How to Excavate for a Flagstone Patio

A flagstone patio needs a solid foundation.

The most important part of installing a flagstone patio is laying a strong, level foundation. Properly screeded sand on a layer of well-compacted gravel is essential. This will not only make it easier to lay the stones, it will also guard against the flagstones shifting and rainwater pooling. Before starting, ask your local utility company to mark the location of any gas, water or electrical lines in the area to be excavated.

Things You'll Need

• Aerosol can of bright red paint
• Wood stakes
• Strong nylon cord
• Long builder's level
• Steel measuring tape
• Shovel
• Hose pipe
• Vibrating plate compactor
• Long straight plank
• Geotextile fabric
• A or B grade gravel
• Rake
• Coarse sand
• Lengths of PVC pipe

Instructions

1. Mark the outline of your patio on the ground or grass with a spray can of bright red paint. Drive 12-inch-long wood stakes into the ground at the starting and finishing points nearest the house and at the points or corners of the patio farthest from the house. Tie a piece of strong nylon cord around the first stake nearest the house. Stretch the cord tightly around the two farthest stakes and to the last stake nearest the house. Level the line with the aid of a long builder's level by sliding the cord up or down the stakes.

2. Mark the farthest stake to determine the drainage slope of the finished patio. This should be 1 inch for every 4-foot length of flagstone. Measure this calculated distance down from the level string on the stake farthest from the house with a steel measuring tape and mark this point with a pencil. Repeat with the second corner stake. Tie a second line of cord around the starting stake. Stretch the string tight and loop it around the mark on the farthest stake. Stretch the line tightly across and loop it around the mark on the second corner stake. Stretch the string tightly and tie the end around the last stake nearest the house. This will be the finished height of your patio.

3. Excavate the patio to a depth of 7 inches with a shovel. Dig inward from about 2 inches outside the red line. Measure the depth around the perimeter with the metal tape measure. Use the long builder's level to check the base level as you excavate. Lift the surface sod carefully and save it for later lawn patching or extensions. Pile the sod in a convenient place and water it to keep the lawn alive. Place the subsoil in a different area for later use.

4. Water the base of the excavation liberally with a hose pipe and allow the water to soak in. Compact the excavation with a vibrating plate compactor. Keep checking the surface with the long carpenter's level, laid on the top edge of a long straight plank laid across the width of the excavation. Have two helpers stretch a line between the two side leveling strings. Tell them to follow you as you work. Measure the distance from the string to the surface from time to time to follow the drainage slope as closely as possible. Fill in indentations and remove high spots with the shovel. Go over the surface at least three times, until you have a smooth, hard base with the correct degree of slope for your foundation.

5. Roll out layers of geotextile fabric over the base to prevent future weed encroachment. Overlap the edges by 2 to 3 inches. Pour A- or B-grade gravel on top of the fabric to a depth of 4 inches. Use a rake to smooth out the surface, and place your builder's level on the top edge of the long straight plank to check the level across the width of the patio. Soak the gravel well with the hose and compact with the vibrating plate compactor. Check the slope every few yards as described. Go over the surface several times, until it is firmly compacted.

6. Pour the final 1-inch-thick layer of coarse sand on top of the compacted gravel and rake smooth. Place long lengths of 1-inch PVC pipe spaced 2 yards apart across the width of the patio, running away from the house. Have your helpers stretch the string across the width as described. Bed the pipes into the sand, taking extra care to measure the distance from the back and front ends of the pipes to the string so that the top surface of the pipes follow the slope.

7. Place the edge of a long straight plank on the first two pipes across the ends nearest the house. Kneel in the sand and move the plank toward you and from side to side across the pipes to level the sand and to move the excess backward. This process is called "screeding." Work your way backward between the first two pipes. When the first row is level, move across to the second row and repeat. When the sand is level between the first rows of pipes, lift the pipes out of their trenches and bed them into the sand on the next pass away from the house as described. Continue until the entire surface of coarse sand is screeded, but still loosely packed and ready to accept the flagstones.

Tips & Warnings

• Adjust the thickness of the final layer of coarse sand in accordance with the thickness of your flagstones. The 1-inch depth described in Step 6 allows for 2-inch-thick flagstones to be level with the surrounding terrain.

References

• YouTube: How to Lay the Foundation for a Patio or Walkway (Video)
• Lowe's: Laying the Foundation for a Patio or Walkway (PDF)
• Sturgis: Flagstone Patio
• The Family Handyman: Build a Stone Patio or Brick Patio