Tips for locating laboratory plumbing risers and stacks By Joel B. Wells In planning a laboratory building, a fair amount of discussion is devoted to some of the basics of the mechanical and electrical systems—where mechanical and electrical rooms and chases or shafts will be located and how large they need to be, for example. Often overlooked but no less important are the plumbing risers that distribute water and gases and the plumbing stacks that convey liquid waste and associated sewer gases out of the building. The choice of location can be affected by the layout of the laboratories and support spaces as well as the building itself. Pressure piping risers A two-story lab building will require one or more sets of risers to distribute plumbing utilities to the upper floor. It is preferable to have dedicated shafts or chases for the risers, so that conflicts with other engineering trades are avoided and so that impacts on usable space (both offices and labs) are minimized. However, space in smaller buildings is often at a premium, so the architect may not be able to provide the risers with their own space. In those instances, the risers can be located in a janitor’s closet, storage room, or a mechanical ventilation duct shaft. The plumbing engineer should coordinate with the architect before commandeering space in a janitor’s closet or storage room, however, to avoid use-of-space conflicts (and, perhaps, an upset owner or user group). Similarly, should the engineer choose to place the risers in a duct shaft, he or she should verify with the mechanical engineer that there is enough room in the shaft to run the risers and that there is a way for the risers to exit the shaft. As with a one-story facility, the horizontal branches off the risers should be provided with valves to isolate various parts of the systems. The risers themselves should also have at least one set of valves at their bases to facilitate shut-down of the upper floor without affecting the lower one. While providing one set or a few sets of risers may work for a small, two-story lab building, the same approach often becomes impractical for a larger facility. One reason is that a multi-story building often has multiple pressurized services. Besides hot and cold water for laboratory sinks, vacuum and natural gas, a facility may have a high- and low-pressure compressed air system, a pure water system, a vivarium water system, and cylinder gases. Fig. 1. Examples of waste stacks and pressure riser locations. Courtesy: HOK (Tampa and Atlanta). Used by permission.Click to enlarge. A second reason that a single set of risers is not as practical for a multi-story building is the arrangement of the labs themselves. Labs can be clustered around the building’s core and spread out around its perimeter. Attempting to supply all the labs with one or even two or three sets of risers can reduce the ability of the owner or users to shut down labs easily for maintenance or reconfiguration. Using more risers to supply fewer individual labs can help alleviate this problem. The location decision can be influenced by several factors. Below are some questions that the engineer should consider when deciding this issue. • Do labs “stack?” That is, do they have a similar layout, and do they occupy the same location on each floor? If the answer to both these questions is “yes,” then there is a very good chance the stacked labs have similar utility requirements. It may be worth having a set of risers dedicated to these labs. Risers may be able to be located in a wall or chase near the entrances of the labs. The architect (and lab planner, if necessary) should be consulted to make sure the wall or chase can be made large enough to accommodate the risers without disrupting the layout of equipment and casework. • Is there a shaft or chase near a set of labs that is in the same location and roughly the same size on each floor? If so, this might be a good location for a set of risers, even if the labs served by them do not necessarily stack or require the same utilities on each floor. (Assigning a set of unrelated labs to be supplied by common risers can be thought of as “zoning.”) However, the engineer would need to verify whether another trade (or trades) intends to use some or all of the space in the chase or shaft before settling on this location. If there is a large shaft or chase available, chances are that more than one trade will covet it. Additionally, the engineer should determine whether horizontal branches fed from the risers can get out of the shaft or chase easily. These locations are no good for risers if all paths of exit are blocked by other tradework such as HVAC ductwork or piping. Finally, the engineer should verify that local or national codes and the local authority having jurisdiction will allow risers to be run in shafts. For example, Section 5.3.4.5 of NFPA-90, 2002 edition (“Installation of Air-Conditioning and Ventilating Systems”) only allows piping made of noncombustible materials carrying nonhazardous or nontoxic materials in ventilation duct shafts. In certain circumstances, chases dedicated solely to plumbing risers may be required. The engineer should coordinate with the architect regarding the chase size and placement. • How crowded are the ceiling spaces above labs and corridors? Lighting, electrical conduit, sprinkler piping, ventilation ducts, chilled water and hot water piping, and waste and vent piping are but some of the services that must share the above-ceiling areas with the various pressurized plumbing utilities to serve a particular floor. In addition to constraints imposed by walls, these trades must also contend with the structure of the floor above and the height of the ceiling itself. While it is good practice from a cost standpoint to serve multiple labs from one set of risers, doing so requires longer horizontal runs of piping, which in turn can make coordination efforts among trades more difficult. Locating risers as close as possible to the labs they serve can reduce these troubles. • What type of structure do the upper floors have? If the floor relies primarily on the slab to sustain its own weight (with the occasional beam between columns, of course), the factors mentioned previously have more influence on where the risers can go. If the floor is supported by concrete beams, however, the structure becomes an additional limiting factor. Risers can sometimes be placed next to columns or in walls located on top of beams, but this practice should be limited to use only when chases or shafts are not available. Fig. 2. Stack composite diagram. Courtesy: HOK (Tampa and Atlanta). Used by permission.Click to enlarge. Structural engineers are generally willing to sleeve or block out a small portion of a concrete beam to allow risers to pass between floors, but they often limit the number and size of the penetrations. Steel beam structures cannot be sleeved or blocked out, so they must be avoided. The architect may need to thicken walls or provide extra chases to accommodate the risers. Close coordination between the plumbing engineer, structural engineer, and architect can generally avoid potential conflicts between the risers and the building structure. Waste stacks and vent stacks Like the pressure piping distribution system, the waste and vent systems for a small lab building usually have more horizontal than vertical elements. Even if the building has two floors, it is rare for labs to be arranged so that sinks or equipment requiring floor drains line up with each other between floors. In this situation, it is more practical to collect the waste from the upper floor’s sinks or drains in the ceiling of the lower floor, drop the common waste line in a nearby wall, and connect it to the under-slab waste piping. Similarly, the vents from individual sinks can be tied together above the ceiling and discharged through the building’s roof. It may be possible to connect the common vent from the lower floor’s sinks and/or drains to the upper floor’s common vent using a vent stack. Multi-story buildings present a better opportunity to use waste and vent stacks than does a two-story building. The following are some issues that should be kept in mind when choosing where to put the stacks. • Drainage systems work by gravity, so a horizontal waste line from a sink or floor drain must be run at a constant slope to a stack. Most plumbing codes require waste lines 2 in. and smaller to have a slope of 1/4-in./ft and waste lines 3 in. and larger to have a slope of 1/8-in./ft. Over long distances, this rate of fall can accumulate quickly and lead to conflicts between a waste line and architectural elements such as casework (if the line runs above the floor from a sink to a stack, for example) or ceilings (if the line runs from a floor drain in the ceiling space below) or between the waste line and other trades (HVAC ductwork, for example). Therefore, it is a good idea to minimize the horizontal distance between a fixture or floor drain and a waste stack. • A waste stack’s size is determined by the number of drainage fixture units connected to it. A drainage fixture unit (DFU), as defined by most plumbing codes, is based on the volume rate of waste discharge of a plumbing fixture, how long the discharge lasts, and how frequently a fixture may be used. Local plumbing codes should be consulted for the DFU value for a particular fixture or floor drain and the maximum number of drainage fixture units allowed for a given waste stack size. • All fixtures and drains that have p-traps must be vented, so at least one vent stack must be provided for the building. To reduce the length and size of branch vents, several vent stacks may be necessary. Depending on the number of floors that the waste stacks traverse, a dedicated vent stack may be required for each waste stack. Local plumbing codes should be consulted for guidance on this issue. With these factors in mind, waste and vent stack locations can be determined. Common places for stacks follow: • Walls behind sinks. This option works well if sinks line up fairly closely from floor to floor and if the architect is willing to provide walls thick enough to accommodate a 3- or 4-in. nominal diameter waste stack (and vent stack, if necessary). This is one of the more preferable locations, since it eliminates the need for waste piping in the ceiling of the floor below and since it results in relatively short waste arms that can run in casework from the sink to the stack. • Structural columns within a lab. This is an alternative when lab sinks do not line up well from floor to floor or if several sinks in a single lab need to be routed to a common stack. The column chosen for the stack should be as close as possible to the sink (or sinks) to help minimize horizontal runs of waste piping in casework or the ceiling of the floor below. Concealment of the waste stack should be coordinated with the architect. Also, it should be verified that the waste stack’s location does not create structural conflicts. If a companion vent stack is provided for the waste stack, coordination with the building structure becomes even more important. As with risers, the structural engineer may limit the number, size, and location of penetrations at a column, particularly if several beams intersect there. • Chases remote from labs. These locations are generally less desirable than the options mentioned above, because they can require long runs of horizontal waste and vent piping back to the stacks. However, they can be used if the architect cannot provide walls thick enough within the labs for stacks or if the building structure will not allow stacks at the columns. (This usually occurs with a steel-structure building.) In general, any of the issues mentioned above cannot be used as the sole factor in determining riser or stack placement. All of them must be considered, and priorities must be established from the answers. Above all, communication and coordination between trades are key. A well-laid-out system of risers and stacks and their accompanying distribution network of horizontal pipes can help result in a building that is more easily constructed and that uses its space more efficiently. Joel B. Wells, E.I.T., LEED AP, is an associate of Newcomb & Boyd Consultants and Engineers, Atlanta (www.newcomb-boyd.com).