Sewer Roaches Coming Up the Drain: Why American Cockroaches Show Up After Monsoon Rain

American cockroaches show up in bathrooms and garages after monsoon rain because the rain floods the sewer system they live in and pushes them up through floor drains, shower drains, cleanouts and any P trap that has gone dry. The fix is to treat the drains and the exterior, keep every trap full of water, cover floor drains with mesh, and replace any cracked cleanout cap.

What a Sewer Roach Is

The American cockroach (Periplaneta americana) is the big one, 1.5 to 2 inches long, reddish brown with a pale band behind the head, and it can fly short distances, which is why one landing on a bathroom wall gets a reaction. In Arizona it is called the sewer roach because that is where it lives. The sanitary sewer under a neighborhood is warm, wet and full of organic material year round, and colonies of American cockroaches live in the mains, the laterals and the manholes, moving up into houses through the plumbing when conditions push them.

It is a different animal from the German cockroach, the small tan roach with two stripes that infests kitchens and never came from outside, and from the Turkestan cockroach, the dark outdoor roach in the block wall. Sewer roaches are not living in your kitchen cabinets. They are visitors from the pipes, and the program to stop them is about the pipes and the exterior, not gel bait behind the refrigerator.

Why Rain Brings Them Up

From late June through September the first big monsoon storm dumps an inch or two of rain in an hour, and in older flood irrigated neighborhoods in Phoenix, Mesa, Tempe and Glendale that water gets into the sewer system through manholes, cracked laterals and storm cross connections. The mains surge, the air in the pipes is displaced, and the roaches move up ahead of the water into the house laterals and out of every opening that is not blocked by a water seal. The same thing happens on a smaller scale when a neighbor runs a plumbing snake or when the city jets the main.

The openings are the traps. Every drain in a house has a P trap, a U bend that holds water and blocks sewer gas and insects. A shower in a guest bathroom that has not been used in 3 months, a laundry room floor drain, a utility sink in the garage, a tub that nobody uses: the water in those traps evaporates in Arizona heat within a few weeks, and the pipe is then open straight to the sewer. The cleanout in the yard is the other door. It is a capped pipe at ground level, and when the cap is cracked, missing or loose, roaches walk out of it every night.

Where to Look

The pattern points to the source. Roaches in one bathroom and the garage after rain means the drains in those rooms. Roaches along the exterior foundation and in the meter box means the yard cleanout or the block wall. Roaches everywhere in a kitchen at all hours, small and striped, means German roaches and a different program.

• Guest bathroom showers and tubs that go unused.

• Floor drains in the laundry room, garage and patio.

• The sewer cleanout in the front or side yard, usually a 4 inch capped pipe near the foundation.

• The water meter box and irrigation valve boxes, which are damp and connected to the same soil.

• Gaps at the slab edge and the garage door seal, where roaches from the block wall walk in.

What Stops Them

1. Run water down every drain once a week. A gallon down each shower, tub, sink and floor drain refills the trap and closes the door. This is the single most effective thing a homeowner can do and it costs nothing.

1. Mesh drain covers on floor drains and shower drains that are rarely used, so a dry trap is still a blocked opening.

1. Check the cleanout cap. Replace a cracked or missing cap with a tight threaded one.

1. Drain treatment. A licensed technician treats the drains, cleanouts and the meter box with products labeled for that use, and a microbial drain cleaner keeps the organic film that feeds them down.

1. Exterior residual and granular bait along the foundation, the block wall, the garage perimeter and the meter box, so roaches that come out of the ground or the wall do not make it inside.

1. Seal the slab edge and the garage door, since Turkestan and American roaches both walk in under a lifted seal.

Sewer roaches carry bacteria from the sewer on their bodies and legs, and a roach that walked out of a floor drain and across a bathroom counter is a sanitation problem, not just an ugly one. People with asthma react to roach allergens, and a heavy population in a garage or a restaurant is a health inspection issue.

FAQ

Are sewer roaches the same as the roaches in my kitchen? Almost never. Kitchen roaches that are small and striped are German cockroaches and need a gel bait and growth regulator program. Big reddish roaches in the bathroom after rain are American cockroaches from the sewer.

Why do they show up in the same bathroom every time? Because that bathroom has the drain with the dry trap or the cracked cleanout nearest it. Run water in it weekly and put a mesh cover on the drain.

Will bombing the house fix it? No. Foggers do not reach the sewer, and the next storm brings the next wave. Treating the drains and the exterior, and keeping the traps full, is what holds.

If the roaches in the bathroom arrived with the first storm, a cockroach control visit from Green Ninja Pest Control treats the drains, the cleanout and the exterior and shows you which traps have gone dry.