Site Overlay

Pros and Cons of Flat Roof Drain Systems

Pros and Cons of Flat Roof Drain Systems

Flat roofs offer many benefits for Los Angeles homes and commercial buildings such as an increase in usable space via rooftop gardens or an expansion of living space such as an outdoor living area. The flat roof system is designed for a climate that is hot, dry and without a heavy rain season. While the benefits of a flat roof are many, they do have problems, especially issues that involve drainage and gutters. Learn more about flat roof drainage systems including inner drains, scuppers and gutters by reading this blog.

The Three Basic Types of Drains on a Flat Roof System

Despite their names, flat roofs are not totally flat. They slope slightly towards a roof drain. All rooftop drains work because of gravity which means that the water must be able to flow to the drain and then away from the roof. That seems obvious, but the fact is that drains cause many problems including:
• Water infiltration to living spaces
• Water damage to inner and outer walls, to window casings, and
• Problems with foundations

While drains can be a source of leaking, they provide many benefits too. Let’s explore what types of drain systems are available and their pros and cons.

There are three types of drainage systems for flat roofs. Those include:
Inner Drains

The inner drain is just as the name implies — It sits away from the edge of the roof towards the roof’s center. A roof with an inner drain is slightly dimpled so that gravity feeds water on the roof directly to the drain. Beneath the roof’s surface is a piping system that shuttles the water away from the house. These usually connect to a downspout. A roof might have more than one inner drain, especially on larger roofs found on commercial buildings or large homes.

A problem with inner drains is that if they clog a small lake can form on the top of your roof. Water is one of the heaviest substances on earth, and standing water on a flat roof causes structural problems. Also, the living space in a home or business is a negative gravity zone, which means that gravity tries to fill it. If you have water on your roof, it will eventually find a way into the building, or it will cause structural damage. If the inner drain and its screen covering are kept clean, water will flow out the drain and away from your home.

Pros — Inner drains are perfect for colder climates as the pipes that drain the water will not freeze. They also offer the benefit of a clean roofline since they do not have roof-edge systems.

Cons — These tend to be the most expensive of the three types of flat drains, and if there is a problem, the repairs can be much more costly because the pipes are inside of the building. The most common problem with inner drains is that people forget to clean the screen and they become blocked, causing a large pond to form.


A scupper is a small boxed drain that exits through the parapet wall along the edge of a flat roof. The roof’s slope is such that water and small debris travel down the roof and out the scupper. On some flat roofs the scupper is not a boxed drain in the parapet, but a divot or channel at the edge of the roof that allows water and debris to flow from the side of the roof. This is the case when a parapet wall is not part of the roof’s design.

Pros — Scuppers are a mid-to-low cost for a flat roof drain system. The wider they are, the easier they are to maintain as small twigs and leaves usually do not form a clog.

Cons — The smaller scuppers do clog, and that can cause water to seep under the edge of the roof. When not connected to a downspout they force of the water falling can cause erosion around a foundation. When hooked into a downspout the spout can clog without regular roof maintenance.

Scuppers might drain into a downspout, or they might have a small channel that extends from the roof where water falls freely to the ground below. Like the inner drain, scuppers can become clogged, especially with leaves and then cause water to pool. A problem with water that pools near the edge of the roof is that if the pond becomes too large, it will spill over the roof’s edge. That is an opportunity for water to seep under the edge of the roof and into the building.


Gutters are metal structures that sit along the edge of a roof and collect water as it slides off the roof. On a flat roof, gutters serve the same purpose, but they channel water from scuppers and box scuppers. The benefit of gutters is that they control the flow of water away from the roof. Gutters also help to protect the building’s foundation and help to keep basements dry. Yes, even though gutters are on the roof’s edge, they help protect basements and foundations by preventing roof runoff from pooling or eroding the soil around the foundation.

Gutters also stop roof water from running down the walls of a building. Water that falls off the edge of a roof has a lot of force, and if it connects with windows, it can damage the window’s casing, or slide under the glass through the window channel. Both situations can cause internal flooding, wall damage, and dry rot.

Pros — Gutters are inexpensive, and they do an excellent job of shuttling and controlling water as it flows away from the roof. Gutters help protect the foundation, basement, and the soil around your building from erosion.

Cons — like all flat roof drains, gutters require maintenance. Regular cleaning is needed and is easier when they have leaf guards.

While flat roofs are perfect for many homes throughout Los Angles, they do require regular maintenance. All three types of drains can clog due to leaves or debris on the roof. That means making sure that leaves and debris are cleaned off the roof on a regular basis.

Leave a Reply