Waste-Pipe Trade-offs

Q: My town has recently started allowing plastic waste-pipe instead of cast iron in residential construction, and a lot of my competitors are making the switch. What are the big differences between the two kinds of pipe?

A: PVC PLASTIC, ABS PLASTIC, and cast-iron drain and waste pipe are considered equivalent in most jurisdictions within the U.S., although a handful of municipalities still cling to older code rules favoring cast iron. Any of the three materials can do the job, explains plumbing engineer, licensed plumber, and code consultant Julius Ballanco. But there are some differences that might lead a builder to prefer one to the other. Cost, appearance, weight, strength, labor considerations, and availability could all factor into a builder's decision.

“Plastic pipe is a lot cheaper to purchase,” says Ballanco. Price hikes for PVC have narrowed the gap, and cast-iron suppliers say it adds up to only $150 per bathroom. But Ballanco says complex discount structures mask the actual comparison.

“A typical plumber's discount on cast-iron pipe could be anywhere from 25 percent off up to 45 percent off list price,” Ballanco explains. “Smaller guys get 25 percent off. Bigger guys doing thousands of feet get 45 percent off. But if they use plastic, they can get up to 75 percent off list price. So, if cast iron is listed as four bucks, big plumbers pay $2.50 a foot. If plastic is listed as a dollar a foot, they really pay 25 cents a foot. You see the difference?”

EASY AS PIE: Complex joints and fittings like this are quick and easy to assemble with PVC pipe and special adhesive.

Home-center pricing further complicates the picture. “Any consumer can look up the list price of plastic. If they shop at a big home center, they will pay less than half—like 35 percent or 40 percent of list price. ... But if you look up cast iron's list price, a home center will charge you close to that list price—if they will even sell you cast iron, which not many do.”

So why would anyone pay extra for cast iron? Ballanco says that cast iron carries a customer perception of higher quality. “I try not to jump into that fray,” he says. “But one issue is that PVC pipe can look ugly when it's exposed in a basement.” The white pipe shows dirt, and the purple primer applied at joints can drip and streak. “ABS pipe, which is black like iron and uses no primer, actually has greater consumer acceptance than PVC.”

LIMITED VISIBILITY: Adhesive joints for ABS plastic drainpipe are not as visible as the purple adhesive used with PVC. However, the two types of pipe function equivalently, and the choice is usually based on availability.

Noise is another issue, Ballanco notes. “I've gone into a few new McMansions where, to get the drain from the master bath down into the basement, they have run pipe across the living-room ceiling. Here these people just spent $750,000 on a house, and whenever somebody upstairs flushes, people in the living room hear a whooshing sound coming across the ceiling.”

The greater mass of cast-iron pipe largely silences the rushing sound of a flowing drain. Major supplier Charlotte Pipe and Foundry Co. (www.charlottepipe.com) coined the phrase “Quiet House” for its hybrid drain-waste-vent (DWV) system, which couples cast-iron drainpipes to PVC vents. But a thoughtful floor plan can allow waste pipes to be located at the back of a closet or in an out-of-the-way partition. “And if you're going to run plastic up in the ceiling,” says Ballanco, “throw in some unfaced fiberglass batt insulation, and you'll be amazed at how much it deadens the sound.”

Plastic's light weight means one person can install it alone. But here again, there are tradeoffs. “You always need a two-man crew for cast iron,” says Ballanco. “But using no-hub connectors, two men can install iron in half the time that one man takes to install plastic. So in terms of total man-hours, the two are almost the same. A lot of people don't understand that.”

The no-hub connectors use a flexible synthetic rubber sleeve and metal compression straps. They're reliably leak-proof, but not as rugged as an old-fashioned lead-and-oakum joint. Structurally, no-hub joints are weak points that require support hangers to be located close to each joint. On the other hand, cast iron can span 10 feet between supports, while plastic pipe needs support every 4 feet—but with a lighter-weight hanger. “The cast-iron guys say, ‘We don't need as many hangers,' but then the plastic guys go, ‘Well, our hangers are easier to put in.' And there is truth in both arguments,” observes Ballanco.

Both plastic and cast iron are materials for the ages. “Cast iron, we know, will last for an eternity,” says Ballanco. “There is no denying that. And the plastic guys will tell you, ‘Ours is the stuff that the environmentalists say, when you dig up a dump in a thousand years, will still be there.' So plastic has longevity too.”

But iron's greater strength comes into play when pipes have to be buried. “If you install plastic correctly, you'll have no problem,” says Ballanco, “but it takes more to properly trench plastic than it does cast iron. With cast iron, you can throw basically anything into the trench. With plastic, it has to be a properly bedded trench.” Fill selection is key: “They started out with sand, and then they tried pea gravel. Well, sand didn't work. Pea gravel was terrible, because it moves. Then they found that regular gravel works wonderfully—but it can't have big chunks.”

There's an American Society for Testing and Materials backfill standard for plastic pipe trenches, says Ballanco, but commercial contractors often use gravel. “They don't want to screw around. Home builders typically just say, ‘Okay, keep the big rocks out and start compacting.'”

HANGERS ON: Plastic drainpipe has to be supported at close intervals by approved straps. Iron pipe can span farther between supports, but the heavier iron pipe also requires more substantial support hangers.

Backfilling quality is a particular challenge right next to the foundation. Says Ballanco, “Some plumbing contractors hate the fact that they can't control conditions where the drain exits the foundation. So they use a cast-iron 10-footer at that spot, even if they run everything else with plastic. That's their assurance that the builder can't screw up their installation close to the foundation.”