Why Your Sewer Backs Up After Heavy Rain

Central Texas is known for sudden, heavy downpours, and San Antonio gets its share. A sewer line that drains fine most of the year can back up during or right after one of those storms. It's not bad luck: rain exposes weak spots that were already there.

This guide explains why rain causes sewer backups, what to check when it happens, and how to stop it from happening every time a storm rolls through.

Why rain causes sewer backups

Your sewer line is meant to carry wastewater from the house, not rainwater. When rain finds its way into the line, or when the line is already narrowed, a storm can push it past what it can carry.

Water seeps into cracked pipe

Older cast iron and clay lines often have cracks or loose joints. When the ground around them is soaked, groundwater and rainwater seep in through those openings. The line fills with water it was never meant to carry, and household wastewater has nowhere to go.

Clay soil shifts the pipe

The clay soil common around San Antonio swells when it's wet and shrinks when it dries. After a long dry spell, a heavy rain can make the soil expand quickly, shifting buried pipe and opening joints further.

The line is already narrowed

Roots, grease and hard-water scale reduce how much a line can carry. A partly blocked line may cope with normal use but not with a sudden surge of extra water.

Water gets in where it shouldn't

Downspouts, yard drains or a damaged cleanout cap can send rainwater straight into the sewer line. Even a small connection adds a lot of water during a heavy storm.

Signs rain is getting into your line

• Drains slow down or gurgle only during or after heavy rain.
• Water backs up into the lowest drain in the house, such as a shower or floor drain.
• The cleanout in the yard fills with water during storms.
• Sewage smells after rain.
• Soggy or sunken spots in the yard along the sewer line's path.

If backups only happen when it rains, the line is probably letting water in or is too narrow to handle it, or both.

What to do during a rain backup

1. Stop using water: no flushing, showers, dishwasher or laundry.
2. Keep children and pets away from any sewage.
3. Don't use chemical drain cleaners; they won't help and can harm pipe and anyone who opens the line.
4. Check the cleanout in the yard. If water is standing in it, the problem is in the main line.
5. Take photos for your insurer before cleaning up.
6. Call a plumber if sewage is coming into the house.

If sewage is overflowing from a manhole or the street rather than your house, report it to SAWS on its 24/7 water and sewer emergency line, 210-704-7297. Our sewer backup page covers the first steps in more detail.

Things to check around your house

• Downspouts: make sure they drain onto the ground, away from the house and away from the cleanout.
• Yard drains: confirm none are connected to the sewer line.
• Cleanout cap: make sure it's in place and not cracked. A missing cap lets rainwater pour straight into the line.
• Low spots: notice where water pools in the yard and whether it's along the sewer line's path.

After the storm: finding the cause

Once the water has gone down and the line is clear, a camera inspection is the best way to find out why it backed up. The footage can show:

• Cracks or open joints where water is getting in.
• Roots growing through those same openings.
• A sag that holds water and debris.
• Scale or grease that narrows the line.

Cracks that let rainwater in usually let roots in too, so fixing them can solve both problems at once.

Fixes that prevent repeat backups

• Clearing the line: hydro jetting removes roots, grease and scale so the line can carry more water.
• Repairing cracked joints: spot repair or lining seals the openings where water and roots get in.
• Fixing drainage: redirecting downspouts and yard drains away from the sewer line.
• Backwater valve: a valve that lets wastewater out but stops it from flowing back in. Ask a plumber whether one suits your house and what's required to install it.

The right fix depends on what the camera shows. A line that's only narrowed may need cleaning; a line with cracked joints may need repair to stop the problem for good.

When to call before the next storm

If your line has backed up during rain before, don't wait for the next storm. Have the line inspected while the weather is dry, when it's easier to schedule and the plumber can see the line without standing water. Our guide to jetting older pipes explains what the camera can find in older lines.

Reading the pattern of your backups

Keep a simple note of when backups happen. The pattern points to the cause.

• Only during heavy rain: water is likely getting into the line through cracks or connections.
• During rain and at other times: the line is likely narrowed by roots, grease or scale, and rain makes it worse.
• After long dry spells followed by rain: shifting clay soil may be opening joints.
• Neighbors affected at the same time: the problem may be in the public sewer, so report it to SAWS.

What a plumber will check after a storm

1. Whether the cleanout cap is intact and the cleanout is clear.
2. Whether downspouts or yard drains are connected to the sewer line.
3. The condition of the line on camera, looking for cracks, open joints and roots.
4. Low spots where water and debris collect.
5. How far from the cleanout any problem is, so repairs can be targeted.

Questions to ask

• Is rainwater getting into my line, and where?
• Would cleaning alone stop the backups, or do joints need repair?
• Is a backwater valve a good fit for my house?
• Can I get the footage to keep with my records?

Preparing for storm season

• Check that downspouts drain onto the ground and away from the house.
• Confirm the cleanout cap is in place and undamaged.
• If your line has backed up before, have it inspected while the weather is dry.
• Keep the plumber's number handy, along with where the cleanout is.
• Move valuables off the floor near the lowest drains if backups have happened before.

Insurance after a storm backup

Sewer backup coverage is often an optional add-on rather than part of a standard homeowner's policy. Check your policy before storm season. If a backup happens, photograph everything before cleanup, keep the plumber's written report and footage, and call your insurer as soon as you can.

Why older neighborhoods see more rain backups

Older homes are more likely to have cast iron or clay sewer lines with cracked or loose joints, and older neighborhoods tend to have large trees whose roots widen those openings. The same openings that let roots in let rainwater in during a storm. Newer homes with plastic sewer lines and fewer joints usually see fewer rain-related backups, though downspout connections and damaged cleanout caps can cause problems anywhere.

When to call right away

• Sewage is coming up into a tub, shower or floor drain.
• Toilets won't flush and water is rising.
• Several drains are gurgling or backing up at once.
• You smell sewage inside the house.

In any of these cases, stop using water and call (866) 228-6201 to reach a licensed local plumber.

The bottom line

A sewer that only backs up when it rains is telling you something: water is getting in, or the line is too narrow to handle a surge, or both. Clearing the line stops the immediate problem. A camera inspection on a dry day shows the cause, and fixing that cause, whether it's a cracked joint, a downspout connection or a missing cleanout cap, is what keeps the next storm from ending the same way. Keep the footage and any repair records together so the next plumber, or the next owner, can see what was done.

If you're not sure whether rain is the cause, note the date of each backup and check it against the weather. Two or three backups that line up with storms are usually enough to show the pattern a plumber needs.

Common questions