Learn How to Calculate Gutter Slope

The angle of your gutters plays an important role in how well they drain water and protect your home. Gutters that are at too severe of an angle can make your house look crooked and off-kilter and gutters that are too flat will hold water and eventually flood. Learning how to calculate gutter slope will help you install them in a way that they will function properly and keep your home looking its best. 

Safety Notes 

One of the main things to keep in mind whenever you are working on gutters is ladder safety. Getting on a ladder is always dangerous and you should be careful whether you are going up one step or several. Make sure that you have someone who can hold the ladder whenever you get on it. This will ensure that it will not slip out from under you and that the base will be stable. 

If your home is more than one story, it is a good idea to have a professional handle any work that has to do with your gutters. They will have the safety equipment necessary to do the job without sustaining any injuries. 

Tools and Materials 


  • String level
  • Pencil
  • Chalk Line
  • Ladder 


  • Length of string (at least 30 feet)

How to Calculate Gutter Slope Instructions 

Because your gutters need to be at the correct angle, you must understand how to calculate the slope. Gutters that aren’t at enough of an angle will not drain the water properly when it rains and gutters that are at too much of an angle will affect the look of your home. Knowing how to properly calculate the slope will help you get the best final product that will both protect your home and enhance its look. 

Step 1: Understand the Formula 

As a rule, your gutter system should have a downspout for every 30 feet of guttering. The proper amount of slope is ½” every ten feet. Based on this formula, the end of your gutter that connects with the downspout should be 1 ½” lower than the other end. This will give you enough of a slope to allow the water to flow properly without looking crooked from the street. 

Step 2: Run a Piece of String 

To get the right amount of slope when installing your gutters, run a piece of string from where the end cap is going to be to where the downspout will be installed. Use your string level to make sure the string is completely straight and parallel to the ground. This will give you the exact length of the guttering itself, which will help you figure out how much lower the downspout end will need to be. 

Step 3: Lower One End 

Using the formula, lower the end of the string that is on the downspout side. If, for example, you end up needing 20 feet of guttering between the end cap and the downspout, lower the end one inch. If you will need 25 feet of guttering, lower it by 1 ¼”. This will give you the correct slope and will not be noticeable or come off as crooked when you are looking at the house. 

Step 4: Mark the Line 

Once you have your string placed correctly, use your chalk line marker to make a line along the roofline. This will show you where the gutters need to be mounted and give you a good guide for their angle. Repeat this process for your whole gutter system along the perimeter of the house taking care to make sure that you have a downspout at most every 30 feet. 

When to Contact a Professional

For your safety, it is important that you only try to set your own gutter lines if you have a one-story home. Anything more than one story will require you to get up on a much taller ladder and you could potentially be exposing yourself to serious injuries. A professional gutter installer will have all the proper safety equipment such as harnesses and specialized ladders that will keep them safe when measuring on a two- or three-story home. 

If you want to make sure that your gutter slope is perfectly calculated and set, a professional is your best option. Not only will they be able to install your gutters properly, but they can also solve any issues that might come up due to an unusual roofline or eaves.