Why does my basement floor drain back up when it rains?

The short version

A floor drain that backs up only in heavy rain is often a sign that rainwater is getting into the sewer, either the public one or your own line. Alexandria describes sanitary sewers as designed to carry dirty water used for toilet flushing, bathing and showering, and washing clothes and dishes, and across more than 90 percent of the city by land area a separate system carries stormwater.^[2] When a storm adds more water than the pipes can carry, the sewer fills and pushes back up through the lowest opening it can reach, and in a house with a basement that opening is often the floor drain.

How rain gets into a sewer that is meant to be dry

The City of Alexandria puts it plainly: "Basement back-ups or flooding can occur when water from a heavy or intense rain flows into the sanitary sewer system."^[1] Utilities split the problem into two parts.

• Infiltration. Infiltration is groundwater that seeps in through cracks in the public sewer pipes, in the laterals that connect homes to them, and in manholes.^[2]
• Inflow. Inflow is stormwater that enters the sanitary sewer directly, from sources in the public right-of-way and sometimes on private property.^[2]

Either way the result is the same. As Alexandria puts it, "Rainwater can enter street and house sewers through cracked pipes or leaky joints and manholes."^[1] And in Alexandria's words: "If the amount of rainwater entering the sanitary sewers is excessive, the street sewers become overloaded and can back-up into the basements of homes and businesses."^[1]

The usual suspects

Downspouts, sump pumps and other drains tied into the sewer. Alexandria notes that some homes and businesses connect sump pumps and roof downspouts to the sanitary sewer, that these connections are not allowed, and that they discharge unwanted stormwater into it during rain.^[1] Spotsylvania County's list of improper connections adds foundation drains and drains from window wells and driveways.^[3] Older homes are the ones to check: Alexandria says that if your downspouts discharge into a pipe in the ground, or you have an outside basement stairwell drain, and your home was built before 1970, they may be connected to the sanitary sewer.^[2]

A cracked or root-filled lateral. Your own line can leak groundwater in through the same cracks that roots use. Broken laterals can let tree roots or debris into the pipe, causing blockages and building backups.^[3] A partly blocked line can drain well enough on a dry day and still back up when a storm adds water. Our guides to signs of a broken sewer line and keeping tree roots out of a sewer line go further.

An overloaded public sewer. Sometimes the problem is the main, not your house. In Alexandria's Old Town, sanitary and storm sewers are combined in the same pipe, and backups there can happen when the combined pipe's capacity is exceeded.^[2] Alexandria also maps the areas of the city subject to sanitary sewer backups.^[1] Ask your own utility whether your street has a history of surcharging.

Low ground. The City of Salem notes: "Homes that are subject to flooding are also more likely to experience a backup."^[4]

Why the floor drain goes first

Water finds the lowest opening on the line. Salem's guidance states it simply: "Backups will enter through the lowest drains first."^[4] In a house with a basement, that is usually the floor drain, then a basement shower or toilet.

Telling your problem from the city's

Watch what happens during the next storm. Fairfax County explains the difference this way: if the backup occurs only when fixtures such as the toilet, tub, washer or sinks are used, "it is possible that you need a plumber." If sewage backs up when you are not using any of them, "it is possible that there is a problem in the county main line."^[5] Ask neighbors on the same street whether their basements backed up too. If they did, report it to your sewer utility; if only yours did, start with your lateral and your own connections.

While it is happening, stop running water in the house. If your basement has a sump pump, Salem advises turning it off during a backup, because sump pumps are meant to pump clear water to a green space or storm drain.^[4] Only do this from a dry floor. FEMA's flood guidance warns of the risk of electrocution and says not to touch electrical equipment if it is wet or if you are standing in water.^[7] If water has reached the pump's outlet or cord, or you would have to wade in, leave it alone and stay out of the basement. Our guide to what to do when sewage backs up covers the first hour and the cleanup.

What fixes it

The fix depends on which cause a plumber finds. A wrong connection is rerouted to drain outside. A cracked lateral is repaired, lined or replaced. Where the public sewer itself surcharges, a backwater valve or backflow preventer is designed to keep it out of your house. In the City of Alexandria's description, a backflow preventer protects the house only while it is closed during the sewage overload, a period that varies with the storm but generally lasts 2 to 6 hours, and the plumbing cannot be used while it is closed.^[1] Our guide to preventing sewer backups covers valves, local cost-sharing programs and insurance.

When to call a licensed pro

Call a licensed plumber after the first rain-related backup, not the third. Ask for a camera inspection of the lateral and a check of where your downspouts, sump pump and stairwell drains discharge. For the lateral and any valve, choose a plumber you can look up on DPOR. Virginia's Board for Contractors regulates plumbers as tradesmen, and state certification of the trade is mandatory, not optional.^[6]

Ask whether the work needs a permit. Under Virginia's Uniform Statewide Building Code, a permit is required for installations or alterations involving the sanitary drainage system, while some ordinary repairs are exempt, such as replacing plumbing fixtures without altering the drainage system.^[8] Alexandria also notes that the property owner or plumber has to get the installation permits from the city for a backflow preventer.^[1]

If you suspect the public main, call your sewer utility.

Get help from a licensed pro

Loclis lists licensed businesses from Virginia's DPOR register. Find plumbers in Blacksburg, book drain cleaning, or read more plumbing repair answers. When you are ready, describe the backup once and licensed local businesses quote it.