Flat roof drainage systems are difficult to put in place because of the lack of slope. While high-pitched roofs have natural runoff from a sharp angle, low-slope roofs require more effort to remove water effectively.
If you have a low-slope roof and have dealt with issues from standing water, you may be in search of an effective drainage system to solve these problems. In this case, it’s important to understand which solution will be most suitable for your home.
Drainage system options for flat roofs include inner drains, gutters, scuppers, and siphonic roof drains. Inner and siphonic drains are the most effective because they don’t rely on an angled surface. However, the other options are suitable for low-slope roofs at up to a 10-degree angle.
For a homeowner with a low-sloped roof, proper drainage is a valid concern. While flat roofs come with a wide range of benefits, certain aspects can require special attention. Read on to learn about all the possible drainage options and how you can apply them to your living space.
Why Do Flat Roofs Pose a Drainage Problem?
Flat roofs pose a drainage problem because there is nowhere for the water to go after it is collected. On a slanted roof, any water that reaches the top of the surface will slide down into the gutter or off the side of the home.
Low-slope roofs do not have this angle, so any standing water will sit in the same location unless there is a drainage system in place. A flat roof can be compared to a level surface, like a street, when it rains. As water continues to pour down, it collects in puddles on the ground and stays in the same place.
This is one of the biggest drawbacks when it comes to flat roofs. Although they do not have adequate drainage by default, a low-slope roof comes with a long list of benefits. For instance, flat roofs are much easier to install and cost less than the alternatives.
Similarly, flat roofs are visually pleasing. They fit with the modern aesthetic and are required to complete modular home designs.
However, it should be noted that the lifespan of a low-slope roof is shorter, averaging 10 to 15 years. They require more frequent maintenance and are prone to leaks from standing water due to inadequate drainage.
What Types of Drainage is Best For a Flat Roof?
Despite its natural inability to drain water, systems can be put in place to improve the practicality of your flat roof. Gutters, inner drains, scuppers, and a siphonic roof drain are among the best options to eliminate standing water on a low-slope roof.
Gutters
Gutters are U-shaped components that run along the perimeter of the roof. They sit just below the overhang on each side to catch water, preventing it from entering the home.
There is a downward spout at the end of the gutter system that disposes of the excess water on the ground. Without gutters or a significant roof overhang, water from rain or melted snow would seep into the walls of the home, causing basement flooding or structural damage.
When choosing a gutter system for your home, there are several options to choose from. Seamless gutters, for example, are all one piece that runs around the entire home with no gaps. While this option is the least prone to leaking, professional installation is required, and any issues will require a full replacement.
Other types of gutters include double spout and two-piece systems. These come in several separate components and are attached by joints and connectors. While these options are easier to install and repair on your own, they require frequent maintenance and are more likely to experience leaks.
| Pros | Cons |
| •Cost-effective •Easy to install (DIY or professional) •Long-lasting •Direct water away from home (preventing roof damage and flooding) | •Can decrease curb appeal •Maintenance and cleaning required •Not ideal for cold weather |
Inner Drains
Inner drains are another popular solution to drainage problems caused by flat roofs. Instead of sitting on the edge of the roof’s overhang, an inner drain is installed toward the center of the roof.
They work by collecting water from the top of the roof and channeling it through a piping system to dispose of it, away from the house. This is great for curb appeal because the system is not visible from the outside like a gutter. However, if anything clogs up the small impression where the drain lies inside the roof, puddles will form on the top and lead to other issues.
When you have large collections of standing water on your roof, it’s inevitable that it will begin to leak into other areas, whether it’s the interior of your home from the ceiling or a structural problem within the walls. Contrary to an exterior gutter system, inner drains work well during the winter and in cold climates where water tends to freeze in the pipes.
| Pros | Cons |
| •Ideal for cold climates •Not visible from the exterior | •Expensive installation and costly repairs •Puddles can form when clogged •Possibility of interior leaks |
Scuppers
Scuppers are an easy and cost-effective way to support the drainage of water from your flat roof. They are typically used in tandem with a traditional gutter system.
A scupper is basically a hole that allows water to travel through and exit the roof. This is especially helpful for roofs without a significant angle, as water can easily form puddles when there’s not enough rain to push it downward toward the overhang.
In addition to being an affordable option, scuppers are easy to maintain. Any obstructions or buildup can easily be removed by hand from the opening.
It’s important to keep the size of your scuppers in mind, however. A size of at least 12 inches is recommended for easy maintenance, as clogs are more likely with smaller scuppers.
| Pros | Cons |
| •Cost-effective •Easy to maintain •Clogs and blockages are unlikely (large scuppers) | •Prone to clogs (small scuppers) |
Siphonic Roof Drain
Siphonic roof drain systems are similar to inner drains in terms of water removal. However, siphonic drains are not gravity-fed. While other classic drains rely on the weight of the water to push itself through the pipes, a siphonic drain is fully engineered.
It sucks the water in at high speeds and in large volumes. It also prevents additional air from traveling through the pipes along with the water. Similar to a shop vacuum, this method is quick and highly efficient.
Siphonic roof drains require fewer pipes and therefore take up less space as a whole. The installation can be complex, so professional help will be necessary for the best results.
| Pros | Cons |
| •Highly efficient •Space-saving •Quick drainage | •Expensive •Complex installation |
What is the Best Flat Roof Drainage System?
The best flat roof drainage system for your home will depend on a variety of factors, including cost, installation time, and suitability for residential properties. Scuppers and gutters are the most cost-effective options, while siphonic roof drains are best suited for commercial use.
The table below will give you a better understanding of where each drainage system option falls in each of these categories.
| System Type | Cost | Installation Time | Commercial | Residential |
| Gutters | $800-$4,000 | 1-4 hours | X | |
| Inner Drains | $2,000-$5,000 | 1-2 days | X | |
| Scuppers | $400-$800 | 2-3 hours | X | X |
| Siphonic Roof Drain | $10,000-$50,000 | 2-5 days | X |
What Type Of Drainage Systems Don’t Work on a Flat Roof?
While you can apply several drainage systems to a flat roof, not all of them will be as effective. Most flat roofs are angled at 10 degrees or less, with some being completely flat.
In the event of a roof with no slant, a gutter system may not work as well, depending on where it is placed. Since this is a gravity-based system, a completely flat roof will not be able to fully take advantage of it.
Similarly, scuppers are not the best option for flat roofs. Again, low-slope roofs at a slight angle of up to 10 degrees may still be able to use this drainage method, but several openings would be required to effectively trap the water.
Inner drains at the top of the roof and siphonic roof drains are best applied in these scenarios, as they make it easiest for water to travel through the system from a flat surface.
What Happens If You’re Not Using a Drainage System?
Not using a drainage system on the roof of your home comes with expensive consequences. The purpose of setting up drainage from your roof is to avoid the collection of water in unwanted areas.
If puddles of water are allowed to sit on the surface of your roof, they will begin to travel elsewhere eventually. This can cause leaks into your living areas or the foundation of your home.
When water sits on the surface of your roof for extended periods of time, it can damage the membrane. This contributes to cracks and can eventually present the need for repairs or replacements.
With such frequent maintenance requirements due to puddles of water, you can expect the lifespan of your roof to decrease dramatically. Using a drainage system that directs standing water away from your flat roof can eliminate the need for costly repairs and will allow you to enjoy your roof throughout its entire life expectancy.
This is why it’s essential to have a drainage system installed for your flat roof as soon as possible. Consult with a professional in your area to determine the best application for your household.
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Ruben has a diverse background in the home services industry, with experience running a construction company, a kitchen and bath showroom, and a moving and relocation company. This breadth of experience has provided him with a wealth of knowledge and expertise in various areas of home improvement in general and specifically in the heating and plumbing niche.