Drain backs up during heavy rain

Quick Answer:

If a basement drain starts to back up while it’s pouring outside, the most likely causes are stormwater getting too close to your foundation or the municipal/storm system overwhelming the sewer. Start with simple checks you can do from the yard and the exterior cleanout during the storm. If water is reaching floor level or the exterior cleanout is surcharging, call a professional right away.

Why This Happens

• Heavy rain can overwhelm municipal combined or separate systems so sewage and stormwater push back into low interior drains.
• Downspouts, roof leaders, or gutters that dump water next to the foundation can push extra groundwater toward the lateral and increase pressure on your lines.
• A broken or clogged lateral, a cross-connection, or poor yard grading can make the problem worse during high intensity storms.
• Timing matters: backups that happen only during the storm peak usually indicate system overload, while backups during steady rain can suggest a partial blockage or poor slope.

For related situations, see Backup after summer storms and Drain backs up only after rainfall.

Step-by-Step What to Do

1. Watch and note timing

Observe when the backup starts: during peak intensity (short heavy bursts) or during long steady rain. That pattern helps decide if this is system overload versus a local blockage.

2. Check gutters and downspout routing

• Walk the perimeter and look for downspouts that discharge close to the foundation or splash blocks that channel water toward the house.
• If water is dumping at the foundation, move extensions or reroute downspouts away from the house to reduce infiltration.

3. Inspect roof leaders and ensure extensions direct water away

• Confirm roof leaders (the vertical pipes from gutters) are connected and draining away from the foundation. Temporary extenders or a few feet of flexible downspout can help during the storm.
• Clear debris from gutters so they don’t overflow down the wall and saturate soil near the lateral.

4. Safely open the exterior cleanout to check for surcharging

• Locate the exterior cleanout (usually a capped pipe near the foundation or at the property line). Wear gloves and eye protection.
• Carefully loosen the cap a little to let pressure escape — do this while standing to the side, not over the opening. If wastewater rushes out forcefully, that is surcharging.
• If the cleanout forces water or sewage out, stop and call a professional. Replace the cap securely if you can do so safely.

5. Note other signs and limit interior attempts

• Check whether multiple drains back up at once and whether there is a sewage smell — both suggest a main-line or system issue.
• Do not run appliances (washing machine, dishwasher) during active backups — that only adds flow into the problem.

What Not to Do

• Do not attempt to excavate the property lateral without utility locates — hitting utilities can be dangerous and illegal.
• Do not run a high-pressure power jetter into a line that is actively surcharging — the added pressure can damage pipes or push sewage into your yard and home.
• Do not wait if water reaches floor level or if surcharging happens during moderate rain — call a professional immediately rather than attempting risky DIY repairs.

When to Call a Professional

• Wastewater is reaching or is likely to reach floor level.
• The exterior cleanout is surcharging (forceful flow from the cleanout) during the storm.
• Multiple fixtures are backing up, there is a strong sewage odor, or the problem recurs after cleanup.
• You are unsure where the cleanout is, or the cleanout cap cannot be removed safely.

Safety Notes

• Avoid contact with backed-up water — it can contain harmful bacteria and chemicals. Wear gloves, boots, and eye protection if you must inspect.
• Turn off electricity in a flooded basement only if you can do so safely from dry ground or by shutting off the main breaker outside. Do not touch electrical panels or cords while standing in water.
• Contain and dry small, non-sewage water quickly to prevent mold. For sewage contamination, professional cleanup is recommended.

Common Homeowner Questions

• Is this a sewer backup or just stormwater? Short heavy-peak backups often mean the public system is overloaded; repeated backflow or sewage smell points to sewer issues.
• Can I fix this by myself? You can check gutters, reroute downspouts, and inspect the exterior cleanout, but call a pro if the cleanout surcharges or water reaches the floor.
• Will a sump pump solve it? A sump can help groundwater but won’t stop sewage backflow from a surcharged sewer or a broken lateral; a plumber is needed for those problems.

More in this topic

For more related fixes and similar symptoms, see Basement Drain Backs Up Only After Rain.