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What is Roof Drainage?

By Jennifer Voight
Updated May 16, 2024
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The primary purpose of a roof is to protect a structure from weather. Rain and melting snow can quickly damage a structure if they are allowed to stand and seep into a building, thereby causing decay and weakening the structure. A roof drainage system redirects water from the roof to the ground surrounding the structure to protect the structure from moisture damage.

Types of roof drainage systems vary depending on the type of roof and local climate. In dry areas of the world, roofs tend to be flat with little to no drainage system. Roofs tend to have a sharper pitch, or slope, in rainier areas of the world to allow rain to flow off of the roof.

In general, roof slope tends to be in proportion to the amount of rain an area receives. Some structures have large overhangs, or soffits, that deposit rainwater far enough away from the building to prevent water damage to the foundation. If a structure’s overhang is shorter, as in much of the United States (US) and Europe, then a drainage system may be installed to redirect water flow away from the roof and into the ground. Sometimes, a system of pipes will be connected to the drainage system to direct rainwater far enough away from the building that the lawn does not flood.

Several factors typically need to be considered when installing a roof drainage system. The roof slope determines how gravity directs water flow from the roof. Adequate drain size prevents water backup and standing water on the roof. Depending on the surface area of the roof, multiple drains may be installed. An adequate number of drains usually will prevent backed up water due to an overload on one or two drains, which could result in possible leaks to the underlying structure.

Local building codes determine proper installation and placement of roof drainage systems by taking a variety of factors into account. A contractor will consider the grade of the lawn, the type of soil, and whether the building has a basement. To determine the type of drain system to install, he or she may also consider the amount of rainfall, how long it tends to rain, and the rate at which rainfall will collect at a drain.

Once a roof drainage system has been installed, it should be inspected regularly to prevent blockage from leaves and debris. In cold winter climates, snow may melt if the attic is not adequately insulated. The melting snow can run under the remaining snow and refreeze at the eaves where the temperature is cooler, thereby causing ice dams to form and block the drain. When melting water collects against the ice dam, it has nowhere to go and may seep into the roof, causing leaks.

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Discussion Comments
By Feryll — On Oct 27, 2014

I have a sun room/exercise room that used to be a side porch. The porch was built with a flat roof and the water stands on the roof after a rain. Trying to get water to drain from a flat roof is not easy. I have been told that the roof should have been built with a slight slant to promote roof drainage, but the roof appears to be almost perfectly flat and the water goes nowhere.

By Drentel — On Oct 26, 2014

@Sporkasia - Any good roof drainage system in an area that gets a lot of snow and ice should have some type of heating system on the roof to help melt the frozen precipitation. In most cases, a solid roof can withstand snow and ice, but when it stays on the roof for weeks and months without melting, or when you simply get too much of it at one time then that's when the big concerns start.

A good heating coil system will melt the snow and ice and when you have a good water drainage system like mentioned in this article in place this will remove the water, and you are not going to have to worry about a collapsed roof or leaking roof caused by standing water or slow melting snow.

By Sporkasia — On Oct 26, 2014

I was in the Northeast in the United States one year after there had been a series of heavy snowfalls. I don't remember the amount of snow they received all totaled, but I do know that they were counting in terms of feet and not inches.

Anyway, we were driving from the airport to one of my friend's houses and along the root we were able to look at the houses and building with all of the snow piled up. There must have been several dozen that had some type of serious roof damage from all of the snow and ice. Some of the roofs had totally caved in under the pressure of the snow, rain and ice.

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