Why Sewer Lines Back Up in the First Big Storm

Sewer backups cluster around the first heavy rains because rain puts water into a sewer system that was never meant to carry it. Groundwater seeps into cracked pipes and separated joints, and stormwater gets in through bad connections — sewer utilities call this inflow and infiltration. Older sewer laterals, the buried pipe that carries everything from your house to the city main, are one of the biggest entry points. Many Central Coast homes built before the 1970s have clay laterals whose joints separate over time and let tree roots in. Between storms, those roots grow at the joints and catch debris, so the line narrows while it still seems to drain fine. When the rain arrives, the flow in the lateral and the main rises, and a line that was barely keeping up can't. Because a sewer drains by gravity, the backup shows up first at the lowest fixture in the house — usually a shower or a downstairs toilet.

What is inflow and infiltration?

Infiltration is groundwater leaking into sewer pipes through cracks, holes, failed joints, and faulty connections. Inflow is stormwater entering the sewer directly, through things like improper drain connections or manhole covers. Either way, clean water ends up in pipes sized for wastewater, and during a storm the extra volume can overwhelm them.

Public sewer utilities point to private laterals as a major source: broken laterals, faulty lateral connections, tree roots, and broken cleanouts all let water in — and can cause backups into homes and businesses when the system is overloaded.

Is my home at higher risk?

The pipe material usually tells the story, and it tracks with when the house was built:

Your risk goes up if the house is older, if there are mature trees growing over or near the sewer line, if you've had a backup before, or if the property sits low relative to the street. Clay laterals under mature trees are the classic recurring root job in older San Luis Obispo County neighborhoods.

What are the warning signs before a backup?

A sewer line usually tells you it's in trouble well before it fails in a storm:

• More than one drain is slow at the same time, or toilets gurgle when another fixture drains.
• The same backup keeps coming back every few months — roots regrowing at a joint.
• A wet spot, sewage smell, or unusually lush green patch in the yard over the line.
• A soft spot or small sinkhole in the yard or driveway — the line may be washing out soil.

One slow sink is usually a branch clog. When it's the whole house, the problem is almost always the main sewer line out to the street.

See the full list of signs →

What should I do before the rainy season?

• Get the line on camera. A sewer camera shows the actual condition — roots, cracks, bellies, separated joints — so you're deciding from footage, not a guess.
• Clear roots properly. A root-cutting blade opens the line but the roots grow back. Hydro-jetting clears roots, grease, and scale more thoroughly — and if the camera shows a break, the lasting fix is repairing that section.
• If you have a backwater valve, check it. Debris can keep the flapper from closing. Inspect it at least once a year and flush it out.
• Stop feeding the problem. No “flushable” wipes, no grease down the kitchen drain.

How a camera inspection works →

Does a backwater valve help?

A backwater valve is a one-way flapper on your building sewer line. Sewage flows out normally, and if the city main backs up, the flapper closes so it can't flow back into your house. Some low-lying areas require them. They only protect you if they're maintained — a valve jammed open with debris slows a backup instead of stopping it.

What should I do if my sewer backs up during a storm?

Stop running water everywhere in the house — every sink, shower, and flush adds to the backup. Keep people and pets away from the sewage; it's a health hazard. If there's a soft spot or sinkhole in the yard or driveway, keep people and vehicles off it. Then call a plumber to find the cause — clearing the line gets you through tonight, but the camera tells you whether it will happen again.

Common questions

Why do only my lowest drains back up?

A sewer line drains by gravity. When it's blocked, water backs up to the lowest opening first — usually a shower, tub, or downstairs toilet — before it reaches anything higher.

Will snaking the line fix tree roots?

It buys time, but the roots grow back. A camera shows whether the roots are coming in through a crack or separated joint; if they are, that section needs repair to stop the cycle.

Who is responsible for the sewer lateral — me or the city?

It depends on the city and where the problem is. In most Central Coast jurisdictions the property owner is responsible for the lateral from the house to the connection at the main, and the city maintains the main itself.

Here's how it usually breaks down →

Want to know what your sewer line looks like before the rains? We camera the line and show you the footage. Call (805) 788-8444 or book a camera inspection.

Book a camera inspection →