PVC pipe shows up in nearly every residential plumbing project: drain lines, vent stacks, irrigation systems, and cold water supply lines all rely on it. The material is inexpensive, durable, and DIY-friendly, but there are a few sizing rules and installation techniques that separate a leak-free system from one that fails at the worst possible time.
Schedule number refers to wall thickness. A higher schedule means a thicker wall. Both Schedule 40 and Schedule 80 share the same nominal pipe sizes and the same fittings (they're interchangeable), but Schedule 80 has a thicker wall, a smaller inside diameter, and a higher pressure rating. For residential drain, waste, and vent (DWV) applications, Schedule 40 is the standard and is what all residential building codes allow. Schedule 80 is used where pipe is exposed to potential physical damage, typically in commercial settings or where pipe runs along a wall and could be hit. For irrigation and low-pressure water supply, Schedule 40 is again the correct choice for most homeowners. The gray PVC you see in electrical conduit applications (Schedule 40 conduit) is not rated for plumbing use. It's a different compound.
Like lumber, PVC pipe nominal sizes don't match their actual dimensions. A 1/2-inch nominal PVC pipe has an outside diameter of 0.840 inches, not 0.5 inches. The nominal size roughly tracks the inside diameter, but not exactly. What matters in practice: always buy fittings that match the same nominal pipe size, and when connecting to copper or threaded fittings, use a transition fitting designed for that connection. Slip fittings (the most common type) are sized to accept pipe by outside diameter. The socket opening is machined to receive the OD of the matching nominal size.
Drain lines must be sized to handle the fixture units draining into them and must run at the correct slope to carry waste without clogging. The standard drain slope is 1/4 inch per foot of horizontal run, enough to keep solids moving without running so fast that liquids outrun solids. A 1.5-inch pipe is the minimum for a single lavatory or utility sink. A 2-inch line handles a shower, floor drain, or two lavatories. A toilet requires a minimum 3-inch drain. The main house drain (building drain) is typically 3 or 4 inches. Never reduce drain pipe size in the direction of flow. A 4-inch main connecting to a 3-inch branch creates a blockage point.
Every fixture drain must be trapped (usually with a P-trap) and vented. The vent keeps the trap from being siphoned empty by the suction created when water drains. The distance from the trap weir to the vent connection (the trap arm length) is limited by code, typically no more than 5 feet for a 1.5-inch line and 8 feet for a 2-inch line. Longer unvented trap arms lead to slow drains and gurgling from the trap being sucked dry. AAVs (air admittance valves) are an alternative to running vent pipe through the roof and are code-approved in most jurisdictions for individual fixture venting.
PVC joints are chemically welded, not mechanically sealed. The solvent cement partially dissolves both surfaces so they fuse into a single piece of plastic when the solvent evaporates. The process is simple but unforgiving. Once the joint is set (typically within 30 seconds), it cannot be adjusted. Cut the pipe square with a pipe cutter or fine-tooth saw and deburr the cut end. Apply purple primer to both the pipe end and the fitting socket; primer removes surface oxidation and softens the PVC for better fusion. Apply cement to both surfaces, insert the pipe with a slight twist, hold for 30 seconds, and wipe excess cement from the joint. Never skip the primer step on pressure-rated lines. It's required by code and significantly improves joint strength.
Standard white Schedule 40 PVC becomes brittle when exposed to UV radiation over years. If a run of pipe is outdoors and exposed to sunlight, paint it with latex paint or use CPVC (chlorinated PVC, which is more UV-resistant) or a UV-stable pipe cover. PVC also becomes brittle below freezing. Irrigation lines that cannot be fully drained before a hard freeze will crack at the fittings. Drain all outdoor PVC irrigation before winter and use blow-out winterization if there are sections that can't drain by gravity. For indoor hot water lines, standard PVC is not rated for hot water. Use CPVC (cream colored) or PEX, which are rated for temperatures up to 200°F and 180°F respectively.
