Commercial buildings have flat or low-sloped roofs that need special drainage solutions. This article provides an overview of the types of drainage systems suitable for commercial buildings and the materials and components of those systems. Also, we’ll cover what kind of maintenance a flat roof drainage system requires.
The basic function of a roof is to protect the building from the elements. As the roof collects rain, the water must be transported quickly and safely away by a drainage system. While pitched roofs can rely on gravity to move water, flat roofs can’t just rely on a typical gravity‐driven gutter system. Commercial flat roofs need interior drains and scuppers to collect water instead.
Though many think flat roofs are completely level, they are actually designed with a slight pitch. Architects and structural engineers add the pitch in order to create positive drainage. This small slope will direct water towards the drains, whether they’re in the centre of the roof or to one side. Positive drainage should allow the water to drain off the roof 24-48 hours after it stops raining.
Architects must also take into account the weight of water on the roof, including near the drainage areas. Water adds significant weight to a building, especially if the drains are clogged or poorly implemented. Water collection can overload a roof’s weight capacity and can even lead to a collapse. This can happen if the water simply doesn’t drain fast enough, creating a backlog of water. Backup drains can mitigate this problem. The backup drains are positioned higher than the main drain, just like the backup drain in your bathtub. If the main drains can’t drain water quickly enough, the water level will rise to the height of the backup drains, where it can flow out safely.
Water may also collect on the rooftop in areas where it is too flat to have positive drainage or where a depression has formed. This water collection is called ponding. Over time, materials, such as wood and concrete, will sink under the load of the water, and insulation may compress. This will create low points on the roof, which may collect even more water.
Ponding, or any standing water on a roof, can contribute to several different problems:
Positive drainage should dry the roof completely in a day or two after a rain, assuming the weather holds. If the roof still has ponding after this time period, then it and the drainage system need to be assessed.
Water collection can be more serious than ponding. Overflow can create leaks by pushing water into areas it would not normally go. The severity of the damage depends on where the water ends up. All drainage systems should have backups to prevent overflow in the case of severe rain or in the case of a clog in the main drainage system.
To find out if the building’s drainage system is performing as it should, you first have to know which type of drainage system the building has and if it’s the right one for the roof. While each drainage system will be different, to best accommodate the roof it drains, there are only two primary drainage systems that are suitable for commercial flat roofs or low slope roofs:
1. Gravity Drainage
A gravity drainage system simply relies on gravity to move the water from the rooftop to the ground. Usually, a gravity drainage system has four sloping drain segments that connect to an interior drain, which then carries the water outside of the building.
A gravity drainage system on a flat roof differs from the gutter system you find on residential roofs in two key ways:
Gravity drainage systems are only recommended for flat or low-slope commercial roofs that have a surface area of less than 150m2 per drainage point. Larger roofs collect more water that may not drain quickly enough with just a gravity system. Those roofs benefit from siphonic drainage systems.
2. Siphonic Drainage
A siphonic drainage system has, on the drainage points, a baffle that prevents air from entering the system. As a result, the pipes have lower atmospheric pressure than the outside environment. Thus, when water enters the drain, it meets an area of lower pressure in the drain and is pulled in faster than it would be by gravity alone. In fact, a siphonic drainage system can achieve 100 times the drainage speed of a gravity drainage system.
In a siphonic system, several drainage points will meet up and deliver the water out of the building. However, this internal drainage pipe doesn’t have to run on an incline, as the system does not need gravity to function. Instead, the collector pipe is installed at the highest possible position and then runs flat until it connects with the “leader” vertical down pipe. This allows for more room for other building features below the drainage system. Also, the roof itself may have a smaller slope and still drain properly.
A building with a large roof footprint, such as an airport, mall, warehouse, convention center or factory, is well‐suited to a siphonic system because the weight of water on these buildings is a bigger concern. If the water on these roofs does not drain fast enough, then it can add significant weight to the roof. In fact, a single inch of water on a 20-square foot area of the roof weighs 2,000 pounds. That is a challenge for architects to plan around, so roofs need to drain water as quick as possible. Siphonic drainage systems are generally recommended for larger roofs of 150m2 or more.
Both types of drainage systems have similar components. It’s wise to understand those parts because armed with knowledge, you will be able to make the best decisions possible for the drainage system:
1. Interior drain: This is a drainage point that can be installed across the roof’s surface, even in the centre of the roof. If water collects in one spot in the middle of the roof, then installing an interior drain there to move the water is a good solution. Interior drains work best when the roof is sloped to allow water to gather at the drainage point. However, that isn’t always enough; so interior drains can be used in combination with scuppers, which drain water at the edge of the roof.
An interior drain has several components:
Different drainage systems require slightly different interior drains. Your roof might need a high-capacity drain, or a dual-outlet drain, to move more water off the roof faster.
Another interesting interior drain modification is a sump pan. These square pans create a recessed drain, which draws water towards drains that otherwise might be too high for smaller amounts of water to reach.
2. Edge drains: Edge drains collect water at the edge of the roof. To understand the need for edge drains, think of a public shower. Just like a commercial roof, the ground in a public shower is flat. If enough shower heads are on, the water will spread out away from the drains, and a backlog of water will develop. On a roof, this is a problem because you don’t want water to reach the edge and run down the side of the building. To solve this problem, professional roofers install edge drains to catch that excess water and ensure that no one on the sidewalk below the roof gets splashed.
There are a few different types of edge drains: scupper drains, gutters and roof drain leaders (also called downspouts):
You also have to consider the material these parts are made of. Drains and pipes of different materials can be of vastly different quality and can solve different problems you might face with your drainage system. Each of these materials has its own benefits and drawbacks.
Interior drainage piping made from any of these materials will require insulation to keep the water from freezing in cold temperatures.
Why do you need to insulate your drainage system in the first place? Drains are roof features that penetrate the roof and may allow conditioned air to escape. Without roof insulation, drains could cost you a fortune in heating and cooling costs. Also, water may freeze in the pipes and damage your system.
How much insulation your drainage system needs will depend on your climate. In fact, in 2012 and 2015, the International Code Council changed its rules in the International Energy Conservation Code (IECC), requiring more insulation for roof drains.
The first change was an increase in thermal R-value a roof has to achieve in each of the American Society of Heating, Refrigeratng and Air‐Conditioning Engineers (ASHRAE) regions. The second change is that this R‐value must be achieved within four feet of the roof drain. Formerly, you only had to achieve the R‐value as an average across the roof. Now, in order to meet the value within four feet of the drain, most roofs will need considerably thicker insulation, or a new type of insulation, such as polyiso insulation.
Your state, province or other local jurisdiction may have adopted the IECC’s changes, or it may not have. It is incumbent upon the roofing professional and building owner to find out which insulation rules apply to their drainage system and to abide by them.
Your locality may have also implemented changes to drainage rules to help deal with urban runoff problems. Rooftops in dense city areas collect a large amount of water and not all municipalities have siphonic drainage systems that can move this vast volume of water quickly. Instead, the water backs up and may flood into people’s homes, as well as rivers, streams and bays. As this water is untreated, it may be a health risk.
To prevent overloading their drainage system, municipalities may require that commercial buildings install control flow drains. These drains actually slow the rate of water flow off the roof, so the volume of water is more manageable for the city’s infrastructure. For a few hours after the rain, a commercial roof with a control flow drain will act as a reservoir, holding the water to prevent damage to property and wildlife.
Another solution that can help storm-water management efforts is vegetative roofing, which can absorb vast quantities of rainwater. Instead of being delivered into the municipalities’ drainage system, the water will be used by the roof’s vegetation, over time.
Building owners will also benefit from familiarizing themselves with the basic requirements of drainage system maintenance. On an annual or semiannual basis, all drainage systems should be inspected and cleaned. If the roof has a history of drainage clogs or problems, then it should be inspected and cleaned even more frequently. Further, if the building is in an area where trees shed their leaves every fall, then one of these cleanings should take place at the end of autumn when the system will collect the most debris.
Roofing professionals shoulds:
Any trees or other landscaping features that are close to the roof should be trimmed. While some roofing professionals may perform this task, it may be better to hire a landscaper to do this work before the roofing contractor arrives. If you wait until after, then the landscapers may unknowingly leave debris on the drain.
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Original article source: IKO