K-style 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.

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Rain Gutters: Basic Information

  • Construction Definition - Gutter: A fabricated channel of metal, plastic, or wood which is installed along and below the edge of a roof to catch and divert rainwater coming off the roof. Also called: eavestrough, rain gutter, or roof gutter.

  • Appearance and Form of Gutters: Gutters are long, shallow troughs made from thin sheets of material. They are commonly made with prefabricated sections, typically anywhere from 8 to 20 feet long, which are joined together during installation. Seamless gutters, almost always aluminum, are formed on-site and can potentially run the entire length of one side of the roof without a joint; as a practical matter, a maximum length of 50 feet is recommended due to performance issues related to thermal expansion and contraction.

    In cross-section, gutters appear semi-circular (half-round profile), rectangular (box profile), or have a straight back and bottom while the front has curves and angle changes similar in appearance to crown molding (K-style profile). The proper depth and width of gutters are calculated using local precipitation rates, the slope of the roof, the size of the roof area draining to the gutter, and the type of profile of the gutter. 6 inches wide by roughly 4½ inches deep are common dimensions for residential K-style gutters.

    Other parts of the gutter system include end caps, drop outlets where the gutter connects to the downspouts, and gutter hangers. Gutters are often painted or coated to match the trim of the building.

  • Gutter Materials: Aluminum (typically coated or painted), copper, steel (various types, typically coated or painted), vinyl (PVC), wood (with waterproof lining), zinc.

  • Where Gutters Are Located: Although gutters are most commonly mounted along the eaves immediately below the edge of the roof, gutters may be located in a couple of other places. Built-in gutters, also called hidden gutters, are built into the supporting roof structure near the roof edge and have a lining which is integrated with the roof covering. On steep-slope roofs, they will usually be located above the soffit, if there is any. On low-slope roofs, they will typically be located right behind a parapet wall. Where two parallel roof sections on the same building slope toward each other, there may be a built-in gutter where the roofs meet. Where a roof section slopes down to a rising wall, there will often be a built-in gutter along the base of the wall.

  • How Gutters Work: A properly sized and positioned gutter catches and contains virtually all water running off the roof. To ensure efficient drainage, the gutter is sloped (typically with a ½-inch fall, or drop, per 10 feet) down to a gutter outlet. The water drains to the gutter outlet, which is connected to a downspout, which conducts the water to a final discharge location appropriate to the site conditions.

  • Purpose of Gutters: Gutters are used to prevent the water that hits a roof from falling to the ground with amplified volume and force in a concentrated mass along the exterior walls of a building. There are a few reasons to keep this from happening. Gutters help keep people from getting drenched when going in or out. Gutters prevent the erosion and damage to landscaping that concentrated falling water would cause. Where there is little or no eave overhang, gutters reduce the amount of water running down the exterior walls and thus the chances that water will find a way into those walls. Most importantly, gutters prevent a build-up of hydrostatic pressure in the soil at the foundation. If water in the soil is concentrated along the building’s foundation, the resulting pressure exerted by the water can overwhelm foundation dampproofing, forcing water into the foundation walls, which can lead to cracks, leaks, and more severe structural damage.

  • Common Issues with Gutters: By far the most common gutter issue is the accumulation of debris within the gutter, particularly tree leaves, but also wind-blown soil and sand, gravel (on built-up roofs), and even sticks and animal bones dropped on the roof by birds. Gutters require regular cleaning. Without regular cleaning, debris will clog the outlets and impede drainage. A clogged gutter means an overflowing gutter, and an overflowing gutter defeats the purpose of having gutters. Even if a gutter isn’t clogged, debris can trap moisture and keep the gutter from drying out properly. The constant presence of moisture shortens the life of most gutter materials.

Related Pages:

Useful Links:

1. General: Downspouts: “Closed Faced vs Open Face Downspouts in Colder Climate Regions” is short but informative. From the Metal Era website.

2. General: Gutters: The Wikipedia article on gutters provides an accurate overview.

3. Building Codes: "1106.6 Size of Roof Gutters" from the 2018 International Plumbing Code. Available on the UpCodes website.

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

5. Technical: “Copper Gutters and Downspouts” over at the Copper Development Association is a thorough (if generic) architectural specification with lots of important information.

6. Technical: “Demystifying Gutter and Downspout Sizing” is a useful little article from the Architectural Graphic Standards website. Helps you understand the equations you need to use and how to use them.

7. Technical: "SMACNA’s Downspout & Gutter Sizing Calculator" is a great tool. Free to use on the website of SMACNA, the Sheet Metal and Air Conditioning Contractors’ National Association.

8. Technical: "Appendix B: Rates of Rainfall for Various Cities" on the ICC website provides the rainfall data for major US cities required for the sizing calculations mandated in the building codes. If your town is not listed, visit NOAA's Precipitation Frequency Data Server (PFDS) to look up the 100-year one hour rainfall data for your area.

9. 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.

10. Manufacturers: See here for gutter manufacturers.