Angie's LIST Guide to
Main sewer drains

This graphic shows the basic paths of a typical home's sewer lines -- key parts of which carry no water at all, but only air to provide proper venting.

(1) All drain pipes should be connected to a network of ventilation pipes that go up through the roof. Venting prevents sewer gases from drifting out of drains into living quarters.  When having a home inspected before making a purchase offer, make sure the inspector confirms that all venting pipes are present.

(2) Most plumbing fixtures have curved "trap" sections that hold a little water forming an airtight seal to keep gases in. This section of pipe is easily removed to clear clogs.

(3) Toilets also have a trap, which is what keeps standing water in the bowl. The toilet drain is the largest drain pipe in the house.

(4) All household drains meet below the house in a main drain that carries the wastewater to the municipal sewer lines or to a septic system. A Y-shaped pipe in a basement or crawlspace provides clean-out access. 

Homeowners can often fix drain clogs themselves if the problem is confined to specific fixtures such as a sink drain or a toilet. However, when all drains are affected – or when water going down one drain backs up in another drain – that generally means the main drain is clogged. Call a plumber who specializes in clearing drains.

If it’s a blockage from something flushed down a toilet, clear the drain by using a plumber’s auger or snake. However, in older neighborhoods, the cause may be tree roots. Older sewer lines were made from fired clay and as they age and weaken the tree roots creep in.

Drain experts are often able to bore through the roots and clear the drain, but in many cases the original drain pipe has become so deteriorated that the new passageway eventually collapses. When this happens the drain line usually needs to be replaced.

When lines become clogged and damages occur, in the majority of cases, the homeowners’ responsibility begins at the connection point to the city sewer system and includes the lateral lines and all drain plumbing in the home. Any maintenance, repairs and unclogging of these lines must be done by the homeowner, often with the help of a plumbing service.

While the indoor piping is relatively easy to repair, underground lateral lines require specialized equipment and tools, so those repairs often require a plumber.

Advice from readers:

A question posted by a reader at answers.angieslist.com prompted several cautionary tales from others about the risks of chemical drain cleaning products -- Join the discussion