How Flat Roof Scuppers Work And Why Sometimes They Don’t – Part II

In a recent blog article we discussed the function of a flat roof scupper. We also talked about some of the inherent problems related to reversed grade at the drainage inlet.  Today we will continue this discussion by talking about the parts and components and material types.

The picture below shows an example of a typical scupper penetration through a masonry wall to the exterior where the water is collected in a downspout  Conductor head. In this particular installation you can see that there is a gap where the scupper is not sealed to the sides or the penetration area of the masonry wall.  This open area has caused leakage of rainwater from the outside of the facade, into the masonry wall assembly. 

scupper penetration

A scupper and downspout system consists of various components and materials designed to convey rainwater from a flat roof through the building’s facade to prevent water accumulation and potential damage, related to water entry. Several different materials join together in a somewhat complex interface at the wall penetration near the scupper inlet and outlet.  Leaks at this area of the roof are extremely common.

The scupper itself is typically made from impermeable materials like metal (such as copper, aluminum, or galvanized steel) or plastic (often PVC). The scupper is an opening or passage that allows water to exit the roof and generally passes through a wall like a small tunnel.  The portion of the scupper that connects directly to the downspout or leader, usually through a horizontal or sloped channel is the scupper outlet. The scupper outlet may be just part of the scupper itself which extends beyond the wall. 

The downspout or leader transports rainwater from the scupper down to the ground level. These components can be made from various materials, including metal, PVC, or even cast iron.  The scupper outlet will often feed into downspout or leader at a widened or enlarged head called a conductor head.  Brackets and straps are used to secure and stabilize the downspout to the building’s facade. These elements hold the downspout securely attached the building facade in proper alignment for efficient water flow.

The picture below shows a historic cast iron rain leader that runs underground.

cast iron rain leader

The collage of pictures below show the different angles of a typical aluminum downspout  

aluminum downspout

Flashing materials, traditionally made from metal (like copper or aluminum), are used to waterproof the area around the scupper inlet where it penetrates the building’s wall. Properly installed flashing prevents water from infiltrating the building envelope.   In many cases commercial roof materials such as built-up, bitumen / tar applications and/or metal roofs have been replaced in modern times single-ply polymer membranes.   The advantage of single-ply polymer membranes such as TPO roofing Systems is that these single-ply membranes can seal well at almost all edges and areas of the Membrane material. Metal can overlap in a successive sequential context but does not seal well at all edges.  

Sealing the wall penetration, particularly near the scupper inlet, can be a complex task due to the nature of flat roofs. A challenge is the common formation of a depression or low point just before the scupper. Water tends to pool or pond in this area, increasing the potential for leaks if not addressed correctly.  Several different approaches can be attempted to apply additional attention to mitigate leakage at a scupper.  These different approaches can often be done together, in concert.  

The picture below shows a modified bitumen roof membrane in Washington DC with a scupper above a condo building.  This area had both issues with ponding and leakage at the orifice of the scopper inlet.  This picture shows the area of damage and deterioration after it had been repaired by the application of an overlay of new modified bitumen granulated membrane.   Modify bitumen membrane is one of the many types of roof materials which cannot be i stalled with a seam laying in a ponding area.  In this case, the pond area relieves before the seam location, yet the added layer above the seam is applied as an additional excess application.

modified roof membrane

Often the scupper itself, in the tunnel area will be metal, because of metal’s inherent durability.  The scupper may be formed of metal flashing on top of the scupper opening, overlapping the roofing membrane. This can act as a protective barrier, but it must be sealed, integrally, to the adjacent or substrate roof membrane to prevent water from entering the wall cavity.

To address the depression issue, flat roofs must have a slight pitch or slope throughout and continuing, generally at minimum ¼” per linear foot of horizontal run, near the scupper. This design facilitates water drainage and reduces the risk of water ponding or pooling.  The issue of a depression at the area before the scupper inlet generally is caused by one if two issues:

  1. Reroofing, done in an overlay, in an effort to save costs will often remove the old roof membrane but leave the existing connection between the old roof and the scupper orifice.  Once the new membrane is applied, that orifice then has the thickness of the additional layer of material creating a small curb which acts like a dam to stop a portion of water from flowing and causes ponding.  
  2. Slight leakage begets more leakage.  Since the area of the interface of the scupper and roof system is complicated, small errors in installation, in the complex layout or configuration of the materials can result in slight leakage.  As slight leakage happens slowly over time, the effects, in some ways, can be effectively worse than faster leaks because slight leaks often go unnoticed.  The materials just below the surface rot slowly, undetected, causing the materials above to subside.

Ensuring that the scupper is installed at the lowest point of the roof, or where water naturally accumulates, helps in efficient water drainage. This allows for proper drainage and minimizes the potential for standing water and leaks.

High performance polymer sealants compatible with the roofing materials are often applied around the scupper penetration to bolster (but not solely create) a watertight seal. In some cases sealant backer rods may be used to bridge intentional gaps (often intentional gaps will be integral in a building expansion joint or often unintentional gaps are found in the case of improper substrate conditions) and provide support and shape for the sealant where a void must be spanned.   However, with roofing, It’s important to understand that sealants are used in one of specific ways but not for the primary or principal method of creating an  impermeable barrier: 

  1. As a reduction overlay for extra strength, or 
  2. As a filler under and as a supportive substrate for overlaid roofing materials.

The picture shows an alternative type of installation where the roof actually flows from a low termination in the horizontal plane of the flat roof system. From there, instead of running through a scupper, the gutter system collects water and feeds it to a conductor head at a downspout, similar to a scupper conductor head.

low termination

Periodic inspection and maintenance are crucial to identify and address any issues promptly before issues compound and become exacerbated. Routine cleaning of scuppers and downspouts of debris prevents blockages that could impede water flow and cause backups leading to leaks.

In summary, a scupper and downspout system involves various components and materials to effectively manage rainwater runoff. Sealing the wall penetration near the scupper inlet is complex due to potential depressions on flat roofs, but techniques like counterflashing, proper drainage, and sealants are employed to ensure a watertight seal and prevent water intrusion into the building envelope. Regular maintenance and inspection are essential to uphold the system’s integrity and functionality.

Smart proactive replacement, construction, upkeep and maintenance of low slope roof and mansard roof systems requires an enthusiastic interest and understanding of historical methodologies, waterproofing principles, and building science.   Here in Washington DC, historic and modern residential and commercial buildings are extremely expensive and the roof and related systems provide the shield that preserves the building. 

We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the value of quality construction and building maintenance, and develop a relationship with our company.  You can learn a lot more on our blog.  Feel free to check it out.  If you have questions about the roof and related systems of your building in Washington DC, contact us or fill out the webform below and drop us a line, it’s quick and easy.  We will be in touch if we can help.

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