8 Signs of a Clogged Main Sewer Line

One clogged sink or toilet is a nuisance. A clog in the main sewer line, the pipe that carries everything from your house to the city sewer, affects every drain and can push raw sewage, with its health risks, back into tubs and showers. In San Jose and South Bay homes, tree roots are the usual culprit.

The good news is that a main line clog almost always gives warning signs first. Catching them gives you time to call a plumber before it turns into a backup and costly water damage. Roots are most common in homes built in the 1950s to 1970s, with clay or cast-iron sewer laterals under mature trees, and when they’ve cracked or crushed an old lateral, sewer line repair or replacement is the lasting fix.

The signs below tend to show up in roughly this order as a clog gets worse: slow drains first, then gurgling and odors, then water rising in the lowest fixtures.

1. Several Slow Drains at Once

Water draining slowly from sinks, tubs or showers is often the first sign that something is restricting the flow. One slow drain is usually a local clog, like hair in the shower drain. Slow drains in the kitchen, bathrooms and laundry at the same time point to the main line. You might notice the bathroom sink draining slowly one week and the kitchen sink or tub the next as the blockage builds.

2. Gurgling Drains and Toilets

Bubbling or gurgling when water drains elsewhere, such as a toilet that gurgles after you use the bathroom sink or drain the tub, means air trapped behind a blockage is being pushed back through the water traps. Gurgling usually follows slow drains, and it gets more frequent as the clog grows and the line has less room for air and water to pass.

3. Sewage Odors

A sewer smell (rotten eggs or raw sewage) from floor drains, tub drains or the yard can mean wastewater is sitting in the line, sewer gas is escaping because of a blockage or a vent problem, or a cracked pipe is leaking. An odor that shows up alongside slow drains and gurgling is a sign the clog is getting worse. Sewer gas is unpleasant and can be unhealthy in an enclosed space, so don’t ignore a smell that keeps coming back.

4. Standing Water in the Lowest Fixtures

Water or sewage collecting in a shower, tub or floor drain, the lowest openings in the house, is a classic main line sign and usually means the line is nearly blocked. Wastewater that can’t get past the clog rises to the first opening it reaches. If you’ve already noticed slow drains and gurgling, standing water is the next step. It’s more likely during heavy winter rain, when water seeping into cracked laterals adds to the load on a line that’s already restricted.

5. Toilets That Overflow or Bubble

Toilets have the largest drain and sit closest to the main line, so they show main line problems first: slow flushing, bubbling, or water rising when other fixtures drain. Bubbling happens when air trapped by the blockage is forced back up through the toilet after you use a sink or tub; overflow happens when waste has nowhere to go. Both can appear suddenly, often right after a dishwasher or washing machine drains.

6. Water Backing Up Into the Tub or Shower

Flush a toilet and watch the tub: if dirty water comes up the tub or shower drain, the main line is blocked. This is one of the clearest signs there is, and it means the clog has moved past the early stage. Stop using water and call a plumber before the next flush or load of laundry pushes sewage out onto the floor.

7. Drains That Clog Again and Again, or Several at Once

If a plunger or snake clears a drain for a few days or weeks and the clog returns, the problem is likely farther down the line. The same goes for several fixtures backing up together, such as the toilet rising while the washing machine drains, or the shower backing up when you flush. Every drain in your home feeds into a single main line (see how the sewer system works), so a blockage there affects all of them. Roots are a common reason for repeat clogs: cleaning removes them, but they grow back through the same joints.

8. Backups When the Washer or Dishwasher Drains

A washing machine or dishwasher sends a lot of water down the drain at once. If a toilet, tub, sink or floor drain backs up while it drains, the main line can’t carry the flow and is partly or completely blocked. For example, dishwasher water may come up in the kitchen sink, or laundry water may rise in a downstairs shower. Heavy rain makes it worse, because the extra water entering an older lateral leaves even less room.

What to Do Right Now

1. Stop using water: no flushing, showers, laundry or dishwasher.
2. Don’t open the cleanout on a full line; sewage can spill out under pressure.
3. Keep people and pets away from any sewage, and don’t use chemical drain cleaners.
4. Call a plumber. CPI answers emergencies at (408) 978-3093; see emergency plumber cost.

Causes of Sewer Line Clogs

Most main line clogs come from what goes down the drains, from roots, or from the pipe itself wearing out. Knowing the cause matters, because it decides whether cleaning is enough or the pipe needs a repair.

• Flushed products. Wipes (including those labeled “flushable”), paper towels and hygiene products don’t break down the way toilet paper does. They catch at joints and bends and build into clogs and sewer backups. Flush only human waste and toilet paper; a small sign by the toilet helps in homes with children or guests.
• Grease. Cooking grease and oil look harmless going down the sink, but they cool, harden and cling to the pipe walls, trapping food and debris until the pipe narrows. Let grease cool in a container and put it in the trash.
• Tree roots. Roots grow toward water and enter through small cracks and loose joints, then expand until they block or break the pipe. It’s most common in older clay and cast-iron laterals near large trees, and in the South Bay’s dry months a sewer line is a reliable water source. Regular inspection, cleaning, root barriers and root-control products labeled for sewer lines help; see how to get rid of roots in a sewer line.
• Mineral scale. Hard water leaves calcium and magnesium deposits that narrow pipes and give other debris something to catch on. Crusty faucets and showerheads are a clue. A water softener reduces scale over time.
• Sags, breaks and collapsed pipe. Older clay, cast-iron and Orangeburg (tar paper) pipe can crack, settle into a low spot (a belly) that collects waste, or collapse under soil pressure. A collapsed pipe stops all flow and needs a repair or replacement, after a camera inspection confirms the damage.
• Foreign objects. Toys, hair, food scraps from a misused garbage disposal, and debris from remodeling can lodge in bends and traps. Drain strainers and a little care during repairs prevent most of these.

Diagnosis and Detection Techniques

A plumber finds the cause before choosing a fix, so you pay for the right repair instead of repeated cleaning. Depending on the symptoms, diagnosis can include:

• Checking the cleanout for standing water, which shows whether the blockage is between the house and the cleanout or farther out toward the street.
• A video camera inspection of the pipe interior to find the exact location and depth of a clog, roots, a sag or a break; see sewer inspection cost and sewer scope inspection.
• Testing fixtures in order, lowest first, to see where water backs up and which branch is affected.
• A water (flood) test, filling a section of drain line to see whether it holds or leaks.
• Locating the line with an electronic locator, which traces the camera’s transmitter from above ground to map the pipe’s path and depth.
• Smoke testing, which pushes harmless smoke into the system to show where gas escapes through cracks or faulty vents.
• Root detection with the camera, before cutting roots out with a cable machine or hydro jetter.
• A pipe condition review: the camera footage shows the pipe material, cracks, offsets and corrosion, which decides between cleaning, a spot repair or replacement.

For more on what comes next, see how to unclog a main sewer line.

How a Plumber Fixes It

• Clearing the line with a cable machine or hydro jetting, usually through the cleanout
• A camera inspection to see why it clogged and whether the pipe is damaged
• A repair or replacement if the pipe is broken, root-filled or sagging; see sewer line repair methods and sewer line replacement cost

For typical prices for clearing a line, see drain cleaning cost.

DIY vs Professional Sewer Line Repair

Whether to try it yourself comes down to where the clog is, how bad the damage is, and the tools and permits involved. A single slow sink is a reasonable DIY job; a main line backup or a damaged pipe is not.

Clearing a main line yourself often treats the symptom and misses the cause, and a mistake can turn a clog into a broken pipe. A plumber can clear the line, show you on camera why it clogged, and recommend the fix that lasts. CPI’s free sewer line inspection coupon covers a camera inspection through an accessible main line cleanout.

How to Prevent Main Line Clogs

• Keep grease, wipes and paper towels out of drains and toilets
• Use strainers in sinks and tubs
• Schedule maintenance cleaning if roots are a known problem
• Have an older line camera-inspected every few years
• Add an outdoor cleanout if your home doesn’t have one

See sewer line maintenance tips.

Sewer Line Clogs in San Jose Homes

• Street trees and clay laterals. Roots are the most common cause of recurring main line clogs in older South Bay neighborhoods.
• Missing cleanouts. Many older homes have no outdoor cleanout, which makes clearing the line harder; adding one helps.
• Who owns the line. In San Jose, the property owner is responsible for the lateral from the house to the main; once an approved wye cleanout is installed at the property line, City crews repair the section from the cleanout to the main. If you suspect the City main is backed up (neighbors are having problems too), call the City’s 24/7 sewer line at (408) 794-1900.
• Holiday and storm season. Big gatherings and heavy rain put extra load on marginal lines.

CPI Service clears and repairs sewer lines throughout San Jose and the South Bay. See drain and sewer cleaning, request a free estimate or call (408) 978-3093.