Standard Roof Drainage Systems

By Craig J Froeter

Roof drains have changed little over the past 100 years. They've traditionally been made of cast iron, although plastics have also come into limited use in the past few decades. Standard roof drainage comes in many forms - gutters, scuppers and internal drains of many different configurations. During the design of the roof drainage system, consideration should be given to roof sumps, insulation elevation plans around drains and the functioning and design of the primary and secondary drainage paths. This article will explore the lessons I've learned about internally piped roof drainage systems.

National and local codes

National codes are typically adopted by states or by local municipalities. They're often modified to suit the needs of the locale, so you need to know the specific code for the area where you are designing. The most recent fundamental change has been the universal requirement for protective secondary drainage systems. Local codes will cover rainfall rates and intensity guidelines so that the proper sizing can be calculated. They will also describe how and where stormwater is to be dispersed through the primary and secondary systems.

Lessons learned

A roof drainage fixture has a brutal and neglected life. In more cases than not it is out of sight and out of mind, rarely cleaned or looked after. It has to stand up to the extreme environmental factors of high heat, frigid cold, acid rain, direct sunlight, hailstorms and flying debris from high winds. Depending on its location, it may also be subjected to salt spray, acids, solvents, plasticizers, animal fats, industrial by-products, decomposing plant debris and constant submersion in water. If all of this isn't enough, every 10 to 20 years it will be subjected to a roof replacement.

During my 25 years in the roofing industry, I've observed countless instances of everything from simple installation errors to unethical corner cutting. While it's not possible or efficient to develop a design specification detailed enough to prevent all of these problems, there are several often overlooked issues that would greatly increase the robustness of the installed roof drainage system. The following sections highlight some of these "lessons learned."

Architect/engineer/contractor

Proper roof drain installation design is a joint effort between the architect, the engineer and the contractor. Many factors need to be considered: the type of construction, type of deck and its thickness, type of roof, type of insulation and its thickness and the purpose of the roof. Even after the drain size is determined it's still not a one-size-fits-all fixture. There are many options available so that the drains can adapt to various conditions of the construction. Not every drain, however, can be adapted to every condition.

While it's very common for the architect to supply the engineer with a plan layout of the roof drain locations for designing the capacity of the drainage system, it's not as common to see an elevation (deck/insulation/roofing thickness) layout at the roof drains. Omitting the elevation layout effectively leaves the design of the roof sump area around the drains up to the discretion of the plumbing and roofing contractors, typically resulting in less desirable drainage system performance.

The development of adjustable drain extensions enabled this behavior by making it quite easy to adapt to building conditions. While this adjustability may, in general, be a good thing, it has opened the door for corner cutting, because many roofing contractors, given the choice, will seldom install a proper roof sump around the drain.

Roof sumps

A roof sump eliminates standing water at the roof drain and is probably the most important factor in preventing debris buildup. Without a roof sump, the drain ring will hold back water, trapping debris. This begins to obstruct the drain, creating more ponding, which in turn collects more debris and so on, until the drain is fully obstructed or cleaned.

A few years back a company I was involved with had the maintenance contract for a local school district. One of their buildings had recently been reroofed, and, for some reason, the contractor installed some of the drains level with the roof surface and some in a 11/2" roof sump. The different configurations were scattered throughout the roof, almost alternating every other drain. The roofing system had 1/8" slope throughout the roof area, so drainage was adequate. There were many trees scattered evenly around the building, so the debris was free to go where it pleased on the smooth-surfaced roof. Every drain that was level with the roof surface had a 15- to 25-foot pond around it, with debris covering the drain strainer. Every drain that was sumped into the roof was perfectly dry, with no debris.

Specification of insulation and/or roof sump plans in elevation as well as plan views is the single most important factor in building a roof drainage system that will perform as designed.

Reroofing

While a building may be remodeled in 30 to 50 years, if ever, the roof will be renovated every 10 to 20 years, on average. Reroofing causes any inherent weakness of a drain's design to show itself. One of the most common problems during reroofing is a bolt breaking off when the drain ring is removed. The roof manufacturer's warranty inspection requires that all roof drain bolts be installed and tight. Thus, the drain must be repaired, retrofitted or replaced.

Repairs take the form of drilling and tapping a new hole in the drain. Retrofitting uses a specially designed drain that fits into the pipe and reduces the pipe size, usually eliminating the drain sump. Replacement of the drain body is the most expensive option, with costs of $1,000 to $1,500 per drain.

The removal of an existing roof can be a very difficult proposition, requiring hammers, spud bars, chisels and pry bars to remove the material from the drain flange. Any money saved initially with a light duty/casting roof drain will be lost during the roof replacement.

Drain options

IRMA (Inverted Roof Membrane Assembly) guards are very useful items when used with IRMA roofs. While they're a product of northern climates and built-up roofing systems, they can also be useful for some configurations of green roofs. A typical IRMA guard is a perforated stainless steel cylinder added to the drain ring. Caution must be observed, however, when specifying these with conventional roofs. The problem is that the holes in the guard are much smaller then the grating on the drain strainers. This will restrict the passage of smaller material that would normally flow through the strainer. The buildup of this material will cause the guard to become blocked more quickly than a typical drain strainer; preventing this problem requires a more frequent maintenance program.

Gravel guards of 3/4" to 1 1/2" height were originally an integral part of all drain rings. Recent drains have developed some styles with rather small guard sizes, which are barely capable of retaining 3/8" ballast, not to mention the ballast of a single ply roof. This is possibly the reason for the confusion of some designers specifying IRMA guards for gravel retention.

The plastic strainers that I've observed simply do not stand the test of time. Odds are that they will crumble and disappear in as short a time as 10 years, leaving the drainage piping exposed to the roofing ballast and anything else that is on the roof. This can cause blockages that are very difficult to clear.

Drainage piping

Internal drainage piping and its design have changed little over the past decades, with perhaps the most significant cost reductions being due to the use of plastic pipe and labor saving connections. A recent modification has been the addition of a secondary roof drain systems requirement. Since its onset, the requirement has been slowly evolving, and it varies from state to state. The requirements can even be very different from one city to the next, so knowing the code for the local area is very important.

Conclusion

The outlets/dispersion points for different drainage systems have different requirements across the United States. Some of the different outlet locations can cause their own problems. While an outlet high on a wall can cause wall damage and/or collateral damage from falling ice, a low outlet can block visibility of the secondary flow, which is generally required by code. Will the landscaping grow to block it or will snow removal cover it? Will connecting the systems in the vertical riser prevent outlet blockages? Does a sensor need to be installed to notify the building owner if no one is there, or to understand the real reason why water is coming out of that drain pipe? All of these issues need to be taken into account when designing the drainage system.

While roof drainage is one of the simpler building systems, it should not be overlooked. Important design considerations should not be left to the whims of contractors. Consideration by the architects and engineers of roof sumps, insulation elevation plans around drains and the functioning and design of the primary and secondary drainage paths will result in a robust design that will serve the building owner well into the future. The resulting design specification will also provide the highest probability that the as-built system will perform as designed.

Craig J. Froeter is an inventor and the founder and managing member of Froet Industries LLC. He has 32 years experience in the construction industry that started with rebuilding equipment for the family commercial roofing company.