Quick Answer:
If several drains only back up when it rains hard, the problem is usually stormwater overwhelming the drainage system or a blocked outdoor drain pushing water toward foundation drains. First note whether the backups match heavy rain. Check exterior storm drains, downspouts and the basement perimeter and sump for extra inflow. Temporarily divert surface water away from the foundation and avoid using indoor water during storms. If you are on a combined sewer system, record storm timing for the utility and contact them. Call a professional if sewage or living-area flooding occurs.
Why This Happens
There are two common causes:
- Stormwater overload: Heavy rain can exceed the capacity of storm or sanitary lines, especially where systems are connected or the municipal sewer is overwhelmed.
- Local drainage pushing water to foundation drains: Clogged storm drains, blocked gutters, or downspouts that discharge near the house can push water into perimeter drains and the sewer lateral, causing backups indoors.
Saturated soil raises groundwater and can overload a basement perimeter system or a sump pump. If your home is connected to a combined sewer, a big storm can force sewage back up into the household plumbing.
Step-by-Step What to Do
1. Compare timing with rain intensity
Watch when backups happen. Note whether they begin during heavy rain, right after a downpour, or at other times. Write down dates and times — this helps you and any utility or plumber diagnose the pattern.
2. Inspect external storm drains and downspouts
- Walk around the property and look for blocked curb drains, clogged grates, or debris in storm inlets.
- Check gutters and downspout outlets. If downspouts discharge close to the foundation, they can push water toward foundation drains.
- Clear leaves and debris where safe to do so. Don’t climb on a slippery roof during a storm.
3. Check basement perimeter drains and the sump
- Open the sump cover and look for an unusually high inflow rate during or after rain.
- Inspect the perimeter drain access points (if present) for sediment or unusual flow.
- If the sump can’t keep up, document how quickly water rises and whether the pump cycles rapidly.
4. Divert surface water away from the foundation
- Use temporary measures during storms: sandbags, heavy plastic sheeting, or temporary berms to slow or redirect surface runoff downhill away from the house.
- Ensure water drains away from the foundation rather than pooling at grade next to the wall.
- These are temporary fixes; plan proper grading and gutter/downspout rerouting once conditions allow.
5. If you have a combined sewer, document and avoid adding load
- Record when backups occur and the local rain conditions. Contact your sewer utility with those notes — combined systems often show a clear pattern the utility can confirm.
- During storms, avoid using showers, toilets, washing machines and dishwashers to reduce load on the system.
What Not to Do
- Don’t assume internal plumbing is at fault—avoid using indoor water during active storms to reduce load; call a pro when backups during storms include sewage or flooding inside living areas.
- Don’t try to snake the main sewer line if sewage is backing into living spaces; that can make contamination worse and is best handled by a trained plumber with proper equipment.
- Don’t enter standing water that may contain sewage or that reaches electrical outlets. Don’t touch contaminated water with bare hands.
When to Call a Professional
- Call a licensed plumber if sewage is backing up into sinks, toilets or living areas, or if backups continue even in light rain.
- Contact a sump pump specialist or plumber if the sump can’t keep up or the pump is cycling unusually fast.
- Notify the municipal sewer utility if you suspect a combined sewer overflow or if you’ve documented repeated storm-related backups — your notes on timing will be useful.
Safety Notes
- Avoid contact with water that may contain sewage. Wear gloves and boots if you must inspect a contaminated area.
- If water reaches electrical outlets, switch off power at the breaker only if you can do so safely and from a dry location. Otherwise, call an electrician.
- Keep children and pets away from flooded areas. Use proper cleanup methods for sewage contamination and consider professional cleanup for large or repeated events.
Common Homeowner Questions
- Why do several drains only block up during heavy rain?
Most often because stormwater is overloading the system or external drainage is forcing water toward foundation drains. - Can I stop this with simple fixes?
Sometimes. Clearing gutters, extending downspouts, and temporary diversion can reduce inflow, but repeated issues usually need professional assessment or municipal action. - Should I inform the utility?
Yes — especially if you are on a combined sewer. Document times and conditions and report the pattern to the utility so they can investigate.
For related patterns that start at night or under different conditions, compare notes with Multiple drains flood at night for helpful context when you talk with a pro.
More in this topic
For more related fixes and similar symptoms, see Multiple Fixtures Backing Up Together.