Slope of driveway to garage in semi-basement

[Rolandd]

Hi,

We are considering a house design that has the garage 3 feet below grade. Access is from the street along a downward sloping driveway that is 15 feet long. The slope is about 20%. I have seen a few houses in my neighborhood with similar or steeper grades. A builder told us that the slope can only be 7%. My question: does this seem right?

Thanks in advance.
Rolandd in Ontario Canada

[Jerry Peck - Codeman]

The best answer I can give you is the attached photo ... the driveway in the attached photo is probably about 20-25 degrees slope.

Now consider trying to go from the street to the drive or the garage to the drive, the car is going to bottom out and get hung up and you will be stuck. Also keep in mind that if you able to not bottom out and get hung up, the car roof would likely hit the header over the garage door opening.

Also consider trying to go up or down a driveway like that when it has been raining and is wet and slippery, now think of doing that with snow and ice.

When going into your sunken garage (the photo would shows the opposite) from the street to the driveway the bottom of the car would hit the edge below the doors, somewhere near the center between the tires. When going from the driveway to the garage the front and read bumpers and overhangs of the car would likely hit the drive and garage floor and leave the tires hanging free in the air.

[Rolandd]

Thanks Jerry,

The photo really helped me "see" the problem.

I talked with the grade engineer at City Hall and he said that 12% is the maximum reverse grade driveway that he will certify. At 12%, the driveway needs to be heated and a drain installed across the base of the slope. The drain needs to connects into the storm sewer. During a one-in-ten year rain storm the streets are expected to flood over and the water will just pour into the garage.

Thanks again, we will alter the design and go for a 10% grade just to be safe.

Rolandd