallons of water away from your foundation and basement or crawl space. However, there's also underground water and soil saturation to consider, which is especially relevant during the rainy season.  

A phenomenon known as the clay bowl effect occurs in the soil immediately next to many homes. Because of this phenomenon, surface water and groundwater can move in different ways. 

The clay bowl effect starts with home construction. The soil that was excavated and then backfilled has different drainage properties than undisturbed soil. Rather than dispersing water, the disturbed soil creates a bowl effect that contains water in the soil next to the house. Even when the terrain slopes away from the house, soil compaction can direct underground water back toward your foundation. 

The increased ground saturation causes a buildup of hydrostatic pressure against the foundation. As a result, homeowners could have basement flooding or foundation damage. Older homes can be especially at risk because buried drainage pipes often become crushed over time. 

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

How Much Can You Save With Effective Drainage and Waterproofing?

There are clear financial advantages to investing in water management for your home before you have a problem. 

Baseline water management costs will vary by location, method, quality, and other variables, but prevention can cost about one-tenth of the cost of damage cleanup and repair. For example, installing rain gutters and downspouts costs between $3 and $40 per linear foot. The average cost of a sump pump can be about $1,000, and basement waterproofing can range in cost based on the home's needs. 

For homeowners that didn't invest in preparation, there are significant costs associated with water damage and repairs. 

calculating cost of water damage and repairs

Cost of Water Damage and Repairs
(2,500 sq.ft. single-story home)

  • 1 inch of water in the home: $26,807
  • 1 foot of water in the home: $72,163 
  • Damaged foundation: Lose up to 30% of your home value.
    That's $105,000 for a $350,000 home.

Even a few thousand dollars spent on water management far outweighs the cost of damage if your home is flooded. Plus, many flood insurance policies provide up to $1,000 toward expenses that mitigate flood loss such as a sump pump. 

Learn the best way to protect your home from water damage with a free inspection from the country's leading basement waterproofing and foundation repair experts.