What is copper pipe Type M vs Type L?
Type M and Type L are residential copper water pipe grades that differ in wall thickness and maximum operating pressure, with Type M having thinner walls and Type L thicker walls for higher-pressure or underground applications.
Copper pipe comes in several wall thicknesses, with Type M and Type L being the two most common in residential and light commercial work across Ontario. The letter designation indicates the pressure rating and structural durability of the tube. Type M has the thinnest walls of the common grades and handles pressures up to 100 pounds per square inch (psi) at room temperature. Type L has thicker walls and is rated for higher pressures, making it suitable for more demanding conditions.
Ontario plumbing code permits Type M copper in indoor water supply lines where water pressure remains stable and does not exceed code limits. It is the most economical choice for typical household distribution within walls and under slabs. Type L is required in certain situations: when running copper outside a building or underground, where physical damage risk is higher, or where municipal water pressure exceeds what Type M is rated for in that specific application.
Pressure, temperature cycling, and mechanical stress all factor into which grade an inspector will accept on a given job. Some municipalities and water utilities in Ontario mandate Type L for main service lines entering a property. Plumbers must verify local requirements, as code adoption varies by region. If you suspect your home has undersized or corroded copper pipe, leak detection and repair specialists can assess water quality and pressure to recommend whether replacement or reinforcement is needed.