Why Does My Floor Drain Overflow When I Run the Washing Machine?

Basement floor drain backing up when the washing machine drains, a common sign of a clogged building drain.

Why Does My Floor Drain Overflow When I Run the Washing Machine?

If you’ve ever started a load of laundry and noticed dirty water coming up through your basement floor drain, it can be alarming.

Most homeowners immediately think something is seriously wrong.

And while it is a warning sign, the good news is this problem usually has a clear cause — and when caught early, it’s often fixable without major repairs.

This is one of the most common plumbing issues we see, especially in older homes.

What’s actually happening when the floor drain overflows

Your washing machine discharges a large volume of water very quickly.

Much faster than a sink.

Faster than a shower.

And faster than many partially restricted drain lines can handle.

When the main building drain is clogged or restricted, water can’t move through the system fast enough. When that happens, it looks for the lowest point to escape.

In most homes, that lowest point is the basement floor drain.

The floor drain isn’t failing.It’s doing exactly what it’s designed to do — acting as a relief point.

Why this sewer backup happens only when the washing machine runs

This is the question homeowners ask most.

“If my toilet and sink seem fine, why my floor drain overflowing when running the washing machine?”

Because a washing machine puts the most stress on your drainage system.

If your main drain line is partially clogged, it may still handle everyday use. But once a high-volume appliance discharges water all at once, the restriction is exposed.

This means the drain isn’t fully blocked yet — but it’s heading in that direction.

The most common cause: a clogged building drain

Over time, main drain lines collect buildup on the inside of the pipe.

This doesn’t happen overnight.It happens slowly.

Common causes include grease, soap residue, laundry lint, sludge, tree roots, and scale buildup inside cast iron pipes.

As that buildup thickens, the pipe’s diameter gets smaller. Eventually, the system can’t handle large amounts of water — and backups start showing up at the floor drain.

Is this considered a sewer backup?

Technically, yes.

But this is the early stage.

This is the plumbing system warning you before a full basement backup occurs. Addressing the issue now is far easier and far less expensive than waiting until sewage is coming up during normal use.

Why snaking the drain often doesn’t solve the problem

Many homeowners call a plumber to snake the drain, only to have the issue return weeks or months later.

That’s because a drain snake usually punches a hole through the blockage without removing the buildup stuck to the pipe walls.

Water flows again temporarily.

But the restriction is still there.

That’s why the washing machine continues to trigger overflows.

How the floor drain backup should be properly diagnosed

The correct first step is a camera inspection of the building drain.

A camera allows the plumber to see exactly what’s happening inside the pipe, including:

• Where the restriction is

• What type of buildup is present• Whether roots or damage are involved

• If the pipe is structurally sound

Without a camera, any recommendation is just a guess.

What usually fixes a floor drain overflow from the washing machine

In many cases, the solution isn’t just opening the line — it’s cleaning it thoroughly.

Depending on what the camera shows, this may include professional drain cleaning, hydro jetting to remove buildup, root removal, or descaling cast iron pipes.

The goal isn’t short-term relief.

The goal is preventing the problem from coming back.

If a clogged building drain is causing your floor drain to back up, hydro jetting is often the solution that doesn’t just open the pipe — it cleans it completely.

Is this backup an emergency?

Not always — but it is time-sensitive.

If water is already coming up through the floor drain, you’re one heavy laundry day away from a full backup.

Catching it now almost always saves money, stress, and cleanup later.

What to do if running your washing machine has caused your floor drain to backup and overflow

If your floor drain overflows when you run the washing machine, the smartest next step is a professional inspection of the main building drain. To Schedule a plumbing service in Pittsburgh click here.

That inspection tells you why it’s happening, how serious it is, and what options actually make sense for your home.

No guessing.

No unnecessary repairs.

Just clear answers