After extended periods of subfreezing temperatures (central New Jersey),
the top of my blacktop driveway raises about 1.5-2.0" above the level of
my garage's concrete floor. (In warm weather, the two surfaces are
flush.) So when I drive into the garage, the weight of the car literally
"snaps" the blacktop at the junction. (Yes, this suggests a void or
inadequate support under the blacktop.)
I'm not sure how water gets under the blacktop. The soil is clay; the
attached house is on a concrete slab. Gutters are working properly to
carry rain away from the house and garage.
At the rear of the 200-foot deep lot, the soil has a 17-degree slope
toward the house and garage. (The length of the slope, not its height,
is about 30 feet.) The distance from the foot of the slope to the back
of the house and garage is about 30 feet; the distance to the front of
the garage and the start of the blacktop is an extra 30 feet. Perhaps
rain from the slope is oozing under the garage slab and making its way
to the driveway. (Only once in four years has any water made its way
into the living area, and that took place in the laundry room that's
behind the garage. So it seems like the water path is right under the
garage. But since their is are no wall cracks, the garage itself doesn't
appear to be raising, unless at the same rate as the whole house.)
The seal where the blacktop meets the garage floor is poor, but the 20"
roof overhang and its gutter keeps most rain out of the gap. Another way
for water to get under the blacktop is where its edges meet the
There is also a very mild slope of the driveway down toward the street.
The driveway raises most within 20 feet of the garage, and virtually not
at all 100 feet away where it meets the sidewalk/street.
I'm planning to replace the driveway in a few weeks. Questions:
1. If I stick with blacktop:
a. Should I make the depth of the gravel under it extra deep to provide
better drainage? What depth? (One contractor proposes 6" of crushed
concrete, called r-blend.)
b. If the new depth is greater than existing depth, all existing gravel
must be removed. Should I allow the contractor to reuse existing gravel?
c. What's the best material to use under the blacktop? As noted above,
r-blend is standard in this area, but I'm not sure it's best for my
d. Should the thickness of the blacktop also be increased? To what
depth? (Same contractor proposed 3 inches.)
e. Any other options for improving drainage?
2. Another approach might be to use a "porous" top, like paver bricks.
Yes, this will allow more water to get below it, but when the soil
freezes and raises the driveway, no lasting damage will occur when I
drive into the garage. Does a porous top make any sense?
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