New Construction Plumbing

All three types of PEX start as the same base plastic, cross-linked polyethylene, and differ in how the cross-linking happens during manufacturing, which affects flexibility and how the pipe is joined.

PEX-A's expansion-fitting method, where the pipe end is stretched over a fitting and shrinks back down to grip it, is the reason PEX-A is the type most home-run manifold systems are built around: the same expandable quality that makes the joint tight also lets the pipe flex rather than split if water inside it freezes briefly. PEX-B and PEX-C, joined with crimp or clamp fittings instead, are common and code-approved choices as well, and the practical difference for most homeowners is smaller than the marketing around each type sometimes suggests.

Trunk-and-branch plumbing runs one larger supply line through the house with smaller branches splitting off to each fixture, connected with fittings along the way; it is the traditional layout for copper and much CPVC plumbing. A home-run manifold system instead runs an individual PEX line from a central manifold directly to each fixture, with no branch fittings in between.

Neither layout is universally right. A manifold system is easy to service fixture by fixture and keeps most fittings out of the walls, but it uses more total pipe and needs a place to put the manifold itself, usually a mechanical room or a closet with wall space to spare. Trunk-and-branch uses less material and is familiar to any plumber, but a leak or a repair on the trunk line can affect several fixtures downstream at once.

Before an inspector approves a rough-in and drywall goes up, the plumbing has to be complete and tested. A failed rough-in inspection is one of the more expensive delays on a build, since it typically means reopening framing that other trades were about to close in behind. A typical readiness checklist looks like this:

• Drain, waste, and vent (DWV) piping pressure-tested, usually with an air or water test held for the period your inspector requires
• Water supply piping pressure-tested and holding steady, with no drop that would indicate a leak
• Vents properly sized and terminated through the roof or another approved location, not just capped off
• Cleanouts installed and accessible, not buried behind a finished wall or under a slab with no access point
• Fixture rough-in locations matching the approved plan set, including toilet and drain spacing
• Pipe properly supported and strapped, with nail plates installed anywhere a run sits within striking distance of where drywall fasteners will go
• No debris, tools, or construction material left inside open pipe ends

New construction plumbing is built around the federal maximum flow standards of 1.6 gallons per flush for toilets, 2.2 gallons per minute for bathroom faucets, and 2.5 gallons per minute for showerheads. Most fixtures installed today go further and carry the EPA WaterSense label, which caps toilets at 1.28 gallons per flush and showerheads at 2.0 gallons per minute while still meeting EPA performance testing for a strong, usable flow.

Choosing WaterSense-labeled fixtures throughout a new home adds up over the life of the house. Starting with an efficient baseline at every fixture means the home is using less water from day one, before any future leaks or inefficiencies are even a factor.

None of this requires unusual equipment. Low-flow fixtures, a well-planned manifold or trunk-and-branch layout, and the correct PEX or copper for each run are all standard materials; the value is in specifying and installing them correctly the first time, before the walls close and every change becomes a repair rather than a plan.