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Moisture plagues garage built into hill

Tim Carter Tribune Content Agency

DEAR TIM: My garage is at the bottom of a hill and is tucked into the soil on three sides. I get some water leakage through the concrete block walls in the spring and during heavy rains. The asphalt floor is always damp even though I run a dehumidifier all summer. Do you have any solution short of digging out the floor and starting over properly with a waterproof membrane? What could have been done when the garage was built to prevent all the water problems? – Howard L., Toronto

DEAR HOWARD: Your garage is like millions of other structures around the world that are built into sloping ground. Builders have been dealing with water issues in these structures for hundreds of years. I was lucky and majored in geology when I was in college. One of my classes was hydrogeology, the study of groundwater. The knowledge I gained in those classes allowed me to build houses and garages that were always bone dry.

Let’s first talk about what’s going on in the soil. Without studying a soil map for your area, I’m going to assume your soil has high clay content because the land your garage is on has experienced four periods of continental glaciation up there in eastern Canada in the past 2 million years. All that ice was created and melted long before man was around. Isn’t that fascinating?

The deeper you dig into the ground, the more compact the clay is. Water has a very difficult time passing through it, so moisture from rain and snowmelt that enters the soil tends to pass through it downslope along the top of the clay. Most of the water moves through the air spaces in the topsoil.

Before your garage was built, the water higher up on the hill just continued down the hill until it connected to a small spring or brook. Your garage is acting like a dam, and water prefers to take the path of least resistance. Cracks in the concrete block walls or between the block walls and the floor are easy entry points for the water.

Here’s what your builder should have done to create a dry garage – and this advice applies to any structure built into a hill. The portion of the concrete block walls below the soil line should have been waterproofed. There are many different products and methods to waterproof a foundation wall. I used a rubberized asphalt spray and stiff insulation boards on the last house I built, and it is still keeping the basement dry.

A perforated drain pipe should have been placed alongside or on top of the footing that supports your concrete block walls. This pipe should have extended around the back and along the two sides of your garage. The pipe should have then extended past the front of the garage, with the trench having a minimal slope.

Because the hillside is fairly steep, within about 10 or 15 feet, the ends of this pipe on both sides of the ground would surface out of the ground. Any water entering the pipe underground would readily flow out of the ends of the pipe exposed to daylight and then go back into the top soil to continue its journey to Lake Ontario and then to the St. Lawrence River.

This pipe should have then been covered with washed rounded gravel the size of golf balls or walnuts. The gravel should have extended up to within 4 inches of the top of the final grade, where you have grass growing. Water passing through the soil would discover this gravel, immediately drop down through it to the perforated drain pipe and then exit to daylight, never having a chance to enter the garage.

To stop water vapor from coming up through the garage floor, the builder should have put down a plastic vapor retarder or barrier under the concrete or asphalt floor. This plastic sheeting is a common product available at any building supply store.

If you want to permanently solve your water issues, you need to dig along the sides of your garage and do all I outlined above. It’s not necessary to put the perforated pipe all the way down to the top of the foundation footing at this point. I’d probably only bury it 1 foot below the level of the soil at the two front corners of the garage.

You’ll have to use a pressure washer to clean all clay and soil from the concrete block walls once you expose them. Allow them to dry and apply the best waterproofing compound that’s available to you. If you choose to hire a company, understand that hot asphalt that’s sprayed on the walls is just damp-proofing. Standard hot liquid asphalt is not a waterproofing material. It will do a great job of keeping dampness from the soil from entering the concrete block, but it will not bridge cracks to stop liquid water.

To stop water vapor from coming up through the garage floor, the best way is to install a thin concrete overlay over your existing floor. You need to put down the 6-mil vapor barrier first and then pour 1 inch of concrete over your existing floor.

This concrete needs to contain very small pea gravel no larger than 3/8-inch diameter. The mix is like any other concrete, but if you’re doing it yourself I’d probably do a ratio of 3 parts gravel, 2 parts medium sand and 1.5 parts Portland cement. The extra amount of cement will give you a very strong mix that will resist the freezing weather you have in Ontario.

Carter’s columns are archived for free at You can also watch hundreds of videos, download Quick Start Guides and more, all for free.
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