Water Volume Vs. Your Drainage System 

Knowing Is Half The Battle

Knowing the correct size gutter to install or have installed on your home can be beneficial to any homeowner. Customers call us about gutter overflow issues everyday. Sometimes debris simply needs to be removed. Other times we simply need to adjust the pitch. Now while we do everything in we can to determine the issue and use the right crew for the job we have times where the homeowner simply doesn’t know. They are not sure why this is happening. They clean the gutters twice a year and someone has checked the pitch and found it to be correct. So now what? While we like to think every home built has the correct size gutter we live in a world where fact-checking has become a necessity. So let’s check the facts on water volume.

Numbers Never Lie, Says Pi

We can easily determine if our gutters are capable of handling the water volume using a simple formula. First we will need to determine the square footage of your roof, we will refer to his as the “roof drainage area”. If you don’t know this your local county auditor should have this information. Now that we have the roof drainage area we need to determine our roof pitch. You can look for a roof pitch tool online to help if you are unsure. Now your roof pitch will have a “roof pitch factor” that correlates to the pitch. These are as follows:

12/12 pitch is 1.3

9/12 or 11/12 is 1.2

6/12 or 8/12 is 1.1

4/12 or 5/12 is 1.05

3/12 is 1

Now multiply your roof drainage area by your roof pitch factor. Now that you have this number we only have one more step, accounting for the maximum rainfall intensity in your area. This is something you will have to research as it varies city to city. Multiply the answer from the previous equation by the maximum rainfall intensity. This will give you the number you’re looking for.

5″ K-style gutters can hold an average maximum ( depending on pitch and amount of downspouts) of 5,520 square feet. 6″ K-style gutters can hold an average maximum of  7,960 square feet. If you find your number is close to the maximum average and don’t want to upgrade your size you can always look to ensure your pitch is optimal for your gutters to increase the rate of drainage or, you can add additional downspouts for the same effect. Downspouts should be placed at least every forty feet of gutter.

https://www.thisoldhouse.com/ideas/how-to-size-gutters-and-downspouts