Also see "Roof Drainage".

Gutter with downspout on a steep-slope roof.

Gutter on a gravel-surfaced built-up roof. Removing loose gravel from the gutters is a common maintenance item with this type of roof.

Useful Links:

1. GeneralThe Wikipedia article here provides an accurate overview.

2. Building Codes: "1106.6 Size of Roof Gutters" from the 2015 International Plumbing Code. Available at the UpCodes website.

3. Maintenance: "General Inspection And Maintenance Of Gutters And Downspouts". Available on the website of the US General Services Administration

4. TechnicalA useful gutter sizing calculator is available at the website of NorthClad Rainscreen Solutions.

5. Technical: "Gutters and Downspouts" can be seen at the website of the Copper Development Association, and includes an explanation of the process of determining the proper dimensions of a hypothetical gutter. Be aware that the data in the Expansion Joint section is specific to copper gutters.

6. ManufacturersGutter manufacturers can be found here.