Why do my drains and toilets back up during monsoon storms?

The sanitary sewer that carries waste from your toilets and sinks is supposed to stay separate from the storm drains that carry rain off the streets. During a heavy monsoon downpour, that separation breaks down. Rain and rising groundwater find their way into the sanitary sewer, the pipe fills past its design capacity, and the extra volume has to go somewhere.

When a sewer pipe fills completely and pressurizes, plumbers and engineers call it a surcharge. A surcharged main has no free space, so wastewater stops flowing toward the treatment plant and instead pushes backward through the branch lines that feed it. The lowest opening connected to that line is where it escapes, which is why a floor drain, a ground-floor shower, or a first-floor toilet floods before anything upstairs does.

The National Weather Service in Phoenix sets the season on a fixed calendar. As the NWS states, "The Southwest monsoon season runs from June 15 to September 30." Peak storm activity lands from mid-July to mid-August, so that window is when surcharge backups cluster.

The sanitary sewer is sized for the steady flow of household drains, not for a flash flood. Two paths let storm and ground water flood it, and engineers group both under the term inflow and infiltration, or I/I.

Infiltration is groundwater seeping in through cracks, bad joints, and deteriorated pipe walls. After a storm soaks the ground, the water table rises and presses water into every flaw in the buried line. Inflow is the faster problem: stormwater pouring in through direct openings. Often those openings are improper connections, where a roof downspout, a foundation drain, or a yard area drain was tied into the sanitary sewer instead of the storm system. In a hard rain, a single roof can dump hundreds of gallons a minute straight into a pipe meant for slow household flow.

Phoenix soil makes infiltration worse. The valley sits on expansive clay that swells when wet and shrinks when dry. The Arizona Geological Survey notes that this kind of shrink and swell movement causes more cumulative damage to structures than floods, tornadoes, and hurricanes combined. Each monsoon soaking and dry-out cycle nudges buried pipes, opening the cracks and joint gaps that let groundwater in.

A clean, open sewer line can usually shed a normal storm. A line that is already half choked cannot. When grease, tree roots, or debris have narrowed the pipe, the surge of I/I water has even less room to move, so a marginal line backs up sooner and harder than a healthy one.

The EPA has measured what clogs sewers. In its 2004 Report to Congress, grease accounted for 47 percent of reported sewer blockages, with roots near one-quarter and grit or debris making up most of the rest. Blockages are the single largest known cause of overflows. As the EPA puts it, common causes of sanitary sewer overflows include "grease blocking sewer pipes," "tree roots that have grown into the pipes," and "infiltration/inflow of excessive stormwater into sewer pipes during heavy rains."

So the monsoon does not usually create a brand new problem. It exposes one that was building all year. A line slowly coated with kitchen grease or invaded by roots had just enough room for dry-weather flow. Add the storm surge, and the same pipe tips over into a backup.

The scale of this nationally is large. The EPA estimates there are between 23,000 and 75,000 sanitary sewer overflows each year in the United States, spilling an estimated 3 to 10 billion gallons. Heavy rain is one of the recurring triggers, which is why a single storm can mean a wave of backup calls across a city in the same afternoon.

The fixture that floods tells you where the trouble sits. A backup that appears at one upstairs sink while everything else drains points to a local clog on that branch. A backup at the lowest fixture in the house, especially during or right after a storm, points to the main line or the public sewer, because that is the first place a surcharge finds an exit.

A classic tell is water rising in a tub or shower when you flush a toilet. Waste that cannot pass a downstream blockage or a full main re-emerges at the lowest opening it can reach. Our page on what it means when water backs up in the shower when you flush walks through that specific symptom and what it says about where the obstruction is.

Other signs to watch during the season include drains across the house slowing at once, gurgling from toilets or floor drains, and a sewage smell near low drains. The gurgling comes from air being pulled through the water seal in a trap as a full line struggles to move waste, and it often arrives just before a visible backup. Any of these signs arriving with a storm is a strong clue that the public sewer, not your private fixture, is surcharged. Tracking which fixtures are affected, and noting whether the trouble starts only when it rains, helps a plumber confirm whether the cause sits on your side of the property line or the city's.

Stop adding water. The instant you see sewage rising at a low drain during a storm, shut off every fixture and tell the household to stop flushing toilets, running sinks, and starting the washing machine or dishwasher. Every gallon you send down adds to a line that already has nowhere to go and pushes more dirty water onto your floor.

Keep people and pets away from the standing water, since sewage backup carries bacteria. Do not pour chemical drain cleaner into a surcharged line, because it will not clear a flooded main and only leaves a hazard for whoever works on the pipe. Then call a licensed plumber. A camera inspection shows whether your line is clear or partly blocked by grease or roots, and that tells you whether the cause was the storm alone or a private-line problem the storm revealed.

If backups keep happening every monsoon, the long-term fixes are mechanical. A backwater valve lets waste flow out but closes against sewage trying to push back in, and clearing a grease- or root-fouled line before June 15 keeps a marginal pipe from tipping over in the first storm. Our guide to preventing sewer backup before monsoon covers those steps, and our broader page on how monsoon season affects plumbing covers the slab and soil effects that go beyond drains.