In recent years, extreme weather conditions have become frequent both domestically and internationally, with torrential rain exceeding the design return period. Countless rooftops suffer from water seepage and leakage due to inadequate drainage. This poor drainage leads to a sudden increase in structural loads, posing long-term, irreversible safety hazards to the building structure, ultimately leading to building collapse.

Single-ply roofing systems offer excellent performance. With over 20 years of development and accumulated experience, their application has expanded, and the area of individual single-ply roofs has also grown. The larger the roof area, the greater the pressure on rainwater collection and drainage. How to rationally design single-ply roof drainage systems, constructing independent drainage systems for both constant flow and overflow, and selecting appropriate and compatible products to ensure a roof system that is both safe, reliable, economical, and aesthetically pleasing is a topic worth studying. 

1. Problems in the Domestic Single-Ply Roof Drainage Market

In roofing projects, drainage systems are a multidisciplinary design process relative to the main building design. Construction processes often suffer from inadequate coordination and oversight. Consequently, while drains serve as outlets for roof water, they can also easily become leaks. These issues are particularly pronounced in single-ply roof systems.

1.1 Due to the lack of prefabricated products specifically designed for single-ply roofing, standardization of construction quality at downspout joints is difficult to ensure.

Single-ply roofing systems offer advantages such as light weight, excellent waterproofing and thermal insulation, quick construction, easy maintenance, and long service life. Since their official introduction in China, they have been widely used in various projects, including industrial plants, stadiums, exhibition halls, airports, and warehouses. However, the construction of detailed joints in single-ply roofing systems is complex, and the lack of effective supporting prefabricated products requires construction workers to perform on-site operations such as cutting the rolls, welding them into shape, controlling the effective overlap width, and installing fasteners. These operations are often limited by workspace, making it difficult to ensure construction quality.

Downspout systems, especially downspouts, are detailed joints in single-ply roofs. Traditional approaches pose significant risks of leakage, uneconomical construction, and poor aesthetics (Figure 1). While some projects utilize prefabricated membranes (prefabricated waterproofing membranes through secondary processing or direct injection molding), significantly improving installation efficiency and construction quality (Figure 2), the waterproofing membrane still relies solely on sealant to secure the interior of the downspout. Due to factors such as aging and cracking of the sealant after long-term use and vibration during drainage, the sealant can lose adhesion or even fall off and clog the downspout, causing water to flow back down the downspout into the roof, creating a potential leak. In many cases, it's difficult to pinpoint the leak point due to this backflow.

Figure 1   Hand-wrapped construction effect

Figure 2   Construction effect of prefabricated coils

1.2 Problems with Existing Siphonic Downspouts on Single-Ply Roofs

Siphonic drainage products are favored by many owners and contractors due to their high drainage efficiency, minimal openings, minimal indoor space occupation, and high system safety. However, their application on single-ply roofs presents some difficult challenges. First, the siphonic system accessory, the rain gutter, lacks development specific to single-layer roofing. Consequently, the siphonic rain gutter still relies on a sealant pressure plate to physically connect the roof to the waterproofing layer. While weather-resistant sealants can be selected from brands with guaranteed quality, the rain gutter will experience long-term rainwater immersion and drainage vibrations. This, combined with freeze-thaw aging caused by climate change, improper sealant application by construction workers, and premature aging of surrounding waterproofing membranes due to prolonged tension and stress, all pose a risk of failure in the seal between the rain gutter and the roof. Furthermore, when a siphonic rain gutter becomes damaged, the sealant is often too strong to be removed for repair. The only solution is to cut the entire gutter and then add a layer of waterproofing membrane to the gap, ultimately leaving the area surrounding the gutter unwieldy (Figure 3).

Figure 3   Details of the siphon downspout on the roof

Secondly, siphon drainage products cannot take advantage of their large coverage area on undulating soft insulation single-layer roofs due to their limited openings and few drainage points. Many puddles will appear on the roof, and long-term water accumulation will affect the life of the membrane and easily become leakage risk points (Figure 4).

Figure 4   Bumpy soft roof

GB 50015 "Design Standard for Building Water Supply and Drainage" stipulates that overflow facilities such as overflow holes or overflow pipes should be installed on building roof rainwater drainage projects; CJJ 142 "Technical Specifications for Building Roof Rainwater Drainage Systems" (hereinafter referred to as "Technical Specifications") also clearly states that overflow facilities should be installed on roofs with pressure flow rainwater drainage systems. However, the design of siphon systems often only focuses on the design of internal water pipes in buildings. There is no clear design plan for the overflow system, nor are there special siphon overflow facility accessories.

1.3 Difficulties in Downspout Renovation for Single-Story Roof Renovations

With the advancement of the 2035 Construction Industry Vision Plan, building quality will significantly improve, and the market for roof renovations will further expand. During roof renovations, the challenges of downspout renovation cannot be ignored.

1) When the building's original drainage capacity cannot meet the requirements of the renovated roof, how can the new design requirements be met without increasing the existing downspouts? Can the addition of overflow facilities compensate for this? These issues need to be considered and addressed in the renovation project design.

2) As a building ages, some downspouts begin to break or corrode, unable to withstand the negative pressure required by siphonic drainage. A more convenient and cost-effective drainage system is needed.

3) Due to non-standard and irregular construction of the original downspout outlets and partial damage during the removal of the rain gutters, the diameter of the drainage bottom pipe varies greatly, making it difficult to use standardized components (Figure 5). 

Figure 5   The dismantled downpipe

4) Due to the main structure of the original building, such as narrow gutters, uneven gutter bottoms, etc., it is not possible to use the existing standard downspout parts on the market. It is necessary to use a special downspout node product that matches it and is flexible to install (Figure 6).

Figure 6 Narrow gutters require specialized downspouts

Regarding the design of the downspout system, the European standard EN 12056-3 "Gravity drainage systems for buildings - Part 3: Roof drainage, layout and calculation" (hereinafter referred to as "European standard gravity drainage system") has a very clear and intuitive design logic diagram for the drainage of flat roofs (including overflow systems), and is accompanied by a detailed and highly operational practical manual. However, domestic design standards are relatively scattered and lack a framework for the design of an overall downspout system. In particular, the concept of the overflow system is vague, and there are almost no complete detailed regulations for semi-pressurized/gravity downspout systems. Overall, there are significant differences between Chinese and foreign standards, reflected in the following key points:

1) Differences in Design Procedures

European standards first determine the maximum design water depth of the roof based on factors such as the building's structural load and roof slope, and then determine the locations of downspouts and overflows. Rainfall is then calculated, and the downspout and overflow locations are adjusted based on product matching.

Chinese standards, on the other hand, lack a systematic and standardized design procedure and typically begin with rainfall calculations. This lacks a standardized interface between the construction and plumbing professions, hindering non-professionals, such as component suppliers, from developing a sound foundation and building a comprehensive downspout system.

2) Provisions on design requirements related to loads on single-layer steel roofs

In the German standard DIN 18531-1 "Waterproofing of roofs, balconies and walkways - Part 1: Requirements and principles for construction and design of unused and used roofs" , it is stipulated that for single-layer steel roofs, the minimum waterproof height at the edge of the parapet is 150 mm. Combined with local load research on steel plate roofs, the maximum water level under the most unfavorable load condition is 75 mm, so a guideline of a maximum design water depth of <75 mm is proposed. Denmark's SBI ANVISNING 273 "Roofing Manual" also stipulates that the maximum design water depth is 100~120 mm.

Domestic standards do not have clear provisions or relatively authoritative guidance. Designers usually make empirical estimates based on building type, whether the roof is accessible or not, and active loads, surplus loads, etc. Although domestic standards are relatively flexible, in actual operation, the risk of system failure increases due to the different demands of various stakeholders and the lack of necessary supervision.

3) Regarding the Location and Number of Downspouts

The European standard for gravity drainage clearly states that "for flat roofs with parapets, each roof catchment area shall be equipped with at least two downspouts (or one downspout plus one emergency overflow)." The Danish standard "Roofing Manual" further specifies the spacing of downspouts and their distance from the roof edge. Such clear guidance ensures a relatively reasonable number of downspouts.

Although domestic technical regulations stipulate that there should be no fewer than two gutter systems per catchment area, and that their location should be determined based on factors such as the load-bearing capacity of the roof catchment structure and piping layout, the lack of mandatory standards and detailed guidelines often simplifies downspouts to a single gutter system in practice. Overall, downspouts tend to be larger in diameter and have fewer openings.

4) Overflow System and Overflow Height

Overflow systems are mandatory in European standards. They serve as a timely drainage measure in the event of exceptionally high flow rates. They also serve as an independent drainage system, ensuring that roof water does not exceed the maximum design depth through the overflow in the event of a failure in the normal flow system.

The European standard EN 1253-2, "Gutters for Buildings - Part 2: Test Methods" (hereinafter referred to as "European Standard Building Gutters") and the European gravity drainage system define the water level differential for gravity drainage: 35 mm for downpipe diameters less than 110 mm, 45 mm for diameters greater than 110 mm, and 55 mm for siphonic drainage. The "Drainage Design Information Sheet" of the Central Association of the German Roofing Industry also provides a standard illustration for an overflow height of 35 mm. These regulations provide strong support for the design of overflow heights for drainage systems. When the overflow is installed at a height that corresponds to the specified water level differential, it can be used most efficiently and the roof water load can be reduced as quickly as possible.

The domestic standards have a vague concept of overflow facilities, lack mandatory requirements and specific practices, and have almost no corresponding atlases, resulting in a variety of overflow system settings that cannot meet their real needs.

5) Integrated prefabricated components for flat roofs and their requirements

The selection of products is crucial to the effectiveness of the entire system. Many European and American standards, such as the European standard building gutter and the American standard ANSI/SPRI RD-1 "Performance Standard for Renovation of Roof Drainage Systems" (hereinafter referred to as the "American Standard Roof Renovation Standard"), have detailed provisions on the performance requirements and test methods of factory-prefabricated downspout products with waterproof skirts to ensure the effectiveness of integrated prefabricated downspout products.

However, the domestic national standards have not yet proposed a definition and requirements for integrated prefabricated components. It is only mentioned in the technical regulations that the edge of the rain gutter should not leak at the connection with the roof. In the context of no standard requirements for integrated prefabricated components, the product itself may have design risks and there will be a greater risk of water leakage after installation.

2 Comparative analysis of differences between Chinese and foreign standards related to water drainage system design

For water drainage system design, the European standard EN 12056-3 "Gravity drainage systems for buildings - Part 3: Roof drainage, layout and calculation" [3] (hereinafter referred to as "European standard gravity drainage system") has a very clear and intuitive design logic diagram for drainage of flat roofs (including overflow systems), and is accompanied by a detailed and highly operational practical manual. However, domestic design standards are relatively scattered and lack a framework for the design of an overall water drainage system. In particular, the concept of overflow systems is vague, and there are almost no complete detailed regulations for semi-pressurized/gravity water drainage systems.

In general, there are obvious differences between Chinese and foreign standards, which are specifically reflected in the following key points:

1) Differences in design steps

In the European standard, the maximum design water depth of the roof is first determined based on factors such as the structural load of the building and the roof slope, and the location of the downspout and overflow is determined. Then, the rainfall is calculated and the location of the downspout and overflow is adjusted while matching the product.

In contrast, there is no systematic and standardized design step in the domestic standard, which usually starts with the calculation of rainfall. This has resulted in a lack of a standardized connection process between the construction and water supply and drainage professions, and has also made it difficult for non-professionals, such as component suppliers, to find a basis for product development and to build a complete water drainage system. 

2) Regulations on the design requirements for loads on single-layer steel structure roofs. The German standard DIN 18531-1 "Waterproofing of roofs, balconies and walkways - Part 1: Requirements and principles for the construction and design of unused and used roofs" stipulates that for single-layer steel structure roofs, the minimum waterproof height at the edge of the parapet is 150 mm. Combined with local load research on steel plate roofs, the maximum water level under the most unfavorable load condition is 75 mm, so a guideline of a maximum design water depth of <75 mm is proposed. Denmark's SBI ANVISNING 273 "Roofing Manual" also stipulates a maximum design water depth of 100~120 mm. Domestic standards do not have clear regulations or relatively authoritative guidance. Designers usually make empirical estimates based on building type, whether the roof is accessible or not, and active loads, surplus loads, etc. While domestic standards are relatively flexible, in practice, the differing demands of various stakeholders and the lack of necessary oversight increase the risk of system failure.

3) Regarding the Location and Number of Downspouts

The European standard for gravity drainage clearly states that "for flat roofs with parapets, each roof catchment area shall be equipped with at least two downspouts (or one downspout plus one emergency overflow)." The Danish standard "Roofing Manual" further specifies the spacing of downspouts and their distance from the roof edge. Such clear guidance ensures a relatively reasonable number of downspouts.

Although domestic technical regulations stipulate that there should be no fewer than two rain gutters per catchment area, and that their location should be determined based on factors such as the load-bearing capacity of the roof catchment structure and piping layout, the lack of mandatory standards and detailed guidelines often simplifies this practice to a single-bucket setup. Overall, downspouts tend to have larger diameters and fewer openings. 

4) Overflow system and overflow height The overflow system is mandatory in the European standard. Its function is to serve as a timely drainage measure when the flow rate exceeds the recurrence period, and on the other hand, it serves as an independent drainage system to ensure that the roof water does not exceed the maximum design water depth through the overflow when the normal flow system fails. The European standard EN 1253-2 "Gutters for Buildings - Part 2: Test Methods"(hereinafter referred to as "European Standard Building Gutters") and the European gravity drainage system define the water level difference of gravity drainage. When the downpipe diameter is less than 110 mm, the water level difference is 35 mm, when it is greater than 110 mm, the water level difference is 45 mm, and the water level difference of siphon drainage is 55 mm. The "Drainage Design Information Sheet" of the Central Association of the German Roofing Industry also provides a standard illustration of the overflow height of 35 mm. These regulations provide strong support for the overflow height design of the downspout system. When the installation height of the overflow is equal to the specified water level difference, the overflow can be used with maximum efficiency and the roof surface water load can be reduced as quickly as possible. The concept of overflow facilities in domestic standards is vague, lacking mandatory requirements and specific practices, and there are almost no corresponding atlases, which makes the settings of overflow systems varied and unable to meet their real needs. 

5) Integrated prefabricated components for flat roofs and their requirements The selection of products is crucial to the effectiveness of the entire system. Many European and American standards, such as European standard building gutters and American standard ANSI/SPRI RD-1 "Roof Drainage System Renovation Performance Standard"  (hereinafter referred to as "American Standard Roof Renovation Standard"), have detailed provisions on the performance requirements and test methods of factory-prefabricated downspout products with waterproof skirts to ensure the effectiveness of integrated prefabricated downspout products. Domestic national standards have yet to define or specify integrated prefabricated components; the technical regulations only mention that the connection between the edge of the rain gutter and the roof should be leak-proof. Without standardized requirements for integrated prefabricated components, the product itself could contain design flaws and present a significant risk of leakage after installation.

3. Application of Gravity Downspouts and Overflow Systems on Single-Ply Roofs

In summary, due to the limited design load reserve (typically on non-walkable roofs), the effective combination of a constant flow system and an overflow system is often crucial for improving roof drainage capacity in single-ply roof downspout designs. Given the structural characteristics of single-ply roofs, the use of prefabricated composite membrane downspouts and overflows is also an effective measure to ensure the quality of waterproofing.

Both the American roof renovation standard and the European building gutter standard have requirements for prefabricated downspouts containing membranes.

1) Roof Waterproofing Seal: The bond between the roof membrane and the drainage product flange must be watertight.

2) Backflow Seal: The backflow seal should extend below the top of the existing drainage system and form a watertight connection with the drainage system.

3) The roof drainage system should be constructed of polymer, metal, or a combination of these materials to ensure good weather resistance. The manufacturer should be contacted to confirm compatibility with the membrane system. 4) Flaps are essential components of downspouts and should promote proper water flow and provide an effective inlet area.

5) American roof renovation standards require a watertightness test pressure of 10 feet (1 foot = 0.305 m) of water column (approximately 30 kPa) for at least 24 hours. European standards for building gutters require a lower pressure of 10 kPa for 15 minutes.

The prefabricated, integrated components of the downspout series are based on these standards, utilizing an integrated metal flange and waterproofing membrane for a fully welded metal flange and pipe. Professionally designed, they feature multi-layered anti-backflow seals and a variety of flappers in various styles and materials. This allows for flexible adjustment of pipe diameter, length, and flange dimensions to suit a variety of application scenarios, providing a reliable downspout product option for single-ply roof professionals. At the same time, the product must be verified and tested for water tightness using professional equipment according to standards (Figure 7), and drainage tests must be carried out on various styles of leaf baffles and pipe combinations of different diameters to ensure product reliability (Figure 8).

Figure 7 Example of watertightness test facilities

Figure 8 Example of water flow test facilities

In addition to commonly used downspouts, overflow outlets used in overflow systems are also worth noting. A variety of overflow outlet types allows overflow systems to be configured to suit different buildings, greatly facilitating design and construction. For example, JUAL offers the Letter Box series of rectangular overflow outlets, the Horizental series of horizontal overflow outlets, and the Cycle series of oval tube overflow outlets. Different products are suitable for different roofing conditions. When selecting an overflow outlet, multiple factors should be considered, including the thickness of the insulation layer, the width of the gutter, the thickness of the parapet, the location and appearance of the overflow drain, and ease of installation.

Overflows and constant flow outlets should be considered as part of a complete drainage system, unified in form, style, and performance, using the same materials and craftsmanship, and complementing and matching each other's performance. For example, the height of the Horizental and Cycle series overflow outlets can be adjusted using a special sealing ring to match the overflow system design, thereby ensuring more effective roof drainage.

4. Gravity Drains with Siphon Effect

Due to their high drainage efficiency, pressure-type siphon systems are widely used on large building roofs. Gravity systems are more widely applicable due to their cost-effectiveness, ease of construction, and universal applicability. Gravity systems offer significant advantages, particularly on single-story roofs, as they can provide both a constant flow system and an overflow system, distributing the water collection area on the roof.

Improving the drainage efficiency of gravity drain systems is a worthy research direction. Through research on the siphon phenomenon, several necessary conditions for maintaining siphonic effects have been identified: 1) the water level difference created by the building roof height; 2) the integrity and sealing of the piping system; 3) reducing vortices, ensuring a full-pipe bubble flow or a mixed flow of steam and water; and 4) ensuring consistent flow in the water flow direction.

These conditions were applied to the modification of a gravity overflow system, resulting in a preliminary product model (Figure 9). The components of this product model feature the following:

Figure 9 Overflow system model

1) The D225 air baffle, larger than the downspout cover, effectively separates air from water entering the rain gutter and reduces vortex formation.

2) The D160 bowl collector, combined with the D75 downpipe, collects water in a large-diameter bowl and separates some air within the bowl, preparing it for entry into the smaller pipe.

3) The 1-meter vertical pipe creates a water level differential, facilitating testing and adjustment of the siphon effect at the roof's designed water depth.

4) The 92° (external r = 180 mm) circular bend ensures continuous water flow and prevents sharp bends that could cause a sudden drop in flow velocity within the pipe and disrupt the siphon effect. The rain gutter portion of this model was tested separately, comparing the popular 87-style rain gutter a, siphonic rain gutter b, and conventional prefabricated rain gutter c with the rain gutter d in this model (Figure 10). The details are as follows (all downpipe diameters are DN75):

Figure 10 Various types of rainwater buckets

01.

Comparison of Water Depths in Front of the Gutter Within a Certain Range

Within the 150 mm water depth range, only the 87-style rain gutter failed to produce a siphon effect. As shown in Figure 10, when a siphon effect occurs, the water depth in front of the gutter c > d > b. This demonstrates that the siphonic rain gutter can achieve full flow at relatively low water level differences and maximize its drainage efficiency, minimizing roof water levels.

02.

Comparison of Drainage Capacity After the Siphon Effect

As shown in Figure 11, the maximum drainage capacity of each rain gutter is b = d > c > a. This indicates that the maximum drainage capacity of rain gutter d in this model is comparable to that of siphonic rain gutter b, demonstrating excellent drainage performance.

03.

Comparison of Drainage Efficiency After the Siphon Effect

It is noteworthy that the curve of the model's rainwater bucket d rises very rapidly, but without the obvious inflection point seen in the siphonic rainwater bucket b. This suggests that its bowl-shaped water collection function plays a significant role. Furthermore, this product is more suitable for quickly reaching maximum drainage flow and meeting drainage requirements when used as an emergency overflow outlet on the roof.

Figure 11    Hydraulic model testing of various rainwater buckets

This demonstrates that gravity drainage systems with a siphon effect are feasible, and in-depth research and application are highly meaningful.

5. Conclusion

For single-story rooftop buildings, the rational design and layout of gravity-based constant flow and overflow systems, along with the selection of matching prefabricated components, particularly prefabricated gravity-based drains with a siphon effect, can achieve excellent economic efficiency and system performance, making it an ideal solution.