Home inspections address drainpipe problems more indirectly than directly. First, much drainpipe is out of sight, and inspectors do not perform any kind of invasive testing. Second, their focus is more on normal functioning of the plumbing system rather than individual components, unless and until they detect a leak or excess moisture accumulation, in which case they will try to discover the cause of the defect and analyze piping. Furthermore, a lot of drainpipe is outside the home and underground, rendering it essentially out of the inspector’s purview.
Drainpipe is used for several different purposes and in different applications, both in and out of the house. Sometimes the terminology is misleading pedestal pumps. Inside, drainpipe is used to empty plumbing fixtures and to funnel waste by means of gravity to a common exit point of lowest elevation, tending to get larger as it goes. (Here, some purists distinguish drain piping from waste piping by claiming that the former does not carry solids.) Other piping used to vent and equalize the air pressure for the drainage system (and connected directly to it) is sometimes found to double as drainpipe.
Outside, drainpipe connects house plumbing to a sewer or septic system. It is used to collect storm water runoff and transport it away from the foundation. It also forms the backbone of the French drain and similar constructs designed to mitigate over-saturated areas in the yard. Drainpipe terminology applies to all of these functions, even though there is considerable overlap in the kinds of professions dealing with them.
Inspecting drainage problems inside the house might touch upon investigating traps, vents, leaks, or floor drains. A clogged drainpipe can result from a trap of the wrong type (i.e., not a P-type), double trapping, not enough slope, or insufficient pressure due to a venting problem. Odor problems stem from a siphoned-off trap (perhaps too much pressure or an S-type design) or possibly an evaporated trap that sometimes occurs in a floor drain that isn’t used very often. Leaks can come from a host of potential sources, including corroded drainpipe fittings and a cracked or damaged drainpipe.
Outside, the inspector checks for unusually wet areas in the yard and especially around the foundation. He checks the functioning of gutters and down spouts and whether a closed system of drainpipe appears to be channeling storm water away from the foundation. (Poor drainage creates saturated soil and great hydrostatic pressure on foundation walls, leading to cracks, tilting, and bowing.) If necessary, he investigates for the presence of a retention pit, rain garden, or drainpipe exiting to daylight. He compares outdoor conditions with what he finds in the crawl space or basement to assess whether storm water is being handled properly.
Additional relevant items that may be included on the home inspection checklist include sump pumps and French drains. Water in the crawl space arises from no foundation drainpipe system, a high water table, leaks, or underground springs. If drainage by means of gravity isn’t possible (because the house is in a bowl), a sump pump is the only option. French drains are sloped ditches lined with perforated drainpipe to improve yard drainage.