Rough-in Plumbing in new Construction - Pro Construction Guide
How to Rough-in Plumbing in New Construction

How to Rough-in Plumbing in New Construction

This high-end rough-in plumbing work combines the right materials, tools and techniques.

By Harvey Adams and Michael Davis

The first step for a successful rough-in plumbing in new construction is to get familiar with the local plumbing codes. States, counties and even municipalities create building codes. Regardless of how you learned to do the rough-in plumbing work, you should study local codes and assure your work is compliant.

When you arrive on site, make sure the house you are plumbing is the house you bid. Have there been changes? When the framing started, did the owner decide he wanted to add a bidet in the master bath and a wet bar in the great room?

These things happen, so you should assure you’ve agreed to any changes. And, assure those changes have been priced into your contract before you start the project. Negotiate up front. Don’t let yourself get halfway through a job and find yourself faced with an unreasonable homeowner who thought you’d just “throw  in” the extras.

After confirming the job matches  your bid, begin laying out the fixtures. Carefully mark the location of tubs, showers, sinks and appliances. Check to be sure everything will fit and necessary backing and support are in place.

Now, the second step for rough-in plumbing is to cut holes for the drains. Using a hole saw sized 1⁄⁄8-inch larger than the pipe or fitting needed  for the drain, cut out the decking, and see what is below. With proper planning and a little luck, you may have a clear path for your drainpipe. Otherwise, you could find yourself looking at obstructions, such as trusses, joists or beams.

Solvent-cement is used to connect ABS pipe and ttings. This product will invariably get on the threads of the can. This will have one of two results: You’ll put the lid back on the can and, next time you need it, the lid will be glued on tight. Or, the adhesive will build up, so that you can’t get the lid on correctly. The solvent-cement will spill out all over your tools. To prevent this, smear a little pipe joint compound on the threads of the can when you first open it. This will prevent the cement from hardening, and you’ll always be able to get the lid off and on.

Shown are a waste and vent for a double lavatory.

How to Rough-in Plumbing in New Construction

Shown is a vent line running out of a roof.

Looking up from below, once the holes for the drains are cut, you can see where you want your pipe to be and determine the best path through trusses, walls, and joists. Insure adequate room exists for fittings. If framing obstructs the route, don’t cut it until you are certain it’s non-structural. Always ask before you cut.

By code, a typical vanity sink can be drained using 1½-inch pipe, but for this job, we used 2-inch pipe. It doesn’t cost much more than 1½-inch, and it insures faster drainage and fewer clogs.

We also used wide-radius fittings, although the oversized 2-inch drain pipe does allow use of short-radius fittings. This can be a real lifesaver, when space inside a floor system or dropped ceiling is limited. If you work in a tract home market, you may not have the luxury of doing this. In an ultra-competitive market, the difference between 1½-inch and 2-inch drain pipe may put your company out of the running.

Once you plan the drain layout, plan the vents. Determine whether a single- or multi-drain and vent system is appropriate. For this job in Colorado, we used the International Plumbing Code (IPC), which allows an entire bathroom – tub-shower combination, toilet and sink – to be vented using a single common vent.

We only use a multi-drain and vent system when the layout of the fixtures and framing beneath lends itself to a common vent system. If a multi-drain and vent system is not configured properly, it can cause a gurgling sound in the pipes.

When you are plotting the course for your drains, you must ensure you have adequate room in the floor system to provide the required fall – or slope – of the drain pipe. Not enough fall may cause the pipes to drain too slowly. Too much of this slow draining can cause waste to be left behind. The standard is a ¼-inch fall for each running foot of drain pipe. This varies, depending on the size of the pipe. Larger pipes require less fall.

To be a good plumber, you must know a little about everything that goes into building a house. You need a basic understanding of wood framing, so you know what can be cut or drilled without damaging  the structure. You must be able to read electrical drawings, so you  can avoid putting pipes in the way of fixtures, switch boxes in walls,  or can lights in ceilings. And, you need to understand mechanical drawings, so you can stay out of the way of ductwork.

A water line and waste stack are going to the second floor.

Here, water and waste lines are going around structure framing.


Often, the builder will ask that you stub out the vents first thing, so the roofing contractor can proceed with his work. Codes vary. But, in cold climates, vents penetrating the roof often are specified to be 2 or even 3 inches in diameter to prevent them from being sheared off by the weight of accumulated snow. Local codes may require that vent pipes be stubbed out within 5 feet of the ridgeline to limit the amount of snow that can accumulate behind them.

It’s important to build a degree of fall into your vents. To comply with IPC code, all piping in a Drain-Vent-Waste (DVW) system must be “washed,” meaning that it will have adequate fall to insure moisture cannot accumulate anywhere in the system.

Three types of plastic DMV pipe are used in residential construction:  polymerized vinyl chloride (PVC), a thermoplastic polymer; chlorinated PVC (CPVC); and acrylonitrile butadiene styrene (ABS). We used ABS piping,  because it’s simpler to install and doesn’t require a primer before applying solvent cement.

On custom homes, it’s common practice to stub out all the vent pipes on the rear of the roof, so they are not visible from the street side. This requires extra labor and materials, so confirm this with your builder when you’re pricing the job.

When you vent a sink in a vanity, consider where the light fixture over that sink will be. You will likely have to elbow your drain pipe to one side, so it doesn’t interfere with the electrician’s work. Although you could avoid considering these steps, it could lead to call backs, should conflicts occur.

If you find a leak in a DVW system, locate the leaking fitting. Attach a shop vacuum to the closest opening. Create negative pressure inside the pipe, and paint the leak with solvent cement. The vacuum pressure will pull the adhesive into the joint, sealing the leak.

Installing water lines

The next step is installation of water lines. Some high-end homes use copper water pipe throughout, but most residential projects in this area use PEX (crosslinked polyethylene). Less expensive, faster and easier to install than copper, PEX is quieter and has proven to be a dependable alternative to copper water pipe. But, before installing PEX plumbing pipes assure it’s approved in your area.

With the PEX water line, we used copper stub outs. That way, if the homeowner wants to change out an appliance or fixture in the future, he doesn’t need a special tool or fitting to do it. A nut and ferrule on a copper line are a lot easier to change out than an expansion ring inside a PEX line.

Code allows a standard three-fixture (bath/shower combination, sink and toilet) bathroom to be supplied with ½-inch water lines. To insure good water pressure, we feed no more than two fixtures from a ½-inch line. Our standard piping for a three-fixture bath is a ¾-inch cold and ½-inch hot water line, since only two of the three fixtures require hot water. Some high-end fixtures require a ¾-inch water line to work properly.

Testing the lines

Rough-in plumbing: Hold-rite brackets on a lavatory rough-in.

Hold-rite brackets are shown on water stub-outs on a lavatory rough-in.

Once the DVW and water lines are installed, they must be tested. This can be accomplished using either water or air pressure. We used air pressure. Caps were placed on all open lines, and an air compressor raised the pressure inside the DVW lines to 5 psi.

Water lines must hold two-times their working pressure. This varies, with some municipalities setting it at 80 psi, assuming a working pressure of 40 psi. We tested all the water lines at 100 psi to meet or exceed code.

The plumbing codes say all DVW and water lines must maintain the test pressure for 15 minutes. In reality, the lines have to maintain the pressure until the inspector can get there. When you call for an inspection, you are rarely told when the inspector might be on site, so you pressurize the system in the morning and go about your business. If pressure is not maintained, the leak must be found and repaired, and the system then retested.

About the authors: Harvey Adams is with Adams Plumbing & Heating in Evergreen, Colo. Michael Davis is with Framing Square Construction in Conifer, Colo.

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