Siphonic Roof Drainage for Building Retrofits

Building renovations typically involve the replacement of the roof waterproofing system and roof drains. The replacement of roof drains may also require the replacement of the drainage piping. Depending on the age of the building being renovated, an upgrade to current building codes may be required and these codes may require roof drains and piping sized for higher rainfall intensity and the use of independent secondary (i.e. emergency) overflow systems.

With smaller pipe diameters and no pitch, siphonic roof drainage is ideal for building retrofits, especially where architectural preservation is desired. Not only can it accommodate tight ceiling spaces and limited chase and wall space, it reduces construction costs. It also makes it possible to install all the new piping overhead (in ceilings) thereby eliminating the need to saw cut existing floor slabs to excavate and replace buried piping.

In the context of the LEED™ rating system, siphonic roof drainage contributes to the reduction in materials used by allowing for smaller piping. And it facilitates the achievement of LEED™ credits for the reuse of existing buildings. Siphonic roof drainage is also useful for the replacement of exterior downspouts particularly along facades that must be preserved due to historic value.

Siphonic roof drainage is environmentally friendly, enables historic preservation, is easier to install, and saves money that can be used on other renovation efforts.

Why You Should Consider a Siphonic Roof Drainage System

Smaller pipe diameters with a siphonic system can be used, reducing material costs.
Full-bore flow within the piping reduces pipe diameter as compared to open-channel, traditional gravity flow. The smaller pipe size equates to savings in material. For example, a traditionally designed system calls for an eight (8”) inch pipe, a siphonic system of equal drainage capacity may need only a four (4”) inch or six (6”) inch pipe to drain the same quantity of water.

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Siphonic action permits level pipe installations allowing fewer vertical stacks, saving ground work and building costs.
Traditional systems are designed to be atmospheric throughout and rely on pipe gradient or pitch to induce flow to the point of discharge. This pitch necessitates the pipe elevation to become increasingly lower as it runs laterally. Full-bore flow is achieved independently of pipe gradient in a siphonic system. The piping can be installed flat like any other mechanical system such as sprinklers and it simplifies coordination with other building elements. With siphonic piping being horizontal, the building height may be reduced by 3 to 4 feet, saving on construction costs.

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Driving head of the siphonic system is up to 100 times that of a traditional system (i.e. height of building vs. depth on roof).
Siphonic roof drainage systems make full use of a building’s roof height to drive the drainage capacity. The resulting higher operating velocities (3 ft/sec up to 30 ft/sec) of a siphonic system further reduce pipe size and promotes self-cleaning of debris.

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In a siphonic system, below-slab installation costs are minimized, thus reducing excavation, backfill costs and exterior underground piping.
Level installation allows for longer lateral runs overhead thereby reducing or eliminating pipe installed below slab and the associated costs of excavation, bedding and backfill. If overhead, traditional drainage pipe has to quickly drop vertically to avoid a conflict with the ceiling, structural elements or HVAC systems. If below grade, the longer the horizontal run, the deeper the pipe trench must go to accommodate pitch. Siphonic systems reduce or eliminate these issues. This means there are lower site preparation costs.

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Within a siphonic roof drain system, stack and horizontal pipe locations are highly flexible.
Level installation and freedom of placement of vertical stacks reduces buried pipe depths and the associated costs of trenching, bedding, shoring, and dewatering. The flexibility of stack placement also facilitates on-site rainwater harvesting by allowing flexibility for cistern locations either below or above ground.

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A siphonic system allows for maximum use of open space without intrusion of drainage piping.
Smaller diameter piping conforming to structural and architectural lines present a less intrusive presence in an open area. Level installation and freedom of placement of vertical stacks reduces the size of exterior storm sewer infrastructure. The point of discharge for the roof can be concentrated to one corner typically rather than out the building at several points.

These benefits enable significant savings in terms of time and money. Large roof construction similar to those found on factories, warehouses, airports, convention centers, stadiums and “big box” retailers will realize the benefits of siphonic roof drainage and favor this type of roof drainage system. However, all buildings regardless of size or height can realize the economic and technical benefits offered by siphonic roof drainage.

Siphonic Roof Drain Anatomy

A siphonic roof drain looks much like a traditional roof drain. The distinguishing feature of a siphonic roof drain is the air baffle. This air baffle is engineered and tested to prevent air from entering the piping system at peak flows.

Other than the baffle, a siphonic roof drain has the same features as a traditional roof drain including a drain body, flashing ring, dome strainer, and fastening hardware.

In contrast to traditional roof drains, siphonic roof drains are not designed with a large diameter or deep sump bowl because their operation is by means of sub-atmospheric pressure generated at the under side of the baffle and outlet. The depth of water maintained on the roof is dependent only on the resistance value of the drain assembly while operating under siphonic conditions. Any viscous weir effect of the drain body becomes minor and the flow is determined by simple inertial hydraulic effect of flow from a high pressure (atmospheric pressure at the roof surface) to low pressure (within the piping system).

Unlike a traditional roof drain system, a siphonic system is designed to operate with the piping completely filled with water during a rainstorm. Several drains tie into a horizontal collector that is routed to a convenient point where it transitions into a vertical stack. This stack, once it reaches the ground, is piped to a vented manhole or inspection-chamber where the water is discharged at atmospheric pressure and low velocity into the storm system.

The Siphon Principle

The principle of the siphon has been recognized for ages. A siphon is created by a tube or other type of conduit filled with the fluid to be siphoned, thereby creating a continuous and closed path. In any siphon, the discharge end of the conduit must be lower than the level of the fluid in the source reservoir. Atmospheric pressure at the reservoir surface becomes the driving force pushing the fluid through the tube to the lower point of discharge. Most examples of siphons include an inverted “U” shape, this configuration is necessary to lift the fluid out of the source reservoir that cannot be tipped, much like a car’s gas tank shown above. However, the actual path of the siphon tube is irrelevant to the fluid’s ability to flow. In the practical case of siphonic roof drainage, the drainage piping is installed in the simplest way possible: flat and level. This allows the piping to drain completely when it is not raining and then to prime full into a continuous and closed path on its own during a rain event.

People know that it is necessary to prime the tube in order to achieve the siphonic flow. Ask anyone who has received a mouthful of gasoline while trying to siphon gas from their car’s gas tank to fill a lawnmower. Therefore, the ability of a siphonic roof drainage system to prime itself might be counter intuitive. However, these systems prime up simply because the roof drain design and flat installation make full-bore flow occur. This tendency is exactly why plumbing codes have a set of rules for venting of sanitary waste systems. Without venting, water flow through even a pitched or vertical pipe will create zones of reduced or increased pressure that defeat fixture trap seals and under the right conditions can cause instances of full-bore flow. Still not convinced? The next time you flush a siphon-jet water closet consider why the water gets drawn out of the bowl.

What is happening in syphonic drainage?

2004 was an important year for the syphonic drainage industry, with the publication of a Dti funded draft standard for Syphonic Roof Drainage, and the formation of the Syphonic Roof Drainage Association.

The draft standard is awaiting passage through the BSI process, but when published will give a real benefit to designers and specifiers, who for so long have not had a source of information and guidance as to correct design and installation procedures.

The SRDA was formed this year with the intention of:

• Raising the profile and status of the syphonic industry

• Providing clear, informative and impartial information to specifiers

• Giving specifiers the reassurance that member companies have signed up to a wide range of regularly monitored standards

Designing the pipe configuration for optimum performance

One of the major benefits of a syphonic system is that the horizontal pipe runs do not have any fall, minimising the space required to accommodate the system. This provides the designer with freedom to route the pipes to any location at high level, before dropping to ground level. The high suction in the system reduces the pipe diameters and number of vertical drops needed compared to a gravity system, providing a reduction in cost for most installations.

The major advantage of a syphonic system is that drainage can be taken to the end of the building, removing the need for almost all under slab drains.

To ensure an efficient system is created the following design criteria must be followed:

a) Prime time – All roof drainage design in the UK is based on a 2-minute storm, so if the system does not operate fully in less than 1 minute, there is a risk the building will flood. As buildings have got larger, so have syphonic systems, and unless the prime time is considered, systems can be created which will not function properly.

b) Balance - Syphonic systems should be carefully balanced, usually by changing pipe sizes, to ensure the correct amount of water enters each outlet. The balance is achieved by trading off the available suction pressure against the pipe and fitting energy losses. Also try to balance the flow rate into each outlet, although it is not critical for them to be exactly equal but large difference will result in air being pulled into the low flow rate outlets.

Materials specification for the pipework

Due to the high negative pressures that can occur in the systems, it is important that a resilient material is used with high resistance to implosion. Good quality HDPE pipe and fittings provide a good solution and ideally they should be made to EN 1519 the product standard for this material.

Another useful property of HDPE is that it can be fusion welded together removing the need for seal ring joints that may fail causing flooding of the building if the internal pressures force the joints apart.

In addition the system must be securely fixed with strong brackets especially if the pipework is hung horizontally some distance from the soffit of the slab. Systems are available which combine a steel rail and pipe brackets that restrain the pipework, which leaves the drop rod to deal only with carrying the dead weight of the pipework.

Calculated method for sizing the pipework

These days’ manual calculations are rarely done; software is now available to speedup the process and removes human error. The primary aim of the software is to size the pipework to ensure it runs full of water as quickly as possible to induce a syphonic action as the water column drops in the main vertical section of the pipe system. On buildings of several stories negative pressure of –800 mbar frequently occurs providing a high level of suction at the outlets.

Calculation software is frequently linked to a drawing package, which provides a diagram of the proposed installation, together with the hydraulic calculations.

The Syphonic Outlet

Syphonic outlets are designed to reduce the entry of air into the system, if air reaches more than 40% of the volume of the pipe the syphonic action will stop. To reduce the amount of air entering the system a baffle plate is usually fitted over the orifice of the outlet, this not only reduces the amount of air being pulled into the outlet opening, it also stop a vortices forming that will draw air into the system rapidly. The best outlets have been tested and approved by the British Board of Agre´ment. Outlets should also be designed to easily accommodate the water membrane of the roof, which comes in many forms.

What is the best approach to ensure all the benefits of a syphonic system are utilised?

The first thing is not to use a higher safety factor then necessary, second to consider how to deal with the occasional high rainfall rate that generally will not last for more than 2 minutes (NOT 1 HOUR).

Solutions:

1) Design the primary system to a realistic return rate, 108mm/hr has proved to work well in most locations in the UK with several 2 minute occurrences annually.

2) If a high rainfall rate is the result of calculation, deal with the excess above 108mm/hr by some other means:

a) Charge it via overflow weirs to the outside of the building

b) Install an internal gravity overflow system

c) Install a secondary syphonic system, which only operates when the water level builds up on the roof to a predetermined level.

d) Allow the water to build-up on the roof for a short period; the high intensity rate will only last for 2 minutes, following which the system will remove the excess quickly.

Rainfall Intensity Rate

To ensure the building is adequately protected from water ingress, the rainwater system must be designed to remove all the water that falls on the roof quickly. BS EN 12056 part 3 provides both data and a method for calculating the highest expected rainfall intensity expected for all regions of the UK plus a method for deciding on the safety factor best suited for various scenario’s.

The designer must decide how best to use this data; the initial calculation may give the designer a very high intensity rate if they input either a high safety factor or an excessively high return period, i.e. 100 years.

Using 100 years as the return period for a project in London would give the following results:

Safety Factor Category 2 222mm/Hr

Safety Factor Category 3 270mm/Hr

Safety Factor Category 4 600mm/Hr

Using the highest intensity rate will result in the designer creating an oversized system rarely, if at all, going syphonic having a dramatic effect on the pipe diameters, however the chance that it will be utilised is extremely unlikely, plus in the normal life span of the property it will probably never operate syphonicaly but only as a gravity system loosing all of the advantages of a well designed syphonic system.

Syphonic Roof Drainage – How does it work?

Originally developed in Scandinavia over 40 years ago, syphonic roof drainage systems have been in use in the UK since the early 1990’s. Since then, many large projects have used the syphonic system to overcome installation problems which would have been difficult to solve using a traditional gravity rainwater system.

Siphonic drainage is actually very simple in principle and all systems work in exactly the same way. Baffle plates inserted in the outlets restrict air entering the top of the system which, when combined with carefully sized pipework, causes the system, horizontal and vertical, to run full. In a very similar way to a simple tube siphon (such as you would use to empty a fish tank), the action of water dropping down the downpipe will cause a negative pressure to form at the top. This negative pressure can be harnessed to suck water along a collector pipe installed horizontally connecting the outlets at high level.