Roof Drain Specification Explained

Michael Tobias
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    There are a variety of roof drains available to specify for construction use in Chicago, suitable for flat and low slope roof applications on new and existing buildings. Leaving a flat roof without a drainage system increases the risk of ponding and damage from water overflow or leakage. Fitting the correct roof drain, to the correct size and quantity will ensure proper drainage helps keep buildings in the city protected.

    Before we explain more about the various roof drain types, their advantages and disadvantages, location suitability and other criteria, it’s important to understand the prevailing regulatory context to their application in Chicago.

    Regulatory Context to Roof Drains

    The regulatory backdrop to roof drain installation in Chicago is mainly governed by the City of Chicago Chicago Building Code but also the City of Chicago Sewer Construction and Stormwater Management Requirements.

    The current editions of this code and regulations should be consulted prior to roof drain specification and installation. These outline the various requirements relating to roof drains including the following relating to drain quantification, downspout traps, overflow requirements and storm design criteria. What follows here is for guidance purposes only.

    Storm design criteria

    Nothing prevents temporary or permanent disconnection of a downspout from a sewer or combined sewer, as long as the disconnection does not result in water drainage beyond the property lines of a building’s lot or create a public hazard or nuisance. Indeed the City of Chicago encourages homeowners to disconnect downspouts from the sewer system and to divert water to yards or gardens, but this should be done in accordance with the Chicago Building Code as follows:

    Provided water discharged can be drained directly to an area drain, catch basin or street gutter connected to a public sewer, without spilling onto adjacent property to cause public hazard or nuisance, single-family (Class A-1) and multiple-family (Class A-2) roofs shall have external downspouts discharging onto a paved or landscaped area.

    Although if this conflicts with a site’s Stormwater Management Plan approved by the City of Chicago Department of Water Management Regulations for Sewer Construction and Stormwater Management, the Stormwater Management Plan prevails.

    Drain quantification

    Minimum number of roof drains: According to the Chicago Building Code, for roof areas 10,000 sq ft or less, 2 or more drains are required, whereas for those over 10,000 sq ft in area, 4 or more roof drains are necessary. Beyond this, all roofs over 750 sq ft in area are to drain to a sewer, where available in any adjoining public way or public place.

    Downspout traps

    Downspout Trap location: According to the Chicago Building Code, Where unexposed to frost, connecting roof downspouts with an open roof connection and set closer than 12 ft to an inside lot line, door or window of a building shall be trapped downspout side of sanitary or combined sewer or drain connections.

    Overflow requirements

    Design load and ponding levels: According to the Chicago Building Code, roof slopes of under 30° pitch are to be designed for environmental rain load, snow load or live load taken to act on the horizontal projection of the roof surface for a roof drain head system regardless of roof drain flow type.

    For roof areas equipped with standard roof drain heads:

    • Design load is to be 25lbs/sq ft of roof area.
    • Maximum ponding shall not be over 3” above the roof surfaces next to a roof drain head.

    For roof areas with controlled flow roof drain heads

    • Design load is to be 35lbs/sq ft of roof area.
    • Maximum ponding is to be limited so that an overflow shall not be over 6” above the roof surface next to a roof drain head.

    Roof Drain Types

    There are various types of roof drains available for flat or low-slope roofs. Roof drain type specification is based on several factors, including: roof design, roof size, roof material, roof pitch, roof load, aesthetics, volume of average rainfall and storm design criteria, drainage rate and overflow requirements, drain locations, maintenance and other features, such as vandal-proofing.

    Siphonic Roof Drains 

    Location: Siphonic roof drains are commonly used on flat commercial roofs.

    Description: They utilize siphoning as a drainage mechanism and function at full capacity when water vacuumed runs into a roof drain via horizontal leaders instead of sloped pipes, which rely on gravity alone. They are often made from galvanized steel.

    Advantages and Disadvantages: Their main advantages are that a minimal number of roof drains can be connected to a single rain leader, they work smoothly and drain faster than any other type of roof drain, require relatively little maintenance and used together can be connected to a single leader. However they are generally more expensive than other drain systems.

    Gutter Systems

    Location: Gutter systems are located at the bottom edge(s) of a roof and are usually fully visible.

    Description: Conventional gutter systems are suitable for some low-slope roofs that have a slope over 2%. Gutter systems collect rainwater from roofs and direct it away from building foundations via downspouts, splash blocks, or underground drain lines. Gutters are usually sloped a minimum of 1” for every 40 ft of run. They are usually sized according to the roof drainage area and made of galvanized steel, aluminium, copper or lead coated copper.

    Advantages and disadvantages: Gutter systems are simple and inexpensive but the require frequent cleaning and maintenance, can be unaesthetically pleasing and be prone to freezing in winter.

    Roof Scuppers

    Location: Roof scuppers are commonly used on flat roofs, and usually placed at or near the lowest roof level.

    Description: A roof scupper allows water to exit via holes in a parapet upstand or roof edge, or through a deck, into a downspout or leader. In these instances, roofs are slightly sloped towards scuppers. They are not drains as such but provide roof drainage.

    Advantages and disadvantages: Scuppers prevent water from ponding by allowing it to exit the roof and the most suitable for flat roofs. They can be aesthetically pleasing, are low maintenance, and large, wide scuppers are rarely blocked by debris. However downspouts installed below scuppers for drainage are vulnerable to blocking and may not protect against heavy rain or snowmelt, if present on flat roofs with inadequate pitch. This means their application requires good detailed design.

    Inner Roof Drains

    Location: Inner roof drains are often used on flat roofs where drainage fixtures are not desired to be seen.

    Description: Inner roof drains include low sections or channels that collect water and feeds to a concealed drain or series of drains that in turn lead to a below ground drainage system. They can be made from stainless steel.

    Advantages and disadvantages: The main benefits of inner roof drains are that they are hidden from view, can be strategically located in several roof areas, they are resistant to cracking or freezing but require careful maintenance to prevent problems. However, interior roof drains are the most expensive flat roof drainage system, requiring the installation of strainers to prevent debris blockage and routine system checks for debris build up.

    Roof Drain Liners

    Location: Roof drain liners are installed in existing roof drains.

    Description: Epoxy liners are installed within existing roof drains used to repair roof drains that may have faults or deteriorated sections.

    Advantages and Disadvantages: Roof drain liners are durable, seamless, and can cover the full length of the drain if needed, or only the sections that have deteriorated, can save a substantial amount of time and ensure that drains are restored back to normal without long delays.

    Roof drain specification – general requirements

    Beyond the regulatory quantification parameters highlighted earlier, there are three key steps to determining the required roof drain specification.

    1. calculate the roof surface area that a drain or set of drains covers and the average rainfall / hour at the building site.

    2. determine the roof area served by an individual leader (drain pipe, scupper, or gutter downspout).

    3. divide roof area / leader area to determine the number of drains required.

    End Note

    The information provided here should equip you to better understand the roof drain options available, their location suitability, characteristics, advantages and disadvantages and allow you to better understand regulatory and general issues affecting their specification, application and maintenance issues. Professional roof drainage engineering support is available for clarification of the same.

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    Tags : stormproof construction Chicago engineers roof drains drainage roof drain types

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