Can Septic Tanks Fill Up With Rainwater? (THREE CAUSES)

If you own a septic system and are expecting a rainy season, one of the things to worry about is stormwater feeding into your system.

So, Can Septic Tanks Fill With Rainwater?

The answer to this is – No, rainwater is not supposed to fill your septic tank. Your septic system is designed correctly by considering several factors, including the type of soil in your property and possible effects from rainwater. And before the site contractor can hand over the project to you, your local Health Department inspects the site to ensure that it meets all the parameters of a safe septic system. Unfortunately, your septic tank can fill up from too much rain if there is a problem with your septic system.

All the wastewater from your house drains into the septic tank, which is partially filtered. Solid wastes are retained in the septic tank while water is absorbed into the ground via a piping network.

If rainwater is allowed to enter your septic tank, it often results in disastrous consequences.

This can happen if there is a lapse in the structure of your septic system.

Three Reasons Why Rain Water Would Fill Up Your Septic Tank

Rainwater can fill your septic tank if it can enter the septic system. This can happen in the following ways:

1. Wrong Connections In The Drainage Plumbing

Rainwater could enter your septic tank if the drainage plumbing into the system is not connected properly. For instance, if the sump pump is connected to the septic system.

In this case, the sump pump will channel excess rainwater in your basement into the septic system.

Your sump pump water is supposed to discharge into a designated area, such as a dry pond or a creek.

Gutter downspouts are connected to the septic system. All the rainwater collected from the rooftop of your home is directed to the septic system.

This will surely fill up your septic tank if the rainfall is heavy.

Water from the downspouts can be used better instead of connecting it to the septic system. You can reconfigure the downspouts to a water barrel where it is collected for later use.

Yard drainage, footer drains, or floor drains connected to the septic system. The flooded yard should drain away from your septic tank.

2. Improper Surface Water Routing

Check the connection of your gutter downspouts if they drop water straight out on top of your septic tank.

And if your septic tank is unlidded, it will fill with rainwater during a heavy downpour.

3. Wrong Underground Drainage

Subsurface drainage pipes connected to your septic system’s sections will allow rainwater to fill your septic tank.

Five Signs Of A Flooded Septic Tank

When rainwater combines with wastewater, it could overpower the capacity of the septic system to treat the sewerage.

This often results in a serious overflow of the system and water backing up into the house.

Rainfall that comes with floods is guaranteed to cause problems if there are flaws in your septic system. Here are some indications of a flooded septic tank:

1. Slow Draining Toilet

This is a significant indication of a disruption in your septic system.

If, after a heavy downpour, you notice that your toilets drain slowly, then there is likely a stoppage on the pipes that drain into your septic tank.

Your septic tank is overloaded and can barely take in different wastewater.

2. Stagnant Water Around The Septic Tank

With a lot of rainfall, the ground could be more saturated.

If you notice pools of water near your septic tank and your toilets draining very slowly, your septic tank is waterfilled.

3. Slow Flushing Toilet

If you notice a slow flushing toilet after a heavy downpour, there may be a drainage problem in the sewer system.

However, if your toilet cannot flush properly, it may not necessarily be an issue with the sewer drainage system. It may be blocked on its own.

4. Gurgling Noise From The Toilet And Drains

Strange sounds originating from the indoor drainage can indicate problems with the septic tank.

So, if you notice odd noise from the toilet and the sinks, check your septic tank to see if it has overflown.

5. Water Back Up Into The House

If the runoff water from the rain gets into the septic tank very fast, it will overload the tank. Since the ground is damp, the water will go back into the house.

Five Septic Tank Preventative Measures Before the Heavy Rains

As the adage goes, prevention is better than cure.

So, before heavy rainfall hits your area, make a date to put your septic system in good condition to prevent rainwater from filling your septic tank.

Here is what you need to do:

  1. Have your septic tank pumped before the start of the rains, especially if it is due for a cleaning. It is advisable to pump every 3-5 years.
  2. Have your sewer system inspected and maintained to ensure it will not be broken by heavy rain. Check the baffle tees to see if they are installed correctly and are not clogged.
  3. Plant grass on the drain field to enhance water absorption during rainfall.
  4. Ensure that the bacteria in the tank are thriving. Add biological additives into your septic tank to support the growth of these bacteria.
  5. Install septic tank lids and risers. It may help to dig up your septic tanks and install raisers with lids. Even if the problem still occurs, repairing your septic tank will be much easier and faster.

What To Do DURING the Heavy Rains

Once the rains have started, you should watch for signs of a flooded septic tank. Here is what you need to do to ease pressure on your septic tank until the system is restored:

  1. Reduce the duration of your shower.
  2. Flush the toilet only when it is indispensable.
  3. Don’t do laundry.
  4. Do not run the dishwasher.
  5. Stay away from flooded water. It may be contaminated with sewerage.

What To Do AFTER the Heavy Rainfall

Once the rains have subsided, check if the septic system is clogged.

If it does not restore itself after the rains or you notice any signs of damage, call for repair.

How To Restore Your Septic Tank After Heavy Rainfall

Wait to pump your septic tank until the floods have rescinded.

At this stage, what you have to do is to make sure that the rainwater is not entering your septic.

Here are some of the steps you can take to minimize water entering the septic tank:

  1. Redirect the water from the roof gutters away from the septic tank.
  2. Channel the basement sump pump water away from the septic tank.
  3. Minimize household water usage. Be keen on how often you run the shower, flush the toilet, and do laundry. Reduce your water output completely.


Rainwater is not supposed to go into your septic tank, considering that it was carefully designed according to the type of soil and the residual water level on your property.

Unfortunately, septic tanks can fill up from too much rain if there is a problem with your septic system.

Thankfully, there are robust measures you can take to ensure that your septic system bounces back after heavy rains, as we have described in this article.

Additional Septic System Topics
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Can You Drive a Lawn Mower Over a Septic Tank?
Can Septic Tanks Fill Up With Rainwater?
Can You Put an Above Ground Pool in a Yard with a Septic Tank?
Can You Move a Septic Tank?
Can You Put a Fire Pit Over a Septic Tank?
How Close Can You Build a Garage to a Septic Tank?
How to Eliminate Outside Septic Tank Odor
Can the Septic Tank be Uphill from Your Home?