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How Do Flat Roofs Drain

Flat Roof Inspect And Maintain Roof Drains On Flat Roofs

Flat Roof Drains design and installation – the Ultimate draining system for flat roofs

Roof drains can also become clogged by debris and cause major damage in a heavy rain. These need to be checked at least 4 times a year. Problem drains need to be checked once a month or once every other month. There is nowhere for the water to go, except to fill up the flat roof. Hopefully, there are overflow scuppers, which can also become clogged.

The roof drain inspections should be scheduled so that they coincide with the rainy season of the area. Checking them before rainy season is best, so they will perform as designed. This is important because, if they get clogged up, could cause major damage. Even with a screen or basket on the drain itself, debris can clog up the flow of drainage. Granules can get past the basket and actually clog up the pipe itself. Debris from trees can get around the basket and clog up the flow into the pipe. There are several different kinds of roof drainage that is used in different situations.

The time when the inspections will come into play is when a storm comes and dumps 8 inches of water in several hours. That is when the performance of your roof system and roof drainage system shows.

Just making sure there are drain baskets on them is a good idea. Many drain baskets are plastic and deteriorate over time, and just blow away. Now larger items can get down there and block it up.

Where Are Roof Drains Specified

1108.1Secondary drains or scuppers. Where roof drains are required, secondary roof drains or scuppers shall be provided where the roof perimeter construction extends above the roof in such a manner that water will be entrapped if the primary drains allow buildup for any reason.

What is flow drain?

Drainage flow is the volume of water diverted by collector and drainage network from a reclaimed area for a certain period of time it is measured in l/s, m3/s, m3/ha .

How do you calculate the flow rate of a roof drain?

Drainage Systems: Gutters And Down Spouts

Gutters and down spouts are perhaps the most familiar type of drainage roof drainage, since the vast majority of residential homes make use of this style. Essentially, the roof of a building is encircled with guttering that catches rain water as it runs off the roof. The water then travels around the guttering until it reaches a down spouta vertical pipe leading down to a designated drainage spot. Of course, for this to work, your roof will need to have a sufficient slope to cause the water to run off to the sides, rather than pool in the middle, but your roof should have such a slope regardless, even if it is a flat roof. The main advantage of this style of roof drainage is that it is inexpensive and easy to repair. If a piece of guttering becomes damaged, it can simply be replaced. Being on the outside of the building makes it more readily accessible than some alternatives, but it is not without its downsides, the main one being the ease with which it can become blocked. Because the guttering is so open to the elements, it often fills up with leaves and other debris that is moved around in the wind, which can clog up the guttering and, in some cases, the down spouts. Worst still, it can become home to wildlife which will only exacerbate said clogging. All in all, gutters and down spouts are an effective and inexpensive drainage system, but you should be prepared for the necessary maintenance to keep flowing freely.

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How Do Flat Roofs Drain

Flat roofs can transform any home by providing an opportunity for a gorgeous rooftop garden or a stylish, weather-resistant outdoor living area but what happens when it rains or snows? How do flat roofs drain?

Its important to know how flat roofs drain so you know how to best maintain them. There are three basic flat roof drain systems: gutters, interior drains, and scuppers. Read on to learn more about how these draining systems work.

Three Common Flat Roofing Drain Systems

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Since flat roofs cannot shed water like a sloped roof, there are three different types of drain systems that are used to drain the water off the rooftop. They are gutters, drains, and scuppers. For each type of draining system there are pros and cons, how well it works is dependent upon your flat roof what roofing system you have, any parapet walls, lots of trees and so on. Regardless of what type of draining system you have, it still needs to be maintained by keeping it clear of debris because any small amount of debris can cause a blockage for the water to exit the roof. Here are the three common drain systems for flat roofs.


Many flat roofs still use the gutters. Because the roof is flat there needs to be some kind of lean to direct the water to the gutters. Most of the flat roofs with gutters are not surrounded in all directions by parapet walls so the gutters are be installed where there is no wall and normally are bigger than the traditional 5 and 6 gutters on sloped buildings. If there are gutters on a flat roof they still need maintenance and care. Depending on the material that the gutters are, major neglect can cause rust, erosion and gutters can begin to rot. In order for them to work properly they need to be clear of debris. If they are not maintained they can get to the condition you see in the photos where the water just ends up sitting in the gutters and not draining down the downspouts.



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Is My Roof Really Flat

While your roof may be considered a flat roof, it is not truly flat. A totally flat roof would never drain. Its recommended that every roof has a slope to direct water to drains at its center or perimeter.

A properly designed flat commercial roof will move water quickly to its drainage system. Water that does not have a drainage pathway will accumulate, unnecessarily adding weight to your roof. If there is any breach in the roof membrane or flashings, this standing water will find its way into the building.

Gravity is your roofs best friend when your drainage system is in good working order. But it can also be your worst enemy as water tries to find its way to the ground. Pooled water will find the most minor crack or hole in your roof membrane or flashing and work its way inside. Standing water also degrades the underlying structure of your roof over time. Its a warning sign that your roof is not draining correctly.

Where Do Flat Roof Drains Go

Flat roofs can have a system of drains located at certain points on the roof. The drains are covered with a strainer to keep out debris and connect to the pipes inside the house.

Flat roof drains can be installed in a number of different ways, but the most common method is to install a drain pipe at the base of the flat roof.

Alternatively, you may be able to use a drainage pipe that is attached to the outside of your flat, which is then connected to your homes plumbing system.

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Adding Drains For Ponding Roofs

It is possible to add new roof drains to relieve ponding on a sagged flat roof, as shown at left.

Notice that the new drain extends to the building exterior where it is conducted to ground or to a storm drain.

Using this method permits adding a drain at the roof’s low point.

When the building is re-roofed, the roofer may install tapered foam insulation to direct water to roof scuppers or to a main or new-main roof drain to prevent future ponding. Sketch courtesy of Carson Dunlop Associates .

Types Of Flat Roof Drainage Systems

Flat Roof Drain – How it works and best practice
  • Clark Roofing & Siding LP., one of the most trusted roofing companies in Edmonton and Calgary providing residential and commercial flat roofing systems, is often approached by home and business owners who want to know more about drainage systems for flat roofing. The best drain for your building will depend on your building style and location, but weve put together a handy guide that can help you decide on the right system for your needs. There are four main types of drainage systems to help you with water flow on your building, including gutters, drains, scuppers and sloped installation.
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    The Right Drainage System For Your Flat Roof

    Any of these systems can effectively divert water from your flat roof, provided it is correctly installed and properly maintained. Its important to strongly consider each drainage systems strengths and weaknesses before making your decision, including cost, durability, and maintenance requirements. When in doubt, consult your roofer and consider your building type and location to help you come up with the best drainage system for your flat roofing system.

    Hire Our Experts To Identify Any Problems In Flat Roofs

    When you work with Integrity Roofers to solve your roof drainage problems, you can count on a team of seasoned professionals to complete any work necessary. We have two decades of service in the business, offering the best roofing advice in the Greater Toronto Area. Our counsel is above reproach, and we will recommend the best course of action for your roofing system. When you bring our company in, we will take a full roofing system view and advise you about your current situation and what you can expect going forward in the area of maintenance and or replacement for your roofing system. To get an idea of the cost for your flat roofing work, please complete our estimate form for a comprehensive look at the total costs. If you want to speak to a roofing expert, call our office today at Integrity Roofers at .

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    Why Is Drainage Important For A Flat Roof

    Because of the nature of their design, flat roofs do not have the natural drainage that sloped roofs offer, where gravity is working to remove water build-up. It is essential that flat roofs are fitted with an effective drainage system to ensure they are removing any moisture from the roof, to avoid serious damage. There are several issues that may result if a proper drainage system is not installed on your flat roof, including:

    • The pooling of water and moisture, leading to leakage in the building or structural damage. Pooled water can also damage your roofing system, including the tar or caulking.
    • The build-up of ice and snow in winter months, placing a heavy weight on the roof and potential structural damage.
    • Leakage through roof ventilation, such as chimneys or air vents.

    The Critical Role Of Flat Roof Drainage Systems

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    Although many people don’t realize it, water can be a roof’s biggest enemy. That’s the reason that you won’t find a roof anywhere in sight without some sort of gutter or drain system. It’s also why people go through so much trouble to keep them from clogging. Water can cause underlying wood to rot, mold growth, and cause damaging leaks to enter the building’s structure.On a flat roof, the risk of water damage is multiplied. That’s because a flat roof is prone to allowing water to pool up, which amplifies its’ damaging effects. On top of that, pooling water on a flat roof is a cumulative problem â the longer it remains, the bigger the affected area gets.

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    Help From Roofing Professionals

    When it comes to choosing a drainage solution for your flat roof, it is important to consider the cost of the system, its durability, and the type of maintenance required. Every drainage system discussed here is vulnerable to clogs from debris, but you can largely avoid this risk by using scuppers with no downspouts.

    If you are not sure which drainage system will work best for your roof for your money, consult the roofing professionals of Reliable Roofing. Our technicians install and maintain all types of flat roofing and we can help you choose, install, and maintain the right drainage solution for your home or building.

    No soliciting please

    How Many Drains Does A Flat Roof Need

    Take the roofs total square footage and divide by the total square footage handled by one drain. The number of drain needed is determined by the result.

    If you want to know how many square feet of roof space is needed for a given amount of water, you can use the following formula: 1 square foot = 1/4 of a square meter.

    So, for example, if you have a roof with a total floor area of 1,200 square meters, then you would need to have 11,360 sq. ft. of floor space to handle the water.

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    Common Flat Roof Drainage Solutions

    By Reliable Roofing

    There are many architectural designs for homes and commercial buildings that incorporate flat roofing. A flat roof not only works with the aesthetic of many building designs, but it is also durable and resistant to environmental factors that can lead to roof damage such as wind, hail, and animals. Flat roofing in residential settings may also be used for an outdoor living space or rooftop garden, adding a unique element to the building.

    The one challenge with flat roofing is ensuring proper drainage. Unlike other roofing types that are slanted or sloped to allow water to drain automatically, flat roofs need a built-in drainage system to help remove moisture and debris. Poor drainage can lead to serious issues with flat roofing, from leaks to structural damage. There are several drainage solutions available for flat roofing and it is important to consider the pros and cons of each system before making a decision. This guide explains the advantages and disadvantages of the three most common flat roof drainage solutions interior drains, gutters, and scuppers.

    How To Drain Water From A Flat Roof

    4 Ways To Drain Your Flat Or Low-Sloped Roof Residential Roofing, Roof Types

    Therefore, if you have decided to install a flat roof, you ought to be aware of how you can drain it. Many flat roofs have been damaged and have eventually collapsed by stagnant water. This article will recommend some reliable ways you can drain water from your flat roof.

    Inner Drains

    Many commercial properties employ inner drains. This has to do with positioning pipes beneath the roof or within the parapet to ensure the space is appealing without showing any pipes.

    This particular draining system is positioned at the roofs middle or anywhere the sloped drain is channeled to. Inner drains are most suitable for large roofs.


    • Inner drains let you use the whole of your roof without any hindrance.
    • It does not cause any damage to the wall because of the pipes.


    • Trying to service and maintain inner drains is an expensive adventure. That is because the pipes are inserted beneath the roof.


    In this drainage system, an opening is created by the walls side, where water is channeled to the downspouts or gutters. Many homeowners love to combine scuppers with downspouts to offer extra protection against water damages caused on the walls. A collector head alongside the scuppers will equally protect the walls.


    • Scuppers dont cost as much as inner drains. Wide scuppers seem like the best choice.
    • It does not allow debris to obstruct water flow.





    Which Method Should I Use?

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    What Is A Flat Roof

    A flat roof is exactly what it sounds like: a roof that is flat without any angles or pitches. Its common for commercial buildings to have a flat roof thanks to their easy, low-cost installation. Theyre also commonly seen in older homes.

    There are several types of flat roof systems available to homeowners, such as built-up roofing, rubber roofs, and modified bitumen systems. Many flat roofs are installed on a slope to allow rainwater to flow off the roof. Some others are installed with a unique flat roof drain system to handle precipitation.

    How Do Flat Roofs Drain Rainwater

    If youre considering installing a flat roof on your home you may well be wondering the obvious question how do flat roofs drain? How do you avoid having water pool on the top and cause damage over time?

    Well weve put together the basic facts here so you can rest assured that a properly installed flat roof will not have any problems draining water.

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    Options To Drain Your Flat Roof

    • Interior Drains

    The roof of your commercial business building is one of the more underrated aspects of your business as a whole, but what it represents is so much more. Sure, it can be viewed by some as just being the roof of a building, but that building represents you as a business. And the roof is what separates you and your clients and customers from the elements.

    One of the most common issues with flat commercial roofs is water accumulation. Water building up anywhere on a homes or commercial buildings roof can damage the roof. This is because water does not drain effectively, pools up, and then causes damage to both the roof and underlying structure.

    This can be especially devastating when there is damage to the façade of the roof. This means that water gets into areas it is not meant to be in and then permeates throughout the rest of the roof. This can lead to additional water damage and potential structural damage that can be dangerous for the building and its inhabitants.

    Thankfully, there are a few ways to ensure that the water drains from your flat commercial roof, keeping it free and safe from potential water damage.


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