Roof Drains: Types, Maintenance, and Installation Guide

Written By Adam Paul - Waterproofing Expert

A roof drain is a specially designed plumbing fixture installed on flat or low-slope roofs to collect and channel rainwater or melted snow away from the roof surface. Roof drains are typically installed at the lowest point of the roof. The main taks of the roof drain is to feed the collected water into piping system. Roof drains prevent ponding water, protect insulation, reduce structural loads dueiring rain fall, enhance roof lifespan and minimize the mold growth to the building.

Roof drain consists of a drain body, flange, strainer or dome, clamping ring, and the tailpiece connecting to the piping. These components ensure a consistent water flow to the pipeline during the rain. The roof drain system is essential for flat and lower slope roofs. A roof drain should have a 1:40 ratio of inclination. It should be located at the lowest point of the roof.

Roof drains are mainly of 5 types; among them, siphonic drains are the most efficient roof drains. The scupper size of the roof drain ranges from 4″×4″ to 12″×12″. The downspout diameter of the roof drain ranges from 2″ to 6″. A well-designed and installed drainage system can handle 100 GPM (378 L/min), which is highly recommended for the large commercial roof areas. If you install EPDM, TPO, PVC, modified bitumen, and BUR, you should install the roof drain properly.

The roof drain can be clogged and overflow due to improper maintenance and repairs. Hence, you should clean and maintain the roof drain periodically. A roof drain has both pros and cons. If there is no roof drain on a flat or low-slope roof, water has no direct path to exit the surface, which leads to a series of serious roofing issues. So Aqviz experts always recommend you keep the roof drain properly to remove the accumulated water on the roof efficiently.

What Is a Roof Drain?

A roof drain is a specially designed plumbing fixture installed on flat or low-slope roofs to collect and channel rainwater or melted snow away from the roof surface. Roof drains are typically positioned at the lowest points of the roof to allow gravity-fed drainage into connected piping systems. Roof drains are installed on residential and commercial roofs to prevent water accumulation, structural damage, and interior leaks. 

These are the 5 major benefits of a properly installed roof drain.

1. Prevents ponding water and leaks on the roof
2. Protects insulation and roofing membranes from saturation during rain
3. Reduces structural load stress on the roof during heavy rainfall
4. Enhances the lifespan of roofing systems
5. Minimizes mold growth and interior water damage to the building

How Does a Roof Drain Work?

A roof drain works by collecting rainwater through a drain bowl or dome grate and channeling it into a vertical leader or downspout, which connects to the building’s internal drainage system. From our years of roof waterproofing at Aqviz, we know the efficiency of roof drains depends on proper slope design, filter domes, clamping rings, and watertight seals that keep debris out and prevent backflow or leakage.

Key parts of a roof drain include the drain body, flange, strainer or dome, clamping ring, and the tailpiece connecting to the piping. These components work together to ensure consistent flow and protection. We always ensure each connection is sealed with waterproofing compounds to prevent hidden seepage beneath the roofing membrane.

What is the Purpose of a Roof Drain?

The main purpose of a roof drain is to collect and discharge rainwater or snowmelt from the roof surface, preventing water accumulation that can damage roofing materials and structural components.

Roof drains help to solve multiple problems, including ponding water, freeze-thaw damage, roof sagging, and fungal growth. They’re essential for flat roofs, low-slope roofs, green roofs, and commercial roofing systems with large surface areas. Membrane roofs like EPDM, TPO, and built-up roofs often require integrated roof drainage systems to maintain performance.

What Is the Slope Gradient of Roof Drainage?

The slope gradient in roof drainage refers to the angled incline of the roof surface that directs water flow toward the roof drain. It’s usually expressed in a ratio such as 1:40 or in inches per foot, like ¼ inch per foot. 

This slope is part of the roof design or created using tapered insulation to guarantee effective water flow. The slope gradient helps roof drains perform efficiently by using gravity to guide water to the drain points. Without the proper slope, even the best drain can’t function properly.

Where to Locate a Roof Drain?

Roof drains should be strategically located at the lowest points of the roof surface to ensure efficient water removal. Proper placement of the roof drain ensures water doesn’t sit or back up, which is a common cause of premature roof failure.

As waterproofing experts, we typically install roof drains near structural valleys, at midpoints between high spots, and around roof edges if scuppers are part of the system. For large commercial roofs, multiple drains are spaced evenly to manage high volumes. Around rooftop HVAC units or skylights, we add auxiliary drains to catch overflow and prevent ponding in isolated areas.

What Are the Roof Drain Types?

There are 5 main types of rain drains, each rain drain has designed for specific roofing systems and drainage needs.

1. Primary roof drains: Primary roof drains are installed at the lowest point of the roof to handle regular rainfall and connect to internal drainage systems.
2. Overflow drains: Overflow roof drains are installed slightly above the primary drain level to manage excess water during heavy storms and prevent roof overload.
3. Scupper drains: Scupper drains are openings in the parapet walls that allow water to drain off the edge of the roof, often used in conjunction with downspouts.
4. Gutter drains: Gutter drains are external systems that collect water from sloped roofs and transport it through downspouts to ground-level drainage.
5. Siphonic roof drains: Siphonic roof drains are designed for large commercial roofs, these use vacuum-assisted systems to move water quickly without the need for sloped piping.

The best roof drain depends on the roof type, but we recommend siphonic drains are the most efficient for large flat roofs due to their high-capacity flow and reduced pipe size requirements. For residential flat roofs, primary roof drains with overflow protection offer reliable performance with proper waterproofing.

What Is Scupper Size?

A scupper is an opening in the edge wall or parapet of a roof that allows rainwater to drain off the side instead of accumulating on the surface. Scupper size refers to the width and height of the opening, which typically ranges from 4 inches × 4 inches to 12 inches × 12 inches.

If the scupper size is too small, it restricts water flow, causing overflow or ponding. If it’s too large, it may allow debris to enter, leading to blockages. Larger scuppers can handle higher volumes of water, which is critical during heavy rainfall or in overflow situations.

What Is the Downspout Diameter of a Roof Drain?

The downspout diameter of a roof drain is the internal width of the vertical pipe that carries water from the drain to the ground or stormwater system. This measurement directly affects the flow capacity and how efficiently water is evacuated from the roof. 

Available downspout diameters typically range from 2 inches (51 mm) to 6 inches (152 mm) or more for larger commercial systems. A 3-inch (76 mm) downspout is common for residential roofs, while 4-inch to 6-inch sizes are typical for larger or high-capacity roofs. Undersized downspouts can cause backups and overflow, especially during heavy storms, so we always size them to match the roof’s drainage demand.

What Is the Drainage Capacity of a Roof Drain?

The drainage capacity of a roof drain refers to the volume of water it can handle and discharge efficiently during a given period, typically measured in gallons per minute (GPM) or liters per second (L/s). The recommended drainage capacity depends on the roof size and rainfall rate. As a general guideline, one roof drain with a 4-inch (102 mm) outlet can handle up to 100 GPM (378 L/min).

At Aqviz, we calculate drainage capacity based on local rainfall intensity, roof area, and slope gradient to prevent water buildup and protect the structure. For larger roofs or high-rainfall regions, multiple drains with higher capacities must be installed. Undersized systems can lead to ponding, membrane stress, and eventual leakage.

What Roof Membranes Require Roof Drain?

Roof membranes that are installed on flat or low-slope roofs require roof drains to ensure proper water removal. So we highly recommend that you to install a roof drain for these roof membranes.

1. EPDM (ethylene propylene diene monomer)
2. TPO (thermoplastic polyolefin)
3. PVC (polyvinyl chloride)
4. Modified bitumen
5. Built-up roofing (BUR)
6. Liquid-applied polyurethane or acrylic membranes
   

How to Prevent Roof Drain Clogging?

To prevent roof drain clogging, you can follow these proven maintenance and installation practices by Aqviz.

1. Install dome strainers: Use high-quality metal or plastic strainers to block leaves and debris from entering the drain.
2. Clean roof drains regularly: Inspect and clean the roof drains every 3 to 6 months, especially after storms or heavy winds.
3. Trim overhanging trees on the roof: Reduce the risk of falling leaves and branches that can clog the drain system.
4. Check scuppers and gutters: Keep all connected outlets clear to avoid water backup into the main roof drain.
5. Flush roof drain with water: Periodically flush the drain lines using a garden hose to ensure free flow and detect hidden blockages.
6. Use leaf guards or debris screens: Add protective screens at roof openings to filter larger debris before it reaches the drain.
   

Why Does a Roof Drain Overflow?

A roof drain overflows when it cannot remove water quickly enough, often leading to ponding or backup into the roofing system. From our years of service life at Aqviz, here are 5 common reasons for the roof drain overflows.

1. Clogged strainer or pipe: Accumulated leaves, dirt, or roofing debris block the water flow path of the roof drain.
2. Undersized drain or downspout: The roof drain cannot handle the water volume during heavy rainfall.
3. Insufficient slope gradient: Poor roof slope fails to direct water efficiently toward the drain.
4. Frozen drain outlet: In cold climates, ice buildup can block the outlet the roof drain and cause overflow.

You should inspect the roof periodically to prevent possible structural problems on the roof.

What Are the Cons of Roof Drain?

These are the 6 cons of a roof drain

1. Clogging risk: Leaves, debris, and dirt can easily block the roof drains. Especially if strainers aren’t installed or maintained properly, it can lead to overflow and water damage.
2. Maintenance demand: Roof drains require routine inspections and cleaning. Ignoring this can result in hidden leaks or structural deterioration under the membrane.
3. Cost of installation: Internal roof drainage systems are more expensive to install compared to external gutters due to piping, waterproofing, and labor.
4. Leak potential at joints: Improper sealing at the drain flange or pipe joints can cause seepage into the insulation and deck, especially in older systems.
5. Complex repair and access: Internal roof drains often run through ceilings and walls, making access for repairs more difficult and invasive compared to external systems.
   

What if there is No Roof Drain?

If there is no roof drain on a flat or low-slope roof, water has no direct path to exit the surface, which leads to a series of serious roofing issues. Without a roof drain, water will accumulate and cause ponding, which increases the roof’s structural load and may lead to deck sagging or collapse over time.

Prolonged exposure to standing water breaks down roofing materials, promotes mold and mildew, causes insulation saturation, and accelerates membrane aging. In short, missing or inadequate drainage shortens the roof’s lifespan and increases maintenance costs significantly.

Are Roof Drains Corrosion-Resistant?

Yes, roof drains are designed to be corrosion resistant, but the level of resistance depends on the material used. At Aqviz, we always recommend installing roof drains made from high-durability materials like cast iron, stainless steel, aluminum, or PVC, depending on the roof type and environmental conditions.

Cast iron drains are highly durable and commonly used in commercial buildings due to their long lifespan and strong corrosion resistance with proper coatings. Stainless steel offers excellent rust protection and is ideal for chemically exposed environments. PVC drains are lightweight, rustproof, and cost-effective, making them suitable for residential roofs. Corrosion resistance is crucial because continuous exposure to rainwater, debris, and UV radiation can degrade metal over time, causing leaks or failures if low-quality materials are used.

What if the Roof Drain is Clogged?

If a roof drain clogs, water can no longer exit the roof properly, resulting in ponding, increased roof load, and eventually leakage or structural stress. At Aqviz, we’ve seen clogged roof drains cause major problems like roof seepage, roof blistering, ceiling mold, insulation rotting, and interior water stains. Overflow water may even back up through secondary outlets, damaging parapet walls and flashing.

How to Maintain a Roof Drain?

You should follow these 6 methods to maintain a roof drain properly.

1. Inspect the roof drain monthly
2. Clean roof drains after storms
3. Flush the roof drain
4. Secure strainers and domes
5. Check for roof drain corrosion or cracks
6. Seal and reseal roof drain joints

What is Roof Pitch?

Roof pitch refers to the angle or steepness of a roof surface, expressed as a ratio of the vertical rise to the horizontal run, such as 4:12 (4 inches of rise for every 12 inches of run). 

A low-slope or flat roof (0.5:12 to 2:12) relies on roof drains or scuppers to manage water flow, while steeper roofs (above 4:12) typically use gutters and downspouts. Roof pitch impacts how quickly water drains off and whether standing water might become a problem. The correct pitch ensures efficient runoff, reduces pooling, and helps extend the roof’s service life.