Siphonic roof drainage and the real world
By Craig Froeter
Siphonic drainage has been available in the United States for a few years now. Standards are in place, multiple articles have been written and almost all of the drainage manufacturers have hopped on the bandwagon. Is it the cost savings system it is touted to be and is it always safe?
While it has been touted that syphonic drainage has been used in Europe successfully for many years, a recent article stated, "Gutters typically are used on roofs in Europe."1 While I may agree that test results for the gutters would apply to flat roofs. The results of a failure would be catastrophically worse with a flat roof than that of a gutter.
For years in the roofing industry we used siphonic action to drain ponds from roofs all the time; it was much quicker than using pumps. Siphoning fluids is a very simple concept, but after all of the engineering design and oversight to assure a working installation, siphonic roof drainage becomes a much more complicated system. Problems begin to arise when you have a building with a roof that will be replaced every 10 to 20 years; the siphonic drainage system has to be taken apart and reassembled each time. You have also taken a system that if it was not for the reroofing every 10 to 20 years it would never be looked at, to a system that is in the forefront of maintenance and maintaining its "as built" status.
There are also a couple of issues in the design standard that conflicts with the building code and roofing manufactures installation guidelines.
Siphonic and the building code
Standard 45 - Siphonic Roof Drainage in sections 6.3.3 and 7.6.3 talks of minimal cricketing and the free flow of water between drains in valleys and gutters with a common stack2. The gutter certainly would not be an issue but the valley could be a major problem.
The building code requires a minimum of 1/8" per foot slope for a coal tar buildup roof system and ¼" slope for all other low slope roofing systems3. A 1/8" or ¼" slope would not allow for free flow between roof drains.
A 1/8" slope does not eliminate all ponding (pond lasting 48 hours) on a flat roof due to variances in the deck and 1/4" gets to about 99% elimination of ponding water.
In a system with a 1/8" slope and the drains were 65 foot apart the cricket/saddle should be approximately 4" high. While this elevation is in the range of overflow dams the roofing system required for this slope is coal tar pitch. Coal tar pitch is known for its low temperature flow characteristics and could easily fill and block a small diameter piping system. There is no siphonic system known that could flush solidified coal tar pitch out of a pipe.
In a system with a 1/4 " slope and drains of the same distance of 65 feet the saddle would be 8.1" high and like the description above would prevent self balancing.
Cost savings real or perceived
While I believe that a siphonic system can save money in certain situations; for example, by making it possible to use smaller gutters to drain a roof. The cost of a siphonic system in a flat roof is not as simple to determine. The costs of the other trades that are involved and the other building systems affected by the roof drainage must be considered.
The common stack with multiple drains is a important factor in saving money with a siphonic system, but as discussed earlier the free flow valley would require a variance to the building code to install the roofing system in that manner. The other cost factor is the accelerated degradation of the roofing system due to the ponding water.
For an example, let's take a roof in the Midwest and figure that the ponding water changes the roof life from 20 years to 15 years. The 8,000-square-foot roof requires two drains. The cost of tearing off and replacing the roofing system would be as follows:
8,000 sf x $6.00 per sf = $48,000
Over a 60-year period
15-year cycle: $48,000 x 4 = $192,000 ($3,200.00 per year)
20-year cycle: $48,000 x 3 = $144,000 ($2,400.00 per year)
This calculation equals $800.00 per year additional cost and a total cost of $48,000. Would the savings of a two-drain siphonic system instead of a two drain gravity flow system save more than $48,000?
With all the interest in LEED4 and building green, it can be quite daunting to sort out which designs or products hold the greatest benefits. The design of a drainage system may, in certain cases, save money upfront but could ultimately cost the building owner in the long run by causing an additional reroofing cycle and adding to our landfills. Does that really fall within the intent of green building design?
Real world conditions
Another result of installing the drains in a valley and allowing free flow between them is that roofing manufacturers require that the roofing system must not pond water for more than 48 hours or there will be no warranty issued for that project.
Standard 45 Section 11.4.1 states that the elevation of the drains in a valley must be set within 3/8" of level5. While the elevation of the drains is not a big deal at the initial construction, the first reroof could cause a problem. The tools used to remove some of the various types of roofing systems can and do flex, move, bend decks and fixtures. If the metal deck at the drain was bent up or down 3/4", would it ever be noticed, and what problems would it cause? What if it was in the middle of a common stack run or at the end? How would the roofing contractor even know that it was an issue? What if the building settled or deck sagged?
What if, at the first reroof, the roofing system was changed, and a completely different tapered system was installed. This practice is the norm in the roofing industry today. What would happen to the drainage of the common stack?
Broken drain rings are a very common problem in the industry. Almost all roofing contractors have a used drain ring wall just for this reason. They will look for a replacement that matches the size and sump flange shape, and they will install it. They will probably also think that their replacement ring will flow much better than the one they took off with the strange cover, resulting in a system failure.
The roof drain is unique such that after the building is built it migrates from a plumbing fixture to a roofing fixture. The roofing trade takes over the care and maintenance of the roof drains. Who is ever going to train them in regard to siphonic drains?
The everchanging building
One thing that you can count on in construction is that someone is always reroofing, remodeling, adding or removing equipment. Who will be in charge of policing the siphonic drainage system? Who will ultimately be responsible if there is a failure?