If you're building a flagstone walkway or patio, it's important to know how to adjust the stones so they are level. Uneven surfaces pose a trip hazard and take away from the appeal of your landscape. The most effective way to level flagstone is to build a strong, level base with edging to keep the stones from shifting in the first place.
Excavate the site where you plan to lay flagstones to a depth of 8 inches, or below the frost line if your area gets freezing temperatures in winter. Building a deep foundation prevents frost-heave damage from dislodging the flagstones.
Install wood or metal edging on all sides of the patio or walkway. Drive 10-inch nail spikes through the edging to attach it firmly to the ground.
Spread 6 inches of crushed rock over the surface and compact it with a hand tamper. Pour a 1-inch layer of course sand over the gravel and spread it even with a rake.
Place the first flagstone in a corner of the site, tapping it into the sand with a rubber mallet. The level of the first flagstone will determine the height of the entire surface. Step back and make sure the flagstone is not too high or low, and add or remove the sand as necessary.
Continue setting the flagstones according to your pattern, keeping an even spacing of about 1/8 inch between each. As you go, place a water level over the stones and tap them firmly into the sand base to level them.
Pack fine sand between the joints of the flagstones on all sides. Continue packing the sand until the spaces are completely filled. The sand joints provide cushioning and deter the flagstones from shifting as they settle.
Apply sealant over the flagstone surface to protect it from water damage and to prevent the sand joints from wearing away.