Laying flagstone patios in stone dust or sand, as opposed to mortar or concrete, is known as dry-set or sand-set. Dry construction is much easier for do-it-yourselfers because you can build directly on the ground, while mortared stone requires a concrete slab foundation to prevent cracking in the mortar. For a lasting installation, a dry-set stone patio does need a foundation, though. It's best to start with several inches of compacted gravel, followed by a layer of stone dust, also called decomposed granite, or DG.
The DG is pulverized stone and contains small pieces along with gravel-size and sand-size particles. The variation in particle sizes allows the material to compact well. And while it is still loose, you can use it to level the stones, just as you would with sand. You can buy flagstone and DG at local stone yards and landscaping materials suppliers.
When choosing stone for the project, select the largest stones you can maneuver by hand, and choose them for consistent thickness and appearance. The stones should be at least 1 1/2 inches thick for strength; 2 inches thick is better. Note that "flagstone" merely describes the stones' wide, flat shape; flagstone comes in many different types of stone.
Codes and Regulations
Check with your city's building department to learn about rules and requirements that may pertain to your project. Chances are, you won't need a building permit for this project, but you may need to gain approval from the zoning department, which governs land use.
Most important, call 8-1-1, the national "Call Before You Dig" hotline, to have all underground utility lines marked on your property. You must do this before breaking ground. The service is free but can take several days, so call well in advance of starting your project.
Equipment / Tools
- Tape measure
- 8 Stakes
- Small sledgehammer
- Mason's line
- Line level
- Carpenter's level
- Hand tamp or plate compactor
- Garden hose
- Compactible gravel
- Decomposed granite (stone dust)
- Fill material (as desired)
- Long, straight 2x4 board
Mark the Patio Edges
Set up mason's lines to represent the perimeter of the patio. Drive two stakes at each corner, about 2 feet beyond the sides of the patio perimeter. Tie a mason's line to opposing pairs of stakes to create a square or rectangular layout (use a square layout if the patio will be circular). You will have a total of four strings; the points where the strings intersect represent the corners of the patio area. This layout method allows you to excavate and prepare the entire patio area without having to move the stakes.
Square up the Layout
Confirm that the string layout is square by measuring diagonally between opposing corners (where the strings meet); the layout is square when the diagonal measurements are equal. Adjust the positions of the stakes, as needed, to square the layout.
Slope the Layout Strings
Level and slope the strings based on the position of the patio: The patio surface should be level from side to side (typically parallel to the house) and should slope down and away from the house from end to end at a rate of about 2 inches per 10 linear feet. For example, if the patio is 10 feet long, it will be 2 inches higher at the house end than at the opposite end. This ensures that surface water drains away from the house (or other structures).
To level and slope the strings, place a line level on each layout string and adjust one end of the string up or down, as needed, so the line is perfectly level, then tie off the string. Keep in mind that all of the strings should be at the same height. Then, readjust the two strings running down the sides of the patio to set the correct slope. For a 10-foot-long patio, lower the strings 2 inches on the stakes at the lower end of the patio. Finally, adjust the perpendicular string on the lower end of the patio so it is even with ends of the sloped strings.
Excavate the Patio Area
Remove all grass and other vegetation in the patio area (defined by the strings), then excavate the soil to a depth of 6 inches plus the thickness of the flagstone. For example, if the flagstone is 2 inches thick, excavate a total of 8 inches. This is to make the patio flush with the surrounding ground; you can dig less deep if you want the stones to lie higher than the ground.
Measure down from the strings to gauge the excavation depth. It's usually easiest to dig to full depth right under the strings, then clear out the soil in between, checking with a long, straight 2x4 and a level to make sure the ground is level from side to side (it won't be level from end to end because of the slope). Tamp the soil with a hand tamp or a rented plate compactor.
Install the Gravel Base
Add a 2-inch layer of compactible gravel over the patio area. Rake the gravel smooth and level, then tamp it thoroughly. Add two more inches, then smooth and tamp. As with the excavation, measure down from the strings to gauge the thickness of the gravel and to maintain the proper slope.
Add the DG Layer
Add a 2-inch layer of decomposed granite over the gravel base. Rake the DG smooth, and level it side-to-side with the long board and level. Tamp the layer thoroughly. Remove the stakes and mason's lines.
Organize the Stones
Stack or lay out the stones you will use for the patio surface on a broad, flat area, such as the surrounding grass or a driveway. Arrange the pieces by size and shape so you can select them readily as you lay them into the patio.
Lay the Patio Stones
Place the stones onto the DG surface, starting at one end and working toward the other. Place the stones as close together as desired. You will fill the gaps later with sand, gravel, or even soil and grass or other plants. Vary the size, shape, and coloring of stones for a random pattern, which looks most natural.
Level the Stones
Level each stone, as needed, by adding or removing DG underneath the stone. Each stone should be stable and at the same height as the surrounding stones. Stone edges that stick up create tripping hazards.
Fill the Gaps
Fill the gaps between stones with sand, DG, or gravel. Sweep the material across the patio with a broom to push it into the cracks. Spray the patio with a garden hose to settle the fill material, then sweep more filler across the stones to fill the gaps as desired. Alternatively, you can fill the gaps with a potting soil mix and plant grass or traffic-tolerant ground cover plants between stones.
Flagstone Patio Tips
Some of the stones in your dry-set patio inevitably will move over time, due to use and to seasonal freeze-thaw cycles. But fixing shifted stones is easy. Simply dig out the sand, gravel, or soil around a problem stone, then pry up the stone using your hands or a flat pry bar. If the stone is too high, remove some bed material from under the stone; if the stone is too low, add some bed material. Reposition the stone and make sure it's flush with the surrounding stones, then repack the joints around the stone.