Can we build it?

By Timothy Allinson, P.E.

Murray Company, Long Beach, Calif.

 

First I would like to take this opportunity to wish everyone a wonderful holiday season and a Happy New Year. As Christmas approaches I am reminded of last year, when my 5-year-old son crawled halfway into the fireplace and, peering up, said, "Are you sure Santa can fit through there?" I explained to him that modern chimneys are much smaller than in the old days, so we would have to leave the slider unlocked for Santa. He accepted that statement, and I was thrilled - a believer for at least one more year.

What I am writing about this month is related to Santa and the chimney: making things fit; specifically, making pipes fit in walls. I remember as a young engineer being told by a plumbing contractor that the 2" waste line we showed on our drawings would not fit in a 2-5/8" stud wall. This sounded wrong to me, so he invited me to come to the site and look at a mockup.

Prior to visiting the site, I looked at a cast iron pipe book. The OD for 2" cast iron is, at its maximum, 2.44". The book does not tell you how much the no-hub band adds to that dimension, but it seems reasonable that it might be more than the 0.19" difference between the stud width and the pipe diameter. So, I headed to the site fully prepared to be wrong, and, in fact, I was. The mockup proved that the sheet rock would not lie flat on the studs with the 2" cast iron in place. Also, the standard 4" sleeve was sticking out of the wall.

Since that day, I have become quite diligent about making sure that our pipes will fit within their respective walls. As a consulting engineer, it was quite frustrating, because architects don't like to be told by the plumbing engineer that their walls have to be fatter than they would like. I find on the design-build construction side that architects are more accepting of what they are told when the information comes from the contractor that actually has to build the stuff.

Some chase walls are quite simple to dimension. For instance, if the chase serves wall-hung water closets, we know that the chair carrier serving the fixtures will determine the dimension and that the dimension can readily be obtained from the manufacturer's literature. But what about a high-rise condo or an apartment with a tank-type water closet on the chase wall: How should that wall be constructed?

We know that a vertical stack of water closets will have a 4" waste (possibly larger, if the stack is very, very tall, but that is unlikely). The vent stack will be 3" or 4", depending on height. The 4" pipes will require 6" sleeves. The ideal wall construction for this arrangement is a 6" single stud wall, provided there is no requirement for the sleeve to be raised above the deck. If this requirement applies, the stud would have to be 8" in size; possibly two smaller back-to-back studs might be used, but the space should be wide enough to conceal a 6" sleeve with a little tolerance - 7" at least.

Since the piping for this stack will likely be prefabricated, it is important that the studs be laid out around the pipes. If the stack of water closets is tall enough to warrant a yoke vent, it will spread out the waste and vent stacks, requiring that the studs flanking the pipes be spaced 24" apart, rather than the usual 16". The water riser and supply can be outboard of this stud arrangement to supply the WC without penetrating the stud.

As the number of fixtures on a chase wall grows, the situation becomes more complicated. Branch lines may need to cross pipes and studs. One of the key issues in high-rise design is to allow for prefabrication without penetrating studs, because a prefabricated assembly cannot penetrate studs without being disassembled and reassembled on site; this, of course, negates the benefit of prefabrication.

Take, for example, a typical water closet, lavatory and bathtub configuration. The wall construction will depend on how it is piped. If it is piped with the water closet wet-vented through the lavatory, a dedicated waste stack for the tub and dedicated water risers for each fixture, then it can be piped in a 6" stud. Both walls A and B would require 6" studs. This may not seem the most efficient solution from a material standpoint, but, from a prefabrication and installation standpoint, it is the most efficient, and it would be the preferred solution for the architect, since it keeps wall dimensions to a minimum.

 

Note that sometimes the ADA requires the bathtub to be flipped, such that the shower controls are unencumbered by the WC. In that scenario, the plumbing can be flipped with the tub, but wall C would have to be provided a 6" stud to allow room for the shower valve and the insulated hot water riser.

 

With a double lav scenario, the wall dimensions must increase. In this case the 1/2" branch water lines must cross the waste and vent risers, and they must run horizontally in the chase. In such a case, a 6" clear space is required between the studs. If 2-5/8" studs and 5/8" sheet rock are used on both sides, the face-to-face wall dimension will be 12 1/2", a significant increase from the 6" stud, all because of the addition of one lav.

In summary, in order to determine the best wall construction and dimension, the plumbing engineer must be able to visualize exactly how the installation will be built. This is not a simple task, but it becomes simpler with experience and exposure. Remember that it is best to configure pipes to facilitate prefabrication. In high-rise situations, run pipes vertically whenever possible. If pipes must run horizontally, configure studs such that the pipes can run in a clear space rather than through the studs, especially if it is a typical, repetitive condition.

Lastly, be sure to document your requirements with the architect. Present the architect with highlighted floor plans indicating wall dimensions or wall types and where they are required. Keep a copy of this drawing for future reference, as it is human nature for the architect to forget or for dimensions to get lost as changes are made down the road.

Timothy Allinson is a Senior Professional Engineer with Murray Company, Mechanical Contractors, in Long Beach, Calif. Prior to entering the design--build industry he worked for Popov Engineers, Inc. in Irvine, Calif, and JB&B in New York City. Tim holds a BSME from Tufts University and an MBA from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed Accredited Professional. Tim is a past-president of ASPE, both the New York and Orange County Chapters, and sits on the board of the Society of American Military Engineers, Orange County Post.