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January 17, 2018

Drainage Systems for Flat Roofs

A “flat roof” is a bit of a misnomer, because no roof is perfectly flat. Flat roofs are defined as having a maximum slope or “pitch” of three inches of vertical rise for every twelve inches of horizontal distance. Most flat roofs have a pitch of much less than that. Some amount of slope is imperative to ensure that water can drain. Water weighs about eight pounds per gallon and it’s not good to have a lot of un-drained water on your rooftop. It adds stress to both the roof surface and building structure and can significantly shorten the life of your roofing system.

So, proper drainage is critical. The three most common roof draining systems are interior drains, scuppers and gutters.

drains on commercial roofInterior drains are located within the roof area. Roofs with drains are engineered with a slope that is intended to let water flow toward the drains. Water is then channeled through the drains and into pipes that run through the interior of the building before emptying at ground level or into a sewer.

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Scuppers are outlets through a parapet wall that the roof is sloped toward. Water runs through the scupper and typically into a collector box mounted on the exterior wall of the building that has a downspout attached underneath, allowing the water to drain away from the building at ground level.

The third most common drainage system for commercial roofs – gutters – are normally more robust than gutters on homes because the volume of water they have to handle is typically much greater. Gutter systems will also have downspouts attached to direct water away from the building.

All these drainage systems have specific installation requirements to ensure they are properly integrated with the roofing system on the building for a watertight fit. In addition, with every type of drain, regular maintenance is important, because if water flow were to be blocked by debris, for example, the accumulated water could create major issues that could severely damage your roofing system and the structure beneath. Having the appropriate drains and maintaining them properly will help promote your roof’s longevity.

Article by Royalty /


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