Building a gravel driveway on a hill is not as simple as you’d think. An improperly built gravel driveway will quickly turn into a rutted, muddy mess. There are several things you should consider before choosing your gravel and especially starting construction.
The best gravel for a driveway on a hill requires 3 different sizes of rock in 3 layers. The bottom layer should be crushed #3 stones, with golf-ball-sized #57 crushed rock in the center. The top layer should consist of finely crushed rock and rock dust. The driveway should be about 12″ thick overall.
This article will explore the factors to consider when choosing gravel for constructing a sloped driveway and some methods of keeping the final driveway dry and stable for as long as possible.
Can You Put Gravel on a Sloping Drive?
If the incline is not too steep, you can build a gravel driveway on a hill. The maximum incline for a gravel driveway is often listed as 1 in 12. For every 12 feet (3.65 m) of horizontal distance, there is a one-foot (0.30 m) increase in elevation/height, or a grade of 8.3% or 4.76 degrees.
And that is if modern industry best practices are followed. If a gravel driveway is built on a steeper incline, it will begin to suffer from erosion in a surprisingly short amount of time.
Factors To Consider When Choosing Gravel for an Inclined Driveway
When choosing gravel for use in a sloped driveway, you should consider the following 5 factors:
It is the 21st century, and it is possible to ship virtually anything between any two locations on the Earth in just a few days; for a price. If you can afford it, you could have several tonnes of driveway gravel shipped across the country or overseas.
But unless you’re rich, you will probably buy your gravel from local sources. Buying locally mined gravel is also more environmentally responsible.
A corollary of availability is transporting gravel to your property. Gravel is made of many small pieces of rock and, therefore, very dense. If we assume your driveway will be 40 ft (12.19 m) long, 20 ft (6.09 m) wide, and 12 inches deep (30.48 cm), you will need about 30 cubic yards (22936.6 L) of gravel.
Depending on the type of rock you choose to use, that equates to between 30 and 50 tons of rock. Dump trucks can have load weight capacities between one and 15 tons, so you are sure to need multiple trips or multiple trucks to deliver your entire gravel order.
It is commonly believed that gravel driveways are less expensive to build than conventional asphalt or concrete driveways. It is more accurate to say that they spread the cost over a more extended time.
Once installed, an asphalt or concrete driveway will require minimal maintenance for its 20 to 30-year lifespan. Eventually, water will seep through small cracks in the asphalt/concrete. The water will either corrode the rebar reinforcement or leave cavities that eventually collapse. In either case, the driveway will crack and begin to subside.
Gravel driveways require much more maintenance. Unless special construction methods are used, the gravel will gradually shift downhill and off of the driveway. Lost gravel will have to be replaced.
Gravel will also shift around every time a car drives over it and needs to be evened out at least once a year. Weeds will also colonize a gravel driveway unless killed or removed. If you do the maintenance yourself, you will have to buy special tools and supplies. Or you can hire someone to maintain your driveway for you.
Either way, the accumulated maintenance costs over a 20 to 30-year period will eventually eat up the money you saved by choosing gravel over more expensive materials. The difference is that you will not have to replace your gravel driveway at the end of the period.
3. Shape and Size of the Rocks
Whether or not your gravel driveway will fall victim to rutting or other forms of erosion will ultimately depend on the shape and size of the rocks that make up gravel and how the driveway is constructed.
The primary thing keeping the individual rocks in a gravel driveway in place is other rocks. Specifically, friction between individual rocks as passing cars’ weight and rolling force pushes them against each other.
One of the keys to building a stable and long-lasting gravel driveway is maximizing the friction between individual rocks. The best types of gravel for use in a driveway are those made of rough, angular rocks.
The size of the rocks is a trick question because a properly constructed gravel driveway will have three different sizes of gravel in three layers.
- The bottom or base layer of a gravel driveway should be made out of #3 crushed stones. Also called “clean stone” or “base stone,” #3 stone consists of chunks of crushed rock between ½ inch and two inches (1.27 cm and 5.08 cm) across. The large chunks allow water to flow through them, improving drainage. The base layer should be about four inches (10.16 cm) thick.
- The middle layer of a gravel driveway should be made out of roughly golf-ball-sized #57 crushed stones. These smaller-sized rocks primarily serve as a transition between the bottom and top layers. The middle layer should be about four inches (10.16 cm) thick.
- The top layer of a gravel driveway should be made of a mix of finely crushed stone and rock dust. The smaller pieces fill in gaps in the middle layer and produce a more even top surface for driving and walking on.
Pro Tip: The top layer can also be made of granite ships, crushed seashells, or even smooth river stones if you don’t mind additional maintenance. The top layer should be between one and four inches (10.16 cm) thick.
4. Color Of The Rocks
If we’re honest, the main reason people choose a gravel driveway over asphalt, cement, or pavers is that they like the gravel look.
Maybe you live in a rural area and like the idea of having your own private dirt road, or maybe you’re into the “suburban cowboy” lifestyle. Gravel offers a broader color palette to customize the appearance of your driveway.
One surprising advantage gravel driveways have over asphalt and concrete driveways is longevity. With proper maintenance and regular rock replacement, a gravel driveway could last for up to a century.
If you aren’t looking to build something that will last that long, gravel driveways are the easiest to remove.
The Best Types of Gravel for Sloped Driveways
A properly constructed gravel driveway will need three different types of gravel in three separate layers:
- The base layer should be made of crushed rock between ½ and 2 inches (1.27 cm and 5.08 cm) across, which in the construction industry is referred to as #3.
- The middle layer should be made of roughly golf-ball-sized #57 crushed rock.
- And the top layer, the layer you see, drive, and walk on, should be made of a mix of finely crushed rock and rock dust.
How Many Inches Deep Should a Gravel Driveway Be?
In total, a gravel driveway should be about 12 inches (30.48 cm), or one foot (0.30 m), thick. The base and middle gravel layers should each be about four inches (10.16 cm) deep to produce a stable and adequately drained driveway. This is after both layers are compacted by heavy machinery.
The top layer of a gravel driveway is primarily there for aesthetic purposes. It does not necessarily need to be as thick as the other two layers. But it is a good idea for it to be about four inches thick.
What Causes Gravel Driveways To Erode?
Sloped gravel driveways are subject to two primary erosion causes, weather and cars. Water and ice work break down your driveway’s foundations, while the weight of your vehicles puts strain and stress on the structure.
Weather is the more straightforward cause to explain. A gravel driveway will be exposed to the full force of the elements, water, to be specific. Rain and ice, if winters are cold where you live, are surprisingly dense and can exert a lot of force on comparatively small rocks.
Every time it rains, water will flow downhill and push rocks along with it. Water also all lubricates rocks, allowing them to move around more when pushed by automotive tires.
Ice is an even worse cause of water erosion. When water freezes, it increases in volume by about 9%. And expanding ice crystals exert a surprising amount of force against any surrounding surfaces.
When water freezes in the gaps between pieces of gravel, the gravel will shift position, potentially damaging any concealed supports. Water can also freeze within cracks in individual rocks, gradually breaking them down into smaller pieces.
Cars, especially American cars, with thousands of pounds and concentrate that weight into four surprisingly small patches. The average automotive tire has under five square inches (32.25 sq cm) of its surface area in contact with the ground at any one time.
That means the entire weight of your SUV or pickup truck is resting on a patch of ground no larger than a standard business envelope. And to make things worse, cars move.
Due to complicated physics reasons, the rolling action of automotive tires on individual pieces of gravel will result in a gradual migration downhill regardless of which direction a car is moving. In the short term, this migration result creates a washboard effect.
And in the long term, it will deposit much of the gravel at the bottom of the incline or in your street.
How Do I Stop My Gravel Driveway From Erosion?
Prevent a gravel driveway from eroding by clearing away grass and soil from the build area. Compact each layer of rock before adding the next. Add stabilization at the base using geotextile fabric, or add edge restraints. For extra protection, you can install drainage below the gravel.
At the very least, building a gravel driveway will require clearing the grass and topsoil from the build area. Additionally, each layer of rock should be compacted before the next layer is added. Both of these steps can be performed with a rented bobcat excavator or other heavy machinery. Additional steps can be taken to improve the stability and erosion resistance of the finished driveway.
One way to improve the finished driveway is to add stabilization to the base layer. There are two main ways to do this.
- The simplest way is to add a layer of geotextile fabric between the cleared dirt and the base layer of gravel. Geotextile fabric is permeable to water which improves drainage but prevents underlying dirt from migrating upward.
- You can also stabilize a gravel driveway by adding edge restraints. Edge restraints are essentially a low wall placed between the surrounding dirt and the gravel. The restraints prevent the gravel from migrating laterally and prevent water from the surrounding dirt from leaking in.
Alternative Stabilization Methods
A more complicated stabilization method is to add a plastic stabilization grid. A stabilization grid consists of a honeycomb framework of plastic rods that hold the first two layers of gravel in place.
The grid prevents them from migrating entirely. The plastic grid will not rust or decompose, unlike the metal rebar used in cement and asphalt driveways. It should last as long as the rest of the driveway.
The final method of gravel stabilization is to cheat. Suppose you want the look of a gravel driveway but don’t want the gravel to shift around. In that case, you can essentially glue the gravel together. Once the top layer of gravel is laid down, you can add a two-part epoxy resin coating to lock the individual rocks in place.
Epoxy resins are some of the most robust building materials the average homeowner is likely to encounter.
Some epoxies formulated for stabilizing gravel may be stronger than the actual rock. When properly applied, a resin coating will still leave gaps for water to drip through while preventing the top layer from shifting entirely.
Due to the strength of epoxy resins, this trick should not negatively affect the longevity of the gravel driveway.
When installing a gravel driveway, you do not have to rely on the gaps between individual rocks for drainage. You can install dedicated drainage systems underneath the gravel. Referred to interchangeably as “French drains” and “trench drains,” drainage systems provide dedicated paths of least resistance for water.
Several types of gravel driveway drainage systems are commercially available. In general, they combine gutters laid on either side of a driveway with buried permeable tubing. Water can either flow to the side or down the buried tubing. Installing these systems adds extra expense and effort but will also improve the erosion resistance of the finished driveway.
Correctly building a gravel driveway on an incline is not a simple process. It requires planning, preparation, and the right kinds of gravel. Several methods can be used to improve the stability and erosion resistance of an inclined gravel driveway. If constructed properly, a driveway made of gravel can outlast one made of cement or asphalt by decades.
This may be one of those jobs that you will want to hire a local contractor to handle.
I hope this article has been helpful. Thanks for reading!
For more, check out Building a Fire Pit on a Concrete Slab | Step-By-Step Guide.
Hey, I’m Jim, and the author of this website. I have always been interested in survival, fishing, camping, and anything in nature. In fact, while growing up, I spent more time on the water than on land! I am also a best-selling author and have a degree in History, Anthropology, and Music. I hope you find value in the articles on this website. Feel free to contact me if you have any questions or input!