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Plumbing in a New Ontario House: Supply, Drains, Vents and What the Inspector Checks
During construction Plumbing From a builder
Plumbing in a New Ontario House: Supply, Drains, Vents and What the Inspector Checks
Plumbing is water in, water out, and air in behind it so the water can leave. Everything else is detail. The detail is where the callbacks come from, so here is the system the way I explain it to a client standing in a framed house.
Short version: A new house has three plumbing systems: the pressurized supply (well or municipal service in, hot and cold out to every fixture, ideally through a manifold), the drain-waste-vent system (every fixture through a trap into drains that fall to the main stack and out to the sewer or septic, with vents up through the roof so the drains breathe), and the fixtures themselves. In Ontario the work is done by a licensed plumber under Part 7 of the Building Code, with a rough-in inspection before the walls close and a final at the end. Decide fixture locations, the manifold, the water heater type and any future bathroom rough-ins before the slab is poured, because moving a drain later means breaking concrete.
When plumbing happens in the build
- Under-slab: the main drain, the stack base, floor drains, basement bathroom rough-ins and the sump pit go in before the basement slab is poured. Get this right; it is the one stage you cannot revisit without a jackhammer. Weeping tile and sump covers the drainage side.
- Tubs and showers: one-piece units go in before the walls are closed because they will not fit through a framed door.
- Rough-in: after framing, with the electrician and HVAC trades. Supply lines, drains, vents and the water heater location are run and capped. The plumbing rough-in inspection happens here, usually with a pressure test on the supply and a water or air test on the drains.
- Final: fixtures, faucets, toilets, the water heater connection, and the final inspection before occupancy. What happens when an inspection fails is worth reading before the rough-in.
The supply side
Water enters below frost depth (four feet or more in most of Ontario) from the municipal service or the well line, through the meter or the pressure tank, and splits to cold and to the water heater. From there it is either a trunk-and-branch system (one big line with tees to each fixture) or a manifold: a panel with a valve for every hot and cold run, each run a continuous PEX line to its fixture. The manifold costs a little more in pipe and saves you forever: shut off one fixture without shutting the house, no joints in the walls, and hot water arrives faster to the far bathroom.
Pipe is mostly PEX now, with copper at the water heater and where it shows. Size matters: a long run to a master shower on 1/2 inch will disappoint. Ask for 3/4 inch trunks and a recirculation line or an on-demand pump if the kitchen is far from the heater. Tankless water heaters and the heat pump water heaters now common in new builds change the piping, so decide the heater before the rough-in.
Drains, waste and vents
Every fixture drains through a trap, the U-bend that holds a plug of water so sewer gas stays in the pipe. Toilets have the trap built in. Drains slope down (a quarter inch per foot is the usual rule for small pipe) to branch lines, to the main stack, to the building drain, and out to the sewer or the septic tank. The stack is typically 3 or 4 inches and runs up through the roof as a vent.
Vents are the part people do not see and the part the inspector looks hardest at. Without air behind it, water in a drain pulls a vacuum and siphons the trap dry, and then the bathroom smells. Each fixture has to be within a set distance of a vent depending on its drain size; far fixtures get their own vent run up to the stack or out the roof. Air admittance valves are allowed in some situations and not others; your plumber will know which, and the inspector will check.
Backwater valves on the building drain are required or strongly advised in many municipalities on municipal sewer; on septic the design is governed by Part 8 and the tank location. Septic in Ontario covers that side of the fence.
Decisions to make before the walls close
- Where every fixture goes, exactly: a vanity moved six inches after rough-in is a wall opened up.
- Rough-in for the future bathroom in the basement, now, under the slab, even if it is ten years away.
- Hose bibs: frost-free, at least two, and one near where the car gets washed.
- Laundry location and a floor drain or pan under it on an upper floor.
- Water softener and treatment space near the entry point if you are on a well.
- Shut-off valves at every fixture and a main shut-off you can reach without moving a shelf.
- Insulate the hot lines and any supply line on an exterior wall; better, keep supply lines out of exterior walls altogether. In an ICF house that is easy, since the interior face is foam over concrete and the lines run in the furred service wall or interior partitions.
- Photograph every wall before drywall. The before-drywall checklist says why.
What the inspector checks
Ontario plumbing is Part 7 of the Building Code, inspected by the municipal plumbing inspector (in some municipalities the building inspector carries both). At rough-in: pipe sizes, slopes, venting distances, trap arms, cleanouts, supports, the pressure test on the supply and the test on the drains, backflow protection, and that the under-slab work matched the drawings. At final: fixtures connected, traps, the water heater relief line, the backwater valve if required, and no cross-connections. Building inspections in Ontario is the hub for every inspection on the job.
Related guides on this site
Sources and further reading
Questions people ask
Do you need a licensed plumber in Ontario for a new house?
Yes. Plumbing in Ontario is a compulsory trade; the work is done by a licensed plumber (or a registered apprentice under one) and inspected under Part 7 of the Building Code at rough-in and at final.
What is a plumbing manifold and is it worth it?
A panel with an individual valve and a continuous PEX run for every hot and cold fixture. It costs a little more in pipe and gives you fixture-by-fixture shut-offs, no joints in the walls and faster hot water. In a new build it is worth it.
Why do drains need vents?
Water leaving a drain pulls air behind it. Without a vent it pulls the water out of the traps, and sewer gas comes into the house. The Code sets how far each fixture can be from a vent based on the drain size.
When is the plumbing inspected?
At rough-in, after the pipes are run but before insulation and drywall, with tests on the supply and drain systems, and at final before occupancy. Under-slab work is inspected before the basement floor is poured.
Got a code question this page did not answer?
Ask it in plain English and the OBC Code Navigator answers with the Part, Section and Article from the Ontario Building Code. Two questions free. Your municipality and inspector still have the final word.
Reviewed and updated October 2026. Costs are Ontario ranges, not quotes; code references are to the Ontario Building Code in force at that date.
I typically like to hire a plumber to come and take care of any plumbing needs that my family may have, but I thought it would be good to do some research on how things are done just in case. I did not know much about vent pipes before reading this. I will look and see if I can find mine behind the sink!
Erika….more than likely you have a vent at your sink. If you don’t it usually likely you can add a new gadget called an Air Admittance Valve somewhere on the sink drain (also called an AAV). As a kid you may have put a straw in a glass of water and ut your thrumb over the end of the straw so the water would not come out of the straw. When you took your thumb off the water would drain out. This same theory applies to your drainage system. If your drain pipes cannot get air you can get drain lock…thus the reason for plumbing drainage system venting.
Very helpful article. Do all plumbing fixtures require a water supply and a drain? Is it up to code to install a fixture without a water supply (ie. Drain only) or a fixture without a drain (ie. Supply only). Two plumbers said 2 different thinks and I was hoping to get the facts straight.
Wow, this article on plumbing basics is super helpful for those of us who aren’t exactly experts on the subject. I love how it breaks down all the different parts and functions of a plumbing system in an easy-to-understand way. I definitely learned a lot and feel more confident about tackling some basic plumbing issues around my home now. Thanks for sharing!
This is a solid introduction to essential plumbing concepts every homeowner should know. I like how the article simplifies things like water flow, pipe types, and common fixtures without getting too technical. Understanding the basics before you tackle repairs or talk to a pro makes a world of difference in avoiding confusion and unnecessary costs. Thanks for breaking it down in a way that’s easy to follow and genuinely useful!