Q: I have seen and read instructions for a mortarless flagstone walkway that stressed the importance of uniform thicknesses, even though the shapes are different.

My hope is to make a mortarless patio, measuring 10 by 14 feet, using 1-inch-thick, 12-inch-square slate. The thicknesses are irregular, which is OK with me. I will carefully select the thickest possible pieces and work adjacent pieces to avoid any tripping hazard. I want these installed so that water will run between the pieces, and I was thinking I should set these slate squares in a sand base — but over what? I thought I would put down a weed-block product under the sand. Would a thick layer of sand over leveled, tamped dirt work? I have clay soil.

How do I create a level pad measuring 10 by 14 feet? One end of this patio will join with the side of a brick-edged slate path that was done with mortar using the same slate. Dirt butts up to this brick edge and dirt will surround the patio, so I thought I would put a row of matching brick at the edge of the sand on the remaining three sides of the patio.

I am trying, in a very small way, to do something environmentally good and let all water return to groundwater.

A: Your instincts are right on the money. A sand base for your new slate patio is definitely the way to go. Since water will drain through the sand joints and won’t puddle, your idea is not only environmentally friendly but also will be more useful because you won’t be sloshing through standing water. The challenge is to get the sand base level so that the pavers rest as flat as they can.

It’s important to start with a flat, compacted base because you can count on some settling and unevenness after the patio ages a bit. Because the stones are uniform in length and width, adjustment in the layout won’t be necessary, but since they vary in thickness, you’ll have to fiddle with each stone to get the depth right. As far as the brick apron, we suggest you set it in mortar. This will give you a stronger border and hold the edges of the patio in place.

The project is a fair amount of work, but when you’re done, you’ll have a handsome and environmentally friendly patio.

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