Installing concrete paver driveways on slopes
Interlocking Stone Systems, Inc.
Installation of concrete pavers on steep slopes creates special needs and concerns for the installer. The major concerns for the construction of a driveway is the ability to divert, drain and remove water away from the base, sand and surface.
One note before we start: please check your local code for the maximum slope allowed. We have noticed some localities do have restrictions on the maximum slope percentage, and doing your homework up front may save you time and money!
The proper techniques and construction detail are critical to insure long-term integrity of the paver surface. Lets look at a typical steep slope installation.
The excavation step of the installation is relatively simple, making sure the subgrade is excavated to the same finished surface, or that the cut maintains at least a minimum of 9 inches from finished grade. During the excavation phase, be sure to extend the cut of the driveway at least 12 inches on both sides of the paving surface, with sections where vehicles will turn and exit the driveway.
Soil, Fabric and Base
We recommend that after the excavation is completed the soil be compacted using a plate tamp. After the excavation and tamping, install a fabric such as Mirafi 500X or equivalent. Be sure the fabric has few wrinkles and extends slightly over the sides of the excavated area. Any excess material can be cut later.
The fabric is essential for keeping the aggregate base from migrating into the soil subgrade. The aggregate base should be D.O.T.-approved base material, placed and compacted in 3- to 4-inch lifts, and the minimum thickness of a base should be 8 inches. In areas with freeze-thaw cycles we recommend installing a thicker base.
When the slope is approaching approximately 8 percent or more, we recommend that a concrete header beam be installed. This header beam will be used to divert water from areas below while maintaining the base and pavers on the upslope side. The beams are typically 6 inches wide and 12 inches deep, formed on-site. After the beam has cured, removed the forms and place a waffle drainage mat such as Miradrain 9000 or equivalent on the uphill side of the header beam. The waffle drainage mat will direct water to the sides of the driveway.
As mentioned earlier, water diversion is paramount to maintaining integrity of the pavers, it is important with the waffle mat to direct the water beyond not only the edge of the driveway surface but also the extended base. Proper planning for removal of excess water is extremely important.
Bedding and Pavers
The bedding sand used should be ASTM C 33, i.e. concrete sand. Do not use what is commonly known as screenings. Depth of the setting bed should be a minimum of 1 inch with the maximum depth of 1.5 inches.
Paver selection is also important for the integrity of the driveway. The optimal paver is the standard 4- x 8-inch paver installed in a 45-degree herringbone pattern. This pattern naturally sends water to the edge of the driveway. With the availability of so many styles and shapes of pavers, the problem we encounter is that most homeowners want no part of a standard paver laid in a 45-degree herringbone pattern. The objective is to find a paver that the homeowner likes that will be acceptable for the sloped driveway application.
Make sure the paver the homeowner selects can satisfy two requirements:
1) the correct aspect ratio of the paver and the spacer bars are at least 2 milimeters thick in order to ensure minimum joint width
2) the aspect ratio which is the paver length divided by its thickness should not exceed 4:1 (Pavers greater than 4:1 do not have acceptable interlocking characteristics for vehicular traffic.)
Laying the Pavers
To start laying the pavers, begin at the lowest end of the slope and work uphill. Starting at the lowest end will ensure a tight fit of the pavers as you proceed up the slope. Approximately every 6 feet, adjust and tighten if necessary. The installation of edge restraints are very important. Whichever type of plastic edging you use, install spikes in every hole, especially if the driveway has a curve to it, dont skimp; those edges need to be very secure.
Another major point: make sure that when you finish for the day, you complete everything, meaning you lay the pavers, cut the edges if necessary, install your edge restraints and backfill--yes, backfill up to the edge restraints. Nothing is more frustrating than wrapping up the day, going home in the evening and during the night having a torrential rainstorm that scatters the aggregate base, paver and sand.
Moral of the story: take the time to finish each section completely. The surface of the pavers should have a crown or crossfall of approximately 2 percent to move water to both sides or one side.
The final procedure for the sloped driveway is to sweep and fill the joints with concrete sand and final plate tamp. Use dry sand for this process, as it will fill the joints quickly. On the sloped driveway it is highly recommended that a sand stabilization material be placed in the joints after the concrete sand has been placed. Make sure the paver surface has been swept and cleaned, then sweep in the sand stabilizer. A word of caution: read the directions supplied by each manufacturer, as the application process can vary.
The sloped driveway also requires the installer to do periodic follow-up visits. We make return visits 30 days and 1 year after installation to check for horizontal creep of the pavers, where the vehicles may have pushed the pavers to the sides and created larger joints between them. If we catch the problem in time, less damage will be done. If left without service, water will undermine the sand setting base and possibly the aggregate base.
Also look for any evidence of other water problems. Water problems which can often be overlooked include leaking irrigation lines and downspouts that exit near the driveway.
Keep in mind that the steps above are just a general guideline. Every job is completely unique and requires the installer use his or her individual judgment. Planning the project is extremely important and remember, the purpose is to remove water from the sloped driveway and to keep the pavers as tight as possible.
For more information on this subject please contact the Interlocking Concrete Pavement Institute at (202) 712-9036 or visit www.icpi.org.