Rough-In Plumbing for New Builds: A Homeowner's Complete Guide

The rough-in plumbing phase of a new build or major renovation is the last opportunity to get everything right before it becomes invisible behind insulation and drywall — and before fixing a mistake involves opening walls. In the GTA's new construction market, particularly in Vaughan, Markham, and Richmond Hill where large custom home builds and builder subdivision projects are ongoing, the rough-in inspection is the formal gateway between framing and finishing. Understanding what happens during this phase, what the municipal inspector is looking for, and what the critical quality control points are gives homeowners and project owners the knowledge to advocate for a correctly executed installation rather than simply trusting that it's being done right out of sight.

What Is the Rough-In Phase?

Rough-in plumbing refers to the installation of all drain, waste, vent (DWV) piping and supply line piping within the wall cavities, floor/ceiling assemblies, and slab — before insulation, vapour barrier, and drywall are installed. At the completion of rough-in, no fixtures are installed: there are no faucets, no toilets, no tubs. What exists is the complete network of piping that those fixtures will eventually connect to — drain stubs projecting through the subfloor at precise heights and locations, supply line stubs capped at the wall plane, and vent piping running to the roof stack penetrations above the sheathing.

The rough-in phase is inspected by the municipal building department before the walls are closed — it is a mandatory hold point in Ontario's Building Code compliance process. The plumbing contractor must schedule this inspection and obtain sign-off before any insulation or wall closure proceeds.

The Drain-Waste-Vent (DWV) System: Critical Installation Parameters

Drain Slope: The 2% Rule and Why It Matters

Every horizontal drain line must slope continuously toward its discharge point. Ontario's plumbing code (aligned with the National Plumbing Code of Canada) specifies a minimum slope of 1:50 (2%) for drain pipes 3" and under in diameter — that is, a minimum drop of 25 mm for every 1250 mm (roughly 1 inch per 4 feet) of horizontal run. For 4" pipe (used for building drains and main sewer connections), 1:100 (1%) is the minimum slope.

Why this specific number? It represents the hydraulic velocity threshold at which drain flow carries suspended solids — tissue, food particles, soap — along with the liquid rather than leaving them to settle and accumulate in the pipe. A drain pipe sloped shallower than 1:50 becomes a sediment collector. A pipe sloped steeper than approximately 1:20 allows liquid to run ahead of solids, leaving material deposited in the pipe — counterintuitively, more slope is not always better.

In a new build context, the rough-in inspector in Vaughan, Richmond Hill, or Markham will verify slope compliance using a level or digital inclinometer at multiple points along each horizontal run. A drain line that passes inspection at the time of framing but was not adequately supported may sag over time as the framing settles — creating a low point that collects solids and eventually blocks. Proper hanger spacing (maximum 1500 mm for ABS pipe under 3") and support at all fittings are not optional details.

Drain Sizing: Getting the Fixture Unit Math Right

Every plumbing fixture is assigned a Drain Fixture Unit (DFU) value — a standardized measure of its drainage load — under the National Plumbing Code of Canada. A toilet contributes 4 DFUs; a standard bathtub, 3 DFUs; a bathroom lavatory, 1 DFU; a dishwasher, 2 DFUs. The drain pipe serving a group of fixtures must be sized to handle the accumulated fixture unit load — larger fixture loads require larger pipe diameters to avoid undersized drain restrictions.

In a custom build with a large master bathroom suite — say, a double vanity (2 DFU), a freestanding soaker tub (3 DFU), a separate water closet (4 DFU), and a walk-in shower (2 DFU) — the branch drain serving this group carries 11 DFUs and requires a 3" branch drain to code. A plumber who installs a 2" branch drain to save material cost on a high-fixture-count bathroom is creating a compliance failure that an experienced inspector will catch — and that an overworked inspector on a large production build site might miss, leaving the homeowner with an undersized drain behind finished walls.

Venting: The Pressure Equalization Network

The vent system is the component of rough-in plumbing that homeowners understand least and that has the most invisible long-term consequences when undersized or improperly configured. Venting performs one critical function: it introduces air into the drain system to equalize pressure as water moves through drain pipes, preventing siphoning of water seals from P-traps and allowing drain flow to occur without pressure buildup.

Every fixture requires venting within a code-specified distance from its trap — the maximum developed length from a trap to its vent connection is a function of the trap arm pipe diameter, ranging from 900 mm (3 feet) for a 1-1/2" trap arm to 3600 mm (12 feet) for a 3" trap arm. Fixtures beyond this distance that are improperly vented will siphon their P-trap dry during drain discharge events, allowing sewer gases to enter the living space through the unprotected trap.

In complex custom builds with island kitchen sinks, distant bathroom branches, or basement fixtures below the main sewer connection elevation, venting configurations can become complicated — requiring air admittance valves (AAVs) where approved by the local building authority, wet venting strategies, or extended vent runs to the primary roof stack. These are decisions that need to be engineered into the rough-in layout, not improvised when the inspector is scheduled for the next morning.

Supply Line Rough-In: Pressure, Sizing, and Material

Pipe Sizing for Flow Demand

Supply piping carries pressurized potable water from the main service connection to every hot and cold fixture in the building. The main service connection for most GTA residential builds is 3/4" or 1" at the street — that sizing must be maintained through the building's main supply trunk to avoid pressure drop at the head end. Branch lines from the trunk to individual bathrooms are typically 1/2"; high-demand fixtures like the primary bath shower or an oversized soaker tub filling system should be supplied with 3/4" branches off the trunk rather than daisy-chained 1/2" runs that compound pressure drop with each fixture served.

Supply line sizing errors in rough-in are functionally invisible until the home is occupied and a homeowner discovers that running the shower and washing machine simultaneously drops the shower to a trickle. At that point, the only resolution is to open walls and repipe the undersized branch — an expensive and disruptive renovation that a correctly sized rough-in would have prevented entirely.

Pressure Testing Before Wall Closure

Before the rough-in inspection is called, the licensed plumber should perform a hydrostatic pressure test on all supply lines — capping all stubs and pressurizing the system to 1.5x the normal operating pressure (typically 600–700 kPa for a residential Ontario service) and holding for a minimum 15 minutes. Any leak at a soldered joint, compression fitting, or push-connect coupling will present under test pressure before the wall is closed. The inspector will typically confirm that a pressure test has been performed; in some GTA municipalities and under some building authority jurisdictions, a witnessed pressure test is required as part of the formal inspection.

Do not allow walls to be closed without a completed pressure test. A supply line joint that is 95% sealed but weeps slightly under full operating pressure can saturate wall insulation over months before manifesting as a visible stain on the finished drywall — and by that point, the mould is already established in the wall cavity.

Pipe Material Selection for GTA Water Quality

Ontario's Building Code permits several supply pipe materials for residential construction: copper (the traditional standard), cross-linked polyethylene (PEX-A or PEX-B), and CPVC. Each has performance and cost tradeoffs relevant to GTA homeowners:

• Copper: The premium choice for supply piping — corrosion resistant, long-lived, and fully compatible with GTA municipal water chemistry. Requires soldering skill and is the most labour-intensive installation method, contributing to higher rough-in labour cost. Copper is the preferred specification for high-end custom builds in Vaughan and Richmond Hill where long-term material performance is a priority.
• PEX-A: Flexible, freeze-resistant, and fast to install with push-connect or crimp fittings. PEX-A's flexibility means it can be routed through framing with fewer elbows and fittings — reducing potential leak points. Its colour-coded design (red for hot, blue for cold) simplifies installation verification. A well-specified PEX-A rough-in with properly torqued crimp rings is a quality installation; poorly executed crimp connections are the primary PEX failure mode to scrutinize at inspection.
• CPVC: Less common in current GTA residential construction; functionally adequate but more brittle than PEX and requiring solvent-welded joints with appropriate cure time before pressure testing.

Passing the GTA Municipal Rough-In Inspection

Municipal plumbing inspectors in Toronto, Vaughan, Richmond Hill, and Markham review rough-in installations against the Ontario Building Code and applicable local amendments. Common rough-in inspection failures that generate a deficiency notice and require re-inspection:

• Insufficient drain slope on horizontal runs (most common failure in builder-grade production construction)
• Missing or inadequate venting — branch drains without proper vent connections within code-maximum distances from the trap
• Unsupported drain runs between hangers (pipe sag between supports)
• Missing cleanout access points at required locations — specifically, a cleanout is required within 600 mm of the building drain's connection to the main sewer at the foundation, and at every change-of-direction exceeding 135 degrees in the building sewer
• Drain stub-outs at incorrect heights or lateral positions for the specified fixtures (a toilet stub-out at the wrong flange height relative to finished floor elevation requires subfloor modification to correct)
• Supply line pressure test not completed or results not available for inspection

A rough-in inspection failure means the inspection fee is forfeited, a re-inspection fee is required, and the project schedule is delayed until deficiencies are corrected and a new inspection is booked — a cycle that on a custom build in a competitive Vaughan or Markham market has real carrying cost implications.

Choosing the Right Plumbing Contractor for Your New Build

Rough-in plumbing on a new build is not a commodity service. The decisions made behind the walls — drain slopes, vent configurations, supply sizing, support spacing, material specifications — become permanently embedded in the structure of your home. Selecting a licensed master plumber with documented experience in GTA new construction and a track record of first-time rough-in inspection approvals is the most important quality control decision you make before the framing crew is done.

At Perruzza Plumbing, we design and execute rough-in plumbing installations for custom builds and additions across Vaughan, Richmond Hill, Markham, and Toronto — coordinating with the general contractor and building department to ensure clean inspection results and properly engineered installations that serve your home for decades.

If your new build includes hydronic radiant floor heating — which must be embedded during the rough-in slab phase — understand the full cost and engineering scope before concrete is poured: How Much Does Radiant Floor Heating Cost in 2026?. Driveway snowmelt systems follow the same timing requirement: Driveway Snowmelt Systems: Everything You Need to Know. And if your build involves a new water service connection or meter upgrade to meet permit requirements: When Should You Upgrade Your Water Meter?

Visit our Custom Plumbing service page to learn more about our new construction capabilities, or contact us to discuss the plumbing scope on your upcoming build project.