Drainage, waste and vent systems and fuel gas

Every litre of water delivered to a building has to leave it again, usually carrying waste. Drainage, waste and vent (DWV) systems do this by gravity, so they depend on slope, pipe size and air movement rather than pressure. They also keep sewer gases out of occupied spaces. Storm drainage removes rainwater and snowmelt from roofs and sites, and fuel gas piping, though it carries something very different, is usually installed by the same trade under its own code. This lesson covers all three from the constructor's point of view.

DWV work is some of the earliest MEP work on site: underground drainage is laid and tested before the slab is poured, so errors in elevation, slope or location are literally set in concrete. The construction manager's job is to make sure underground work matches coordinated drawings, is surveyed, tested and inspected before cover, and leaves room for everything that has to connect later.

How gravity drainage works

Drainage pipes flow partly full, with air above the water. Each fixture discharges through a trap, a U-shaped bend that holds a water seal to block sewer gas. Fixture drains join branches, branches join vertical stacks, stacks join the building drain at the lowest level, and the building drain leaves the building as the building sewer, connecting to the municipal sewer or a private septic system.

Slope is everything. Too little slope and solids settle; too much and water outruns solids, which are left behind. Codes set minimum slopes based on pipe size.

The National Plumbing Code generally requires a minimum slope of 1 in 50 (2 per cent) for horizontal drains of 3 inches (75 mm) and smaller, and allows 1 in 100 (1 per cent) for many larger drains, subject to the code's tables.

Vents and trap seals

When water rushes down a stack it pushes air ahead and drags air behind, creating pressure swings that can blow out or siphon trap seals. Vents connect the drainage system to the outside air, usually through the roof, so pressures stay close to atmospheric. Types include individual vents, common vents, circuit vents, wet vents and stack vents, each permitted under conditions the code defines.

Air admittance valves, one-way valves that let air in but not out, can replace some vent piping where the code permits. They are widely used in renovations, but they cannot relieve positive pressure, so most codes still require at least one open vent to atmosphere and limit where the valves can be used.

Trap seals can also evaporate in floor drains that rarely receive water, letting sewer gas into rooms such as mechanical rooms and parking garages. Trap primers, which add water automatically, or trap seal devices prevent this, and their omission is a common commissioning complaint.

Sizing drains with drainage fixture units

Drain sizes depend on how much flow each fixture contributes. Each fixture is assigned a number of drainage fixture units (DFU), for example about 1 for a lavatory, 2 for a shower and 4 to 6 for a water closet, depending on the code and fixture. The engineer adds the units on each branch and stack and reads the minimum pipe size from code tables, which also limit how many units each size may carry on a branch or stack.

Materials, cleanouts and interceptors

Common DWV materials are PVC and ABS plastic, cast iron (hub-and-spigot or no-hub with couplings) and, for some uses, copper or stainless steel. Combustible plastic pipe passing through fire separations needs approved fire stopping, and in some taller or non-combustible buildings codes restrict plastic pipe in plenums or require cast iron for noise and fire performance. These rules are a frequent source of submittal changes.

Cleanouts let maintenance crews rod blockages. Codes require them at the base of stacks, at changes of direction, at set intervals along long runs and where the building drain leaves the building. They must remain accessible after finishes go in, which is easy to forget when floors and walls close.

Restaurants and food service areas need grease interceptors to keep fats, oils and grease out of the sewer; in Canada these are commonly certified to CSA B481. Garages and workshops need oil interceptors, and laboratories may need neutralizing tanks. Municipal sewer-use by-laws set discharge limits and often require inspection access.

Storm drainage

Rain and snowmelt are drained separately from sanitary waste in most new construction. Roof drains collect water and send it through leaders and storm drains to a storm sewer, a stormwater management system on site, or a combined sewer in older areas. Because ponding water is heavy and roofs can collapse, codes require secondary (overflow) drainage in addition to primary drains, through overflow drains or scuppers.

Storm drains are sized from the roof area and a design rainfall intensity taken from local climate data for a short, intense storm.

Many municipalities limit how fast sites may release stormwater, so storm systems may include detention tanks, green roofs, cisterns or controlled-flow roof drains. These items affect structure, schedule and maintenance, so the construction manager should understand them early.

Fuel gas piping

Natural gas and propane serve boilers, furnaces, water heaters, kitchen equipment, generators and rooftop units. The utility brings gas to a meter and regulator; from there the gas fitter installs piping, usually at low pressure inside the building, sometimes at higher pressure with regulators near appliances. Gas piping is sized from the total connected load, the allowable pressure drop and the length of run.

In Canada, gas installations follow CSA B149.1 Natural gas and propane installation code (the 2025 edition is current) and provincial regulations, and gas fitters are licensed by provincial authorities such as TSSA in Ontario (G1, G2 and G3 certificates) and Technical Safety BC. Installations are inspected before the gas is turned on.

Gas piping is pressure tested before concealment, at a pressure and duration set by the code and the piping system's operating pressure, and appliances need combustion air, venting and clearances that the HVAC and architectural trades must coordinate. Electrification policies are reducing gas use in some new buildings, and some cities restrict new gas connections, so check local rules early.

Testing and inspection

DWV piping is tested before concealment, typically with water held at a head of at least 3 metres for the time the code specifies, or with air in some cases, and a final test may follow when fixtures are installed. Underground drains are often camera-inspected before the slab is poured.

The construction manager should require an as-built survey of underground drainage before the slab is poured, since locating a buried drain later is costly. Record inverts, cleanouts and sleeve locations, and keep test reports for closeout.

Drainage on site: what goes wrong

Drainage problems are expensive because they are usually buried or concealed by the time anyone notices. The most common site errors are lost slope, where a pipe sags between hangers or was set at the wrong elevation; reversed grade, where a section runs uphill; missing or misplaced cleanouts; vents connected below the flood level of the fixture they serve; and debris such as concrete slurry left in pipes after a pour. A camera inspection of underground drainage before the slab is poured, and again before handover, is cheap insurance and is increasingly written into specifications.

Coordination matters too. Gravity drains cannot bend around obstacles the way pressure pipes can, so they should be routed first in the ceiling space and given priority over ducts, cable trays and sprinkler mains. A construction manager who sequences drainage rough-in before other trades, and protects its slope through the coordination model, avoids the classic clash where a duct main forces a drain to rise and lose its fall.