The Washington Post Democracy Dies in Darkness

How to install a floor drain in your garage

This piping looks complicated but it’s not. You’re looking at new garage floor drains, a simple P-trap and a surface clean-out. (Tim Carter)

Q: 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 snow melt 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, Wis.

A: I can tell you, I live this nightmare each winter 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. Water melts from my car and truck, and puddles down at the closed garage doors.

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.

More Builder: Thinking of hanging your own drywall? Do yourself a favor -- hire a pro.

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.

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

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 careful in older garages not to clog the drain line past the P-trap.

I installed modern side-outlet box drains in my daughter’s garage. These measure about 14 inches square and are about 10 inches deep. The drainpipe 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!

More Builder: New tool lets you install leak-proof pipe connections without solder

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. If the P-trap gets clogged, it will be very easy to clean out.

Some people worry about oil getting into the floor drains and 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.

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 the car or truck when it is parked in the garage.

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

Subscribe to Tim’s free newsletter and listen to his new podcasts. Go to:

Floor drains