Your Commercial Roof Drainage System Explained

May 20, 2020

You know that water should drain off the roof completely within 48 hours, but do you know what makes that happen? Commercial and industrial roofs can use a combination of different drainage components. If you’ve got drainage problems on your roof, it’s good to know what each element is and how it works.

One of the most important indicators of your roof’s ability to shed water isn’t the presence of any one single roof element but rather, its design. Low-slope roofs are engineered and constructed with enough pitch to direct water to drains and off the roof. The drainage components, dimensions, layout, quantity, etc. are determined during the design, drafting, and planning phases prior to new construction.

Your roof’s drainage system is likely to consist of any (or all) of the following components.

Internal Drains


Similar to the way the drain in your shower works, internal roof drains also contain plumbing that runs inside the building to divert water. A system of pipes is installed under the roof. Roof drains are commonly found on the roof near the center of the building, but a certified architect or engineer will determine their placement by evaluating the locations on the roof that will collect the most water. Your roofing contractor can clear debris and vegetation from the drain sump area, drain bowl, and drain basket, but if the drain line itself is clogged, a professional plumber needs to be called in.

Drain Basket/Dome/Strainer

Set on the drain and then twisted to lock into the clamping ring, drain baskets are strainers that prevent large vegetation and debris from getting into internal drain pipes. At a price of less than $100 each, replacing drain baskets costs a fraction of what a plumber will charge to snake the drain if it’s backed up. They come in a variety of shapes and can be made of any number of materials – from cast iron to plastic. If a drain is missing it’s drain basket, replace it. It’s easy for spare parts, rags, soda bottles, etc. left behind by rooftop workers to end up in the roof drain.


Crickets are insulation formed or cut into diamond- or inverted v-shaped ridges that direct water to (and prevent ponding between) drains. They’re often placed in front of large curbs and at parapet walls to divert water to a drain or through-wall scupper. They are mostly used on low-slope roofs, but a steep slope can also use crickets to change the direction of drainage.

Drain Sump Pan

Another means to draw water toward roof drains is with a sump pan. These metal plates recess the drain slightly below roof level, which is necessary whenever it would otherwise be too high to be effective.

Perimeter Drainage


Scuppers are edge perimeter drains that are incorporated into parapet walls. In the absence of internal drains (or if they are blocked), scuppers prevent the roof from holding water like a bathtub. These openings are level with the roof surface (or slightly higher in the case of secondary/emergency scuppers) and allow water to drain from the edges of the roof. They may be used in conjunction with a conductor head, gutter, or downspout.

Conductor Head

While these go by many names (e.g. collection box, collector head, gutter head, scupper box, etc.), they function the same. They are catch basins, usually made of sheet metal.

Gutters and Downspouts

Gutters are horizontal metal eavestroughs. They channel water that rolls off the roof to downspouts and away from the building. To work effectively, they must be installed just below the roofline, large enough for the volume of water, and clear of debris. Vertical pipes known as downspouts, carry water from the roof to the ground level. Make sure water can flow freely and they are secured. Don’t neglect maintenance!


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