Why Your Basement Floods Only When It Rains

There is a particular version of this problem worth separating from all the others, because the usual fix does not fix it.

The pattern: your drains are perfect. Sinks run, toilets flush, the washing machine empties without complaint. Then it rains hard, and water comes up through the basement floor drain or the basement toilet. The rain stops, and within hours everything is normal again.

If that is your pattern, cleaning your drain line may change nothing — because your drain line may not be the problem.

Combined sewers

Large parts of older urban New Jersey are served by combined sewers: one pipe carrying both sewage and storm water. It was a reasonable design when it was built and it works fine most of the time.

In heavy rain it does not. Every street drain, every downspout and every roof in the catchment is pouring into the same pipe your house drains into. The main fills. When it is full, the water has to go somewhere, and it goes into whatever is connected to it and lower than the street.

Which is your basement.

This is common in Jersey City, Hoboken, Union City, Bayonne, Paterson and the older parts of Passaic and Newark, and in pockets elsewhere. It is not a fault in your plumbing. Your house is doing exactly what it was plumbed to do; the street is over capacity.

The other rain-driven causes

Not every storm backup is the street main. Two others are worth ruling out:

Groundwater. When the water table rises, the lowest opening in the house is where it appears. Low-lying ground near the Passaic, the Saddle River or the Hackensack — which is much of Lodi, Garfield, Elmwood Park and the Meadowlands boroughs — sits where this happens. A floor drain that is already silted up cannot cope with what it is asked to take.

Downspouts or a sump discharging into the sanitary line. Some older properties were plumbed this way, and some had it done informally later. It is now prohibited in most places, and it means your own roof is surcharging your own lateral every time it rains. This one is worth finding, because it is fixable and it is yours.

How to tell which you have

Watch the pattern, and be precise about it:

• Only during heavy rain, clears quickly afterwards, nothing wrong in dry weather — most likely the street main at capacity
• During rain AND slow in dry weather — your line is restricted as well, and that part is worth clearing
• Only when a fixture runs, rain or not — that is an ordinary main line blockage, nothing to do with weather
• Water seeping at the floor-to-wall joint rather than up the drain — that is groundwater through the foundation, not the sewer

Neighbours are useful evidence too. If the whole street floods in the same storm, that is not twelve simultaneous lateral failures.

What actually helps

A backwater valve. This is the real answer for a combined-sewer backup. It is a one-way valve on your lateral that closes when flow tries to come back toward the house. It does not stop the street main filling; it stops the street main emptying into your basement. Some municipalities have had assistance programmes for these — worth asking yours.

Clearing the line anyway, if it is also restricted. A lateral with roots or grease in it has no spare capacity when it is needed most. Clearing it will not stop a surcharging main, but it removes the part of the problem that is yours.

A clean floor drain. The floor drain is the lowest opening in the house and it is the one nobody ever touches. Silted with dust, lint and rust, it cannot take what it is asked to take. This is the one thing on this list that is cheap and entirely within your control.

Getting things off the basement floor. Not a fix, but the difference between a mop and an insurance claim.

What we will tell you

If the pattern says street main, we will say so rather than sell you a cleaning that cannot help. That is a conversation about a backwater valve, and possibly one to have with the municipality.

If the pattern says your line is restricted as well, that part we can fix.

Call (201) 315-6655, day or night, any day of the week — and describe the timing precisely, because the timing is the diagnosis.

Related: what to do when a drain backs up at night · floor drain cleaning