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To decide how much stone to buy, you'll need to calculate the square footage of the area you plan to cover. You'll also need to make sure your project is safe and legal before you expend your time, energy, and money on it.  Start by consulting local building codes to make sure your planned project doesn't violate the law. This information should be available on your city government's website. If you build something that is in violation of local codes, you may be forced to remove it, or may be unable to rent our sell your home to others. It is also important to make sure there are not water or gas lines running near the surface in the area where you plan to lay your flagstone. You can call 8-1-1 to determine if this is a potential concern.  Use marking paint to draw an outline of the area you plan to cover on the ground. This will make it easier to take measurements. To find the total area, multiply the length of the area you plan to cover by the width. Once you've figured out how much ground you need to cover, you can calculate how much stone you'll need to buy.  For flagstone that is an inch or less in thickness, one ton will cover 180 to 200 square feet. Typically, though, flagstone under one inch in thickness is not recommended as it may crack or break with too much weight or foot traffic. For flagstone that is an inch to an inch and a half thick, one ton will cover 90 to 100 square feet. For flagstone that is two inches or more in thickness, one ton will cover 70 to 80 square feet. It is a good idea to order about 10 percent more stone than you calculate you will need. Use highly visible marking paint or lay garden hoses out over the edges of the area you are going to excavate. Then, cut through the sod and roots along the inner edge of the hose using a flat-blade shovel or edger. Dig out the entire area. Depending on the thickness of the stones you plan to lay, you'll need to dig three to six inches down. To keep the edges of your foundation even, you'll need to line the sides of the excavated area with edging material. You can use treated lumber ("benderboard"), vinyl, steel, bricks, or carved stone. Steel spikes should be driven into the ground to secure flexible edging materials. Your foundation should have three layers: landscape fabric, gravel, and sand.  Landscape fabric prevents weeds and separates your foundation from the native soil. The edges of the fabric can be secured under your edging material.  Next, lay two to four inches of pea stone or small gravel. Tamp it down with a tamper to make it compact and even. Some people opt not to use gravel in their foundation. Gravel makes it easier for water to run off your path or patio, so it is highly recommended that you apply a layer of gravel if you live in a rainy area.  Finally, pour your sand. You should cover your foundation with one to two inches of sand and then smooth it out with a rake. You may find wetting down the sand with a hose makes it easier to secure your flagstones. Place your stones in the pattern you desire with the best-looking side up. Try to keep about the same amount of space between each stone.  Begin by laying stones along the perimeter of the area you are covering, using pieces with edges that fit the edges of your path or patio, and work inward. This will reduce the number of stones you need to cut down to make them fit properly. Because of their stability, large stones should be placed near doorways and other heavily trafficked areas.  Stones that are too large or do not have the desired shape can be cut with a chisel and rock hammer. If your stones vary in width, you may need to add or remove sand from beneath some of them in order to get a level surface. Be sure your flagstone is smooth to avoid potential tripping hazards. Use a rubber mallet to tap the stones into place one by one. The final step is to fill the gaps between the flagstones, both to secure them in place and complete the aesthetic look of your path or patio.  There are many options you can choose from for joint filler, including pea gravel, decomposed granite, or sand. You can also put topsoil between the stones and plant a ground-covering plant, such as wooly thyme or ordinary grass.  To fill your joints, just dump the filling material onto the flagstones and sweep it into the joints with a broom.
Plan your project. Purchase your flagstones. Dig out the foundation. Install the edging. Create the foundation. Lay out your flagstones. Tamp the stones down. Fill the joints between the stones.