Stop Water Mess in Your Garage

Garage Floor Drain | This piping looks complicated but it’s not. You’re looking at two new garage floor drains, a simple running trap, and a surface clean-out for the trap! A garage floor drain is an excellent dream garage idea. (C) Copyright 2021 Tim Carter

DEAR TIM: I’m about to build a new home with both an attached garage and a separate free-standing three-bay garage. The house I grew up in had floor drains in the garage and they were wonderful. My builder has never heard of garage floor drains and insists on sloping the concrete floor so all liquids flow to the doors. I can see this will be a nightmare in the winter when ice and snowmelt from cars and trucks. What would you do if you were me and how do you install garage floor drains in your jobs? Steve H., Madison, WI

Have you run into the situation Steve’s facing? I can tell you I live his nightmare each winter here in my own home here in snowy New Hampshire. I didn’t build the house I live in and my garage doesn’t have magical floor drains like the house I grew up in as a kid. Water melts from my car and truck and puddles down at the closed garage doors. It’s a nightmare.

Is the Building Code a Set of Minimum Standards?

It’s important to realize the building code is a set of minimum standards. You can always build things better than what the building code mandates. The building code is revised every few years and I can tell you that some fantastic things have been forgotten or changed in past revisions.

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I checked several citations of the International Building Code and there was language that garage floors must slope to the door so liquids drain to the door or to a drain. Another key point is that cities or towns can make their own revisions to the code. What’s acceptable in one town, may not be allowed in another city nearby. You always need to check with your local code official and see what you can do.

Did You Install a Garage Floor Drain in your Daughter's New Home?

This past year, my daughter and son-in-law built a new home on Mt. Desert Island in Downeast Maine. I specified that floor drains be put in the garage and made sure with the local building inspector, Ms. Angela Chamberlain, that she allowed this. Not only did she permit the floor drains, she indicated the water from the drains could drain to the surface beyond the house just like downspout water from the roof.

How are Garage Floor Drains Piped?

The floor drains in older homes were piped in almost all cases with a simple p-trap just like you might find under your bathroom sink. The u-shaped pipe under the drain created a water seal but it also allowed sand and dirt to accumulate fast. You had to be really careful in older garages so as to not clog the drain line past the u-shaped running trap.

What Kind of Garage Floor Drain Fixture is Best?

I installed modern side-outlet box drains in my daughter’s garage. These measure about 14 inches square and are about ten inches deep. The drain pipe exits out the side of the box drain and the bottom of the pipe is about 1.5 inches up from the bottom of the box.

This allows sand and grit to settle out and not be carried into the drainpipe leaving the box drain. However, you need to keep up with making sure this sand, dirt, and grit never get any thicker than the 1.5-inch catch area!

Do You Put in a P-Trap in a Garage Floor Drain?

Downstream from the box drains I installed a self-made p-trap using 90-degree fittings. I also used a tee fitting so I could create a surface clean out that rises up from one end of the p-trap under the slab.

This clean-out allows my son-in-law, or a future homeowner, easy access into the p-trap using the hose from a wet-dry vacuum. Should the p-trap get clogged it will be very easy to clean it out.

Is Car Oil a Pollution Hazard?

Some people worry about oil getting into the floor drains and it causing pollution of the ecosystem. That’s a valid concern if you have a massive oil spill in a garage. I maintain this is a rare occurrence. If you’re worried about oil pollution, you can install a commercial oil separator as part of your drainage pipe installation. And as for car and truck oil pollution, imagine how many gallons per day leak onto the roadways around your town or city from pesky drips from engines, transmissions, and hydraulic hoses.

Common sense also is required if you have a car or truck that has a tired engine or transmission. If these do leak small amounts of oil, for goodness sake lay out some cardboard to soak up the leaks and replace the cardboard once it becomes saturated. There are also dry granular products made to soak up oil spills. You see this product at many gas stations when someone spills diesel fuel or other oil products.

How Far Below the Floor Surface Should the Drain Be?

It’s very important to make sure the tops of the floor drains are about 2 inches below the primary level surface of the garage slab. The concrete contractor needs to create large shallow funnels around the floor drains.

The size of the funnel depends on the size of the car or truck. At the very least, the funnel should be a total of 10 feet wide and 22 feet long. Center the floor drains so they’re exactly under the center of where the car or truck will be parked in the garage.

What are Floor Drain Benefits?

Another side benefit of having garage floor drains is you can wash your car or truck inside on blistering hot days or in colder months should you live where it gets cold. You can purchase a special hose bib that has both a hot and cold handle just like any indoor sink. This way you can use warm water to rinse off your vehicle.

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