Type L and Type M copper pipe comparison: Use Type M for most home situations. Use heavier duty Type L for underground and other special situations. But check with your local plumbing inspector for regional requirements.
Type L copper pipe is recommended where you need strength and protection. But for normal “in the wall” household plumbing, Type M copper pipe is just fine.
You may be surprised to find two types of copper pipe on the home center shelf, Type L and Type M. The difference is the wall thickness of the pipe and therefore the pressure it can handle. The exterior dimensions are identical, meaning you use the same copper fittings. You also use the same tools, materials and techniques to cut and sweat them. The beefier Type L is often used underground, in hot water heating systems, for commercial plumbing and for gas line (where permitted). Most plumbers use the less-expensive Type M for residential projects: One told me, “Type L pipe will last 300 years and Type M only 250.” But check with your building inspector; some areas require Type L.
Acidic water with a pH of 7 or less can be hard on copper pipe. If you have a well and find out from testing that the water is acidic, you may want to use the thicker-wall Type L pipe; better yet, use “plastic” CPVC pipe and fittings. Note that most “city water,” even in areas of the country where the water is acidic, is balanced to eliminate this problem.
But according to veteran plumber Charlie Avoles, the best alternative to copper is PEX (flexible plastic pipe). “We’re seeing that many properties of PEX are actually tougher than copper, and PEX doesn’t appear to get fragile with age and exposure to ultraviolet light like CPVC does.”