How to Drain Water from a Flat Roof

There are a lot of benefits to flat roofs. They are easy to repair, easy to maintain, affordable, and eco-friendly. But drainage is a big concern when you have heavy rain. Thankfully, there are a few excellent options for how to drain water from a flat roof.

What is a Flat Roof?

A flat roof, or a low-slope roof, is a roof that is nearly level. This is different from many other roofs, which are sloped. The degree of the slope is known as a roof’s pitch. A flat roof’s pitch is 10 degrees or less, though a flat roof is never truly flat. While there are many types of flat roofs, they’re all excellent for extra living space, rooftop gardens, or a living roof.

How to Drain Water From a Flat Roof

There are four common types of drainage systems that prevent water pooling on your flat roof – inner drains, scuppers, gutters, and siphonic roof drains. While each drain is different, they’re usually constructed from PVC, aluminum, copper, cast iron, or steel. Here’s what you need to know about the drainage systems and their pros and cons.

Inner Drains

Inner drains (also called internal drains) are most commonly found on commercial roofs but can be installed on residential roofs, too. As the name suggests, the water drains internally, so pipes are not visible. 

The drain is similar to a sink and is usually installed in the center of the large roof, with drainage pipes installed under the roof or in the parapet wall. Copper, aluminum, or cast iron are the most commonly used materials for inner drains

Pros of inner drains:

• Pipes are protected by the roof and walls of the building, which reduce the risk of bursting, cracking, or freezing
• Protect your walls and foundation from water damage
• Customizable, boosting your curb appeal

Cons of inner drains:

• Most expensive flat roof drainage systems
• Repairs require a professional roofer
• Susceptible to blockages; regularly clear debris and consider installing strainers to help prevent clogs

Scuppers

Scuppers are openings on the roof’s perimeter where water flows over the side of the building and to the ground below. Often suppers are combined with downspouts or gutters to avoid wall damage. There are many types of scuppers, including open, through-wall, decorative, channel, and more. Scuppers are typically built using galvanized steel or copper sheet metal.

Pros of scuppers:

• Cost-effective
• Easy to maintain
• Rarely clog
• One of the best ways to drain water from a flat roof

Cons of scuppers:

• If installed with gutters or downspouts, they will need to be checked for debris regularly
• If not installed with downspouts, there’s potential for wall and foundation damage where the scuppers drain
• Less effective in downpours

Gutters

Gutter systems are the most common way to drain water from any roof, including flat roofs. They catch the water as it runs off the edge of the roof and carries it to a downspout and away from the home’s foundation. 

There are several types of gutters, but they’re typically installed along the roof’s perimeter, protecting the siding and foundation from rain and water damage. Gutters are normally constructed with PVC, steel, copper, zinc, or aluminum.

Pros of gutters:

• Depending on the material, they can be DIY installed
• Cost-effective
• Effective in downpours

Cons of gutters:

• Depending on the material, they don’t last as long as other drainage methods
• Vulnerable to severe weather, like hail and high winds
• Need regular debris clearing and maintenance (watch for signs of clogged gutters)

A common choice for flat roofs, siphonic roof drainage systems pass water faster than other methods due to the siphoning. The system has only a few outlets, which are connected to one main drain pipe. The drains have no slope because the siphoning removes the water smoothly and efficiently. 

Siphonic roof drains are a good choice if you have other equipment on your roof, for example, an HVAC system. A siphonic roof drain system is typically constructed from PVC, cast iron, or steel.

Pros of siphonic roof drains:

• Few clogs due to siphoning
• Very little maintenance
• Excellent for heavy rain

Cons of siphonic roof drains:

• Small debris can block pipes
• Can be noisy in some situations

Which Draining Method is Right for Your Flat Roof?

Choosing the right flat roof drainage system for your home depends on your priorities. Most homeowners evaluate three things – price, durability, and maintenance needs. Let’s take a closer look to help you decide which draining method is right for your flat roof.

Price

Gutters

The average cost of gutter installation is $1,275. This cost varies depending on the materials and the size of your roof.

Scuppers

Scupper installation costs about $400 to $800 each, not including downspouts or gutters.

Siphonic Roof Drains

The average cost to install siphonic roof drains is $3,850. The price will vary depending on the size of your roof.

Inner Drains

They’re widely considered the most expensive drain type, with an average cost of $4,323.

Durability

Gutters

Gutter durability depends on materials. Vinyl is easily damaged by strong weather, aluminum can dent from hail, and copper stands strong in most weather conditions. While the lifespan of gutters varies widely, their average lifespan is 20 years.

Scuppers

Scupper durability depends on their design and material. A scupper that doesn’t jut from the building is less likely to be damaged by weather and falling debris than a scupper that does protrude from the building. On average, scupper lifespan is about 15-20 years, but this varies depending on materials used

Inner Drains

Because inner drains are inside the roof, they’re not susceptible to weather. Just be careful of clogs, as they can cause big problems. Although their lifespan can vary depending on materials, their average lifespan is about 50-60 years.

Siphonic Roof Drains

Heavy rainfall is where siphonic roof drains shine, but they’re susceptible to high winds, hail, and falling limbs. Their expected lifespan is about 17 years, but this will vary depending on the materials.

Maintenance Needs

Gutters

Maintenance needs for gutters vary depending on the material the gutters are made of. However, a good rule of thumb is to clean your gutters twice a year, once in the spring and again in the fall. For caked-on dirt, use a store-bought or homemade gutter cleaning solution. Fix cracks or sagging as the problems arise.

Scuppers

For scuppers, simply clean and remove debris as needed – no more than once or twice a year. The smaller the opening, the more often they must be cleared. For scuppers that protrude from the building, clean off caked-on grime with a cleaning solution as needed.

Inner Drains

Keep openings to the drain cleared of debris. Strainers are highly recommended.

Siphonic Roof Drains

They rarely clog due to their design, but checking them once or twice a year is a good idea. Clear any debris you see.

FAQ About Draining Water From Your Flat Roof

Get Help Fixing Your Roof Drainage Problems

Unless you have a lot of experience with flat roofs and their drainage problems, you’re best bet is to call a professional roofing company. A vast range of issues could plague a flat roof that isn’t draining right. Plus, roof repair is dangerous. Falling is the second leading cause of accidental deaths in America.

But finding a roofing company you can trust can be overwhelming. If the daunting task of finding a roofing contractor is stressing you out, RoofGnome can help. We’ll shoulder the stressful research and option weighing and connect you with a highly-rated, local roofing professional. Your flat roof will be in tip-top shape again in no time.

Main Image Credit: DenBoma / Canva Pro / License