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.
Put down at least a 3-inch-thick layer of sand as a base. You may need to excavate some dirt to achieve this thickness. If so, do it. We’d skip the weed cloth, though. There’s not enough light reaching the dirt subbase for weeds to root. Any weeds that do grow do so in sand and are easily pulled.
We’ve found that the best place to buy sand is at a masonry yard where you can purchase in bulk. To calculate the amount of sand you need, multiply the length of the patio by the width and the total by the thickness of the bed, in this case 3 inches, or 1/4 foot. Divide the total by 27 to get the number of cubic yards. In this case, the computation is 10 multiplied by 14 multiplied by 0.25, and the total is divided by nine. This totals about 1.29 cubic yards of sand.
To get a flat compact sand base, build a screed from PVC irrigation pipes. You can substitute 2-by-4s if you like. Place a 1-inch pipe or board on each side of the shortest width of the patio and one in the middle. Level the pipes using a flat 2-by-4 and a spirit level. Secure the pipes in place by screwing them to wooden stakes. We suggest you build in a slight slope so that any water that wants to puddle will drain in one direction. Shovel the sand between the pipes. Using the pipes (or 2-by-4s) for support, drag a 2-by-4 to level the sand. Sprinkle water from a garden hose on the sand so that it settles. Shovel more sand between the pipes and repeat the process. Three passes should compact the sand enough for placing the slate.
Remove the center pipe, fill the void with sand and tamp it down with your hand. Begin placing the slate tiles. Use a rubber mallet to tap each tile into place. This ensures that it gets full purchase on the sand base. We suggest you try to butt each tile to the adjacent ones, but in any case leave no more than a 1/16-inch gap between the tiles. If the irregular thickness causes one tile to sit higher than its neighbor, remove a little sand so that it sits level.
Once all the tiles are in place, pour a bucket of fine sand on the patio and sweep it around with a broom, filling the joints between the tiles. After a couple of days, the sand in the joints will settle. Then repeat the process.
With the slate installation complete, install the brick borders with mortar between the bricks. Set the bricks on an 8-inch-deep by 8-inch-wide footing of mortar to provide some stability. To make the footing rigid, drop a piece of No. 3 rebar into the footing at the 4-inch mark.
The project is a fair amount of work, but when you’re done, you’ll have a handsome and environmentally friendly patio.
