flat roof drainage

Types of Drainage for Flat Roofing Systems

All flat-roofs and low-slope roofs have at least one thing in common: they are designed to protect the building from elemental damage. This includes falling debris, hail, snow, and rain, no matter how heavy or how often it happens. 

Here in Colorado, commercial buildings of all shapes and sizes make use of flat or low-sloped roofs because they do a great job of enhancing the durability of the building in question while being cost-effective to maintain over multiple decades. 

One of the challenges in designing a flat roof is planning for water drainage. Failure to do this will essentially defeat the purpose of the roof, as water ponding can cause serious damage to the structure. Roof draining has been a part of modern architecture for as long as human beings have been living and working indoors. Even earthen homes made from thatch and mud must contend with the inevitable downpour, making sure that accumulated water has a planned route for egress from the structure. 

To address the need for effective flat roof water drainage, there have been multiple design schemes developed over the years. In this article, A-to-Z Roofing & Exteriors is showcasing some of the most common types of drainage systems that are incorporated into flat and low-sloped roofs commonly found in Colorado. 

Three Types of Sloping for Planned Drainage

Did you know that the term ‘flat roof’ is a bit of a misnomer? That is to say, flat roofs aren’t actually flat at all. In fact, they are designed with strategically sloped zones that divert water to places where gravity will naturally send it. 

flat roofing drainage

Traditionally, flat and low-slope roofs use at least one of three main types of sloping to ensure effective water drainage: valleyed, centralized, and single-slope. These roof slopes are typically surrounded by something known as a parapet wall, which is intended to stop accumulated water from overflowing over the sides of the roof. Some parapet walls are overbuilt as a safety measure for people who might be on the roof, too. 

Valleyed Sloping

A valleyed commercial roof slope is just what it sounds like: a valley. A very shallow ‘V’ is designed into the slope of the roof, creating a horizontal flow channel that meets at least one (but often two or more) scuppers. A scupper is merely an industry term for a roof drain with a wide-mouth, often vertical intake. 

Valleyed sloping is a great option for small and large flat roofs alike, and the design of the roof eliminates the possibility of water ponding (unless, of course, the scuppers become clogged, which does happen). 

Centralized Sloping

Whereas valleyed sloping is meant to divert water to either side of a flat roof, centralized sloping is meant to bring rainwater into centralized, shallow pits on the roof. In each of these sloped pits, a grated drain is installed and the accumulated water is routed through the interior of the building and out to a storm drain or sewer system. 

Centralized sloping can be thought of as a kind of ‘bathroom sink’ design, where all of the water is brought to a single point where gravity does the work of sending it out and away from the building. 

Single-Slope Roof Drainage

Less common but still just as effective as valleyed and centralized sloping is single-slope roof drainage. 

A single-slope flat roof will send all of the rainwater to one side of the building, where a channel drain will be responsible for collecting the water and sending it to a downspout often installed on the exterior of the building.

Single-slope roof drains are less susceptible to clogs because of the sheer size of the channel drain, but they aren’t always a good fit for some roof geometries. 

Siphonic vs. Conventional Drains 

Regardless how the water gets to the drain on the roof, a well-designed intake system is required for proper drainage. 

Siphonic drains make use of a specially designed baffle that activates when water pressure reaches a significant threshold on the roof. When this happens, gravity takes over and the rainwater is ‘siphoned’ away from the building. 

Conventional roof drains have few if any moving parts and are often constructed using a single grate that must be cleaned seasonally to prevent blockages. 

Generally speaking, siphonic drains are preferred above conventional drains, however, siphonic drains can be problematic when ice damming or debris clotting becomes an issue. 

Have questions about commercial roof drainage? We have answers. Contact A-to-Z Roofing & Exteriors with your toughest roofing questions. We love a challenge!

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