Skip to content
water pipe, fountain, tube-3862489.jpg

What Happens When Your Grey Water Tank is Full? (2022 Guide)

Few things can bring seasoned RVers to a halt faster than the warning light for their motorhome’s grey water tank. When that sensor starts blinking, it’s time to start looking for a dumpsite because your RV’s plumbing is living on borrowed time. If you don’t stop and empty it soon, you risk an overflow of wastewater that could cause significant damage to your furnishings and furniture.

While grey water isn’t the dirtiest of materials – it shouldn’t contain any toilet waste – it’s still full of detergents, shower gels, food residues and all the dirt and grime you’ve scrubbed from your body recently. Mixed together, these substances have a greasy consistency and a pungent odor, two things you want to keep well away from your carpets.

This article talks about why it’s so important to empty your motorhome’s grey water tank regularly and what happens when a tank overfills.

What Happens When a Motorhome’s Grey Water Tank Gets Too Full?

Imagine you’ve got two full glasses of water and you attempt to pour one glass into the other. You’re going to end up with a soggy mess. The same thing happens to grey water tanks when they get too full. Try to take a shower, wash your hands or go to the bathroom and you risk an overflow that saturates your RV’s floors and possibly some of your furniture.

That’s why motorhomes have sensors and warning lights to tell their occupants it’s time to empty the tank. Most RV sensor systems are built with a degree of overflow tolerance. When the light starts blinking, the grey water tank is nearly at critical capacity but there’s still time to drive to a suitable dumpsite.

Think of it like the fuel light on your dashboard. It’s not an emergency yet but it’s a friendly reminder your vehicle needs some attention.

If you disregard the warning light and continue to use your motorhome’s sink and appliances, the tank will continue to fill until it overflows. First, it will back up into the shower tray to try and contain the overspill. If you address the problem at this stage, you’re unlikely to see any damage. It might smell a bit weird but all the grey water will drain back out of the shower as soon as you empty the tank.

But don’t forget shower trays are shallow. As soon as that tray fills up, grey water is going to flow over the edges and start seeping into your floors. Now you’ve got a real problem and possibly some costly damage to your motorhome’s furnishings.

Is It Dangerous to Let a Motorhome’s Grey Water Tank Overflow?

Fortunately, there’s no immediate threat to health like there would be if your black water tank overflowed. There’s no toilet waste in grey water so it shouldn’t make you sick right away. It’s made up of everything except for toilet waste: washing-up liquids, food residues, soap, hair products, shower gels and water used for showering.

Mixed together, these substances create a sludgy wastewater with an oily sheen and a nasty smell. Depending on how carefully the tank is maintained, there may also be some mold deposits. You’ll certainly see leftover pieces of last week’s dinner in a not-so-appetizing condition. It’s not an instant health hazard but it’s pretty gross.

Grey water can become dangerous if it saturates soft furnishings such as carpets because they’re difficult to deep clean. Weeks after a flood, your carpets may look dry but still be harboring microbes, mold and mildew. If you notice musty odors emanating from materials saturated by grey water, you need to get rid of them or risk inhaling mold spores in the atmosphere. This can cause allergic reactions, rashes and respiratory problems.

Such a serious mold problem is unlikely to go unnoticed in a motorhome. So the chances of occupants getting sick are low. It’s more likely you’ll be out of pocket after paying for replacement carpets, rugs and low-down cabinets. All of this trouble and you could have avoided it by paying attention to your tank’s warning light.

Now let’s talk about what to do when that warning light starts flashing.

What Do I Do When My Motorhome’s Grey Water Tank Is Full?

You’ll know the RV’s grey water tank is reaching critical capacity when a warning light appears on its control panel. This is an indication you need to stop soon and find somewhere to empty your wastewater, preferably an authorized dumpsite.

To see how close you’re getting to an overflow, check the shower tray and sinks. If water is pooling around the plugholes, you need to stop as soon as possible. Do not add any more liquids to the tank. Avoid going to the bathroom. If you must, don’t flush. Don’t use the taps or any water based appliances until you’ve emptied the tank.

Authorized RV dumpsites are the best (and often the only) locations where you can deposit your wastewater. However, if you can’t make it to a dumpsite in time without risking water damage, you may be able to empty the tank in a green area away from people. Some states have strict rules against emptying grey water anywhere but dumpsites. Other states allow discreet, responsible dumping as long as it’s not too close to roads or buildings and doesn’t contain toxic black water.

If in doubt, google the dumping rules for the state you’re driving through. If the state allows off-site dumping, you may have the option to open your water tank’s valve while stationary (when you’re camping, for example) so that wastewater flows out of the vehicle and onto the ground outside instead of filling up the tank. It saves space for when you’re back on the road and reduces the number of times you need to empty the tank.

You should only leave your grey tank’s valve open if state regulations permit you to do so. If you’re on a private camp site, be polite and check its rules as well. Make sure you’re not emptying too close to other people or on ground where the water can’t drain away safely. And don’t forget, there needs to be a little bit of water in the tank before you close the valve and drive away to avoid rinsing problems.

Can a Grey Water Overflow Cause Serious Damage to My Vehicle?

The short answer is no, not unless your motorhome has other underlying issues. Even if you experience an overflow, water shouldn’t be able to reach the moving parts of your vehicle. It would take an extremely serious flood and a lot of holes to cause damage to the engine or electrics. If that happens, you’ve got bigger problems than some soggy carpets.

The worst damage you’re likely to face is some ruined soft furnishings. Even this can be avoided if you address the overflow before it spills out of the shower tray. The extent of the damage depends on what materials get saturated – floorboards are more waterproof than carpets – and how quickly you can clean and dry them.

It’s important you clean any porous materials (like carpets) which have gotten wet with a detergent before you dry them. This reduces the risk of mildew and mold spores. Wipe down walls and cabinets and stop in a location where you can open the motorhome’s doors and windows for a time to encourage healthy ventilation.

The good news is that motorhomes are designed to be tough and highly adaptable. Even if some furnishings do get damaged, you can likely remove the damaged sections and replace them. The likelihood of you having to replace a large amount of flooring is small unless the water has been allowed to stand for a prolonged period.


NOTE: If you're looking to purchase a new tiny home, but aren't sure if it's right for you, be sure to check out Escape Campervans. They allow you to rent unique vans and RVs all around the US. Book today!