Regardless of the size and style of patio stones you use, laying black dirt beneath them is not effective. Although you can tamp dirt until it is compact, it is still subject to erosion and settling. A patio built on a foundation of dirt might last for one season, but rain, the freeze-thaw cycle and the weight of people using the patio will eventually cause the dirt to run and the patio stones to settle unevenly.
Dirt of any color does not make a suitable foundation for a raised patio in your yard. Working with dirt as a foundation will initially create a smooth foundation on which to build a patio. Over time, however, the dirt will settle and prove to be a highly inept patio foundation.
Though an uneven foundation is the most significant problem associated with building a patio on dirt, other issues will arise. Because the dirt will also sit between the patio slabs, it will be easy for your shoes to pick up the dirt and track it into your home. Additionally, the dirt between the slabs provides an ideal environment for grass and weeds to sprout and grow, giving the patio an unkempt appearance.
To build a patio, the correct method requires you to dig 8 to 10 inches in depth over the entire area under which you wish to situate the patio. Cover the bottom of the hole with one or more layers of landscaping cloth to prevent weeds from growing up through the patio, and fill approximately 6 inches of the hole with gravel. Use a tamp to compact the gravel as tightly as possible.
After you have tamped the gravel considerably, apply 1 inch of stone dust, water it and tamp it until compact. Repeat this process two or more times, and then lay the patio slabs on the stone dust. Sweep sand into the cracks between the patio slabs to prevent them from filling with dirt and growing weeds over time.
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