Exact Gutter and Drainage Calculations for Western Colorado

Find all the calculations for gutters and downspouts that can help prevent flooding and foundation damage from excess rainfall.

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In western Colorado, rainfall is almost entirely absent from December through February. But when it does show up the rest of the year, it can present a challenge with peaks in September and October.

Any excess moisture can lead to foundation problems from cracking to flooding. That’s why it’s critical to install the necessary gutters, downspouts, and drainage systems. 

Here are the detailed requirements and calculations.

Rainfall Water Management

Rainfall starts at the roof, moves through gutters and downspouts, finally arriving on the ground. There it comes into contact with still more water, all of which needs to be routed away from your foundation.

Here are the critical rainfall water management elements to consider.

  • Total surface area of the roof
  • Pitch of the roof along with the structure of peaks and valleys
  • Size and slope of gutters
  • Size, shape, and number of downspouts
  • Landscape grading around the foundation
  • Waterproofing of basement or crawl space

Rainfall on a Single-Story 1,600-Square-Foot Home

In the example shown here a single-story 1,600-square-foot home accumulates nearly 1,000 gallons of water from just one inch of rain. Add a few more inches and the gallons advance rapidly, hitting nearly 5,000 gallons at five inches.

how many gallons of water after rainfall

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

You can see how gutters can rapidly become flooded in a downpour. That overflow can then drop right on to your foundation. So it’s critically important to have the right capacity gutters and downspouts.

Water Volume and Roof Structure

Wind can play a role in that it blows the rain into any steeply pitched roof, causing more rain to collect and run down into the gutters. Plus, the steeper the roof the more surface area for rainfall along with a swift flow of water off the roof.

A roof’s pitch factor is the amount of the rise over a 12-inch run. If the rise is nine inches, or 9-in-12, the pitch factor is 1.2. This is used in our gutter capacity calculations.

Yet another thing to examine is the roof’s overall structure, including any peaks and valleys. They can act to funnel the rain and increase the water flow to the gutters.

Capacity Calculations for Gutters and Downspouts 

Here is the full range of factors needed for our gutter and downspout calculations.

  • Home square footage or footprint
  • Roof pitch along with any peaks and valleys
  • Gutter shape, K-style or half-round
  • Gutter dimension
  • Downspout size, shape, and slope
  • Expected rainfall intensity

Sample Calculations for Grand Junction

You can find the expected rainfall intensity at the NOAA Weather Service’s precipitation frequency estimates data server. The starting point is Grand Junction’s five-minute expected rainfall burst that’s likely over a 10-year period. That comes in at 0.259 inches. Converting that to inches per hour, 0.259 x 12, yields 3.1 inches.

Taking an example 1,800-square-foot home with a roof pitch of 12-in-12 and a pitch factor of 1.3, the total roof watershed is 1,800 x 1.3, or 2,340 square feet. That multiplied by the expected rainfall intensity of 3.1 inches equals a 7,254-square-foot drainage capacity.

For the gutters, a K-style five-inch gutter has a capacity of 5,520 square feet. This is not enough. Instead, a six-inch K-style gutter with a capacity of 7,960 square feet should be used.

Of course, the downspouts must also handle this same level of rainfall. Rectangular 2 x 3-inch downspouts have a capacity of 600 square feet, while 3 x 4-inch can handle 1,200 square feet. With the 3 x 4-inch rectangular downspouts, we’d need at least seven to handle the expected water flow.

Soil Saturation and Ground Water Challenges

While all this rain is falling on the roof, collecting in the gutters, and running through the downspouts, it is also falling on your lawn. That quickly results in soil saturation and even underground water.

A critical issue is ensuring that your landscape grading helps move all the water away from your foundation. In addition, the water coming from the downspouts should be routed through downspout extensions to move it well away from the foundation before it hits the ground.

Even with those precautions, once the soil is saturated underground water starts moving back toward your foundation. That’s due to the clay bowl effect. This results from the original excavation for basement or crawl space construction followed by backfilling. The soil used in backfilling has a different drainage factor than the rest of your undisturbed landscaping. This allows the water to more easily flow toward the foundation.

That buildup of hydrostatic pressure can cause foundation cracking, and once those exist, water moves in, causing flooding. 

The best way to deal with this is basement or crawl space waterproofing including interior drainage and a sump pump.

Water Damage Repair Costs

FEMA has developed cost estimates of the impact of several different levels of home flooding. Those are highlighted in the chart below.

cost of water damage, repairs and flood cleanup

Cost of Water Damage and Repairs

(2,500-square-foot 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.

Those numbers are a sobering reminder that the costs of prevention are well worth it. Installing the right gutters, downspouts, extensions, and drainage is an excellent approach.

It’s also a good idea to consult with the experts to fully examine your situation and provide sound advice on prevention and any needed repair.

For a free inspection and repair estimate, contact the professionals at Foundation Repair Western Colorado.

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