How to Install a Flagstone Patio

Introduction: How to Install a Flagstone Patio

About: The official instructable for Popular Mechanics magazine, reporting on the DIY world since 1902.

A traditional flagstone patio can be a great addition to just about anybody’s yard. The material is attractive and durable and nicely complements a wide range of other landscaping features. Many varieties of stone are available at landscape and masonry outlets. Slate, a metamorphic rock, is one of the best-known types. Bluestone, which we used on this project, is a sedimentary rock with a very engaging color. When installed, both types yield a relatively smooth, flat surface.

Yet another option is fieldstone, the rocks removed from the soil when land is cleared. Though truly distinctive when installed properly, its finished surface is much rougher than flat stones like slate and bluestone. Flat stones usually are sold in nominal thicknesses from 1 to 3 in. and in 6-in. width and length increments, starting at 12 x 12 in. and going up to 2 ft. 6 in. x 2 ft. 6 in. Anything larger or thicker is usually a special order. Prices vary substantially. We bought ours in upstate New York, where 1-in.-thick bluestone costs about $3 per square foot delivered. Stonework can certainly be beautiful and can add value to your property. Unfortunately, it’s not very easy to install. Most of the problems are in the material itself.

Unlike consistent building units, like bricks or cast concrete pavers, natural stone comes in variable thicknesses. When you order nominal 1-in.-thick stones, for example, you get pieces that have one flat side but that vary in thickness from as little as 1⁄2 in. up to as much as 11⁄2 in. Because your goal is to create a finished surface that is as flat as possible, this means you have to custom fit the bottom of each stone in the sand base, which can be extremely time-consuming even after you get the hang of the job. The material is also pretty heavy. One-in.-thick stone weighs about 14 pounds per square foot. That means that a 1-in.-thick, 2-ft. 6-in. square piece tips the scales at about 87 pounds, and a 2-in.-thick piece of the same dimensions weighs about 175 pounds. For most patios, 1-in. stone is fine. But if you plan to drive over the stone, you’ll have to use material that’s at least 2 in. thick. If installation speed and light work are important to you, then bricks or concrete pavers are probably the better choices.

This project was originally published in the April 2002 issue of Popular Mechanics.  You can find more great projects at Popular Mechanics DIY Central.

Step 1: Layout and Site Preparation

Because flat stones are sold in standard width and length dimensions, you can create just about any pattern from a simple all-one-size approach to a more complicated pattern like the one we used here. Just keep in mind that stone dimensions can vary by as much as 1⁄2 in. from the nominal designation. A 12 x 12-in. stone could be 111⁄2 in. on one side and 121⁄2 in. on the other. These differences in size are accommodated by the grout joint between stones, which is usually 3⁄8 to 1⁄2 in. wide. Work out your pattern on graph paper before you place an order.

Try to minimize the number of pieces that will need cutting on site. If you’re laying a patio that is going to abut the house like ours did, plan to slope the installation away from the house so water can drain off. A 1-in. slope for every 8 ft. of run is a good minimum figure. Proper base preparation is a matter of some debate. A plurality, if not a majority, of sources recommend at least a 4-in.-thick layer of compacted gravel covered by a 2- to 3-in. layer of compacted sand. On this job we used the sand layer because it was much easier to fit the stones in it and because the patio was covered by an enclosed porch on the floor above. But if a patio were to be completely exposed to the weather, we would omit the sand and just use a 6- or 7-in.-thick gravel base because it has better drainage.

Begin site preparation by removing the sod in the patio area. Then spread gravel on the site using a shovel and rake (Photo 1). Next, attach screed guide boards to the house and to stakes driven into the ground on the other side (Photo 2). The boards along the house should be level from side to side. The opposite boards also should be installed level from side to side but located lower than the house boards to accommodate the necessary slope mentioned above. Next, cut a notch in both ends of a screed board to fit over the guides. Pull the screed across the gravel until the surface is flat (Photo 3). Add or remove gravel from areas as needed.

Once the surface is flat, compact it with a plate compactor (Photo 4), available at tool rental stores for about $50 a day. If you haven’t used one of these machines before, it’s a good idea to work in the center area of the patio site first. These machines can be hard to control, so it’s smart to get comfortable with your machine before you work close to the house. Make at least three passes over the entire area, overlapping passes by half a plate width each time. Add a few inches of sand and spread it with a shovel and a rake. Then screed it to finished depth and flatness with another screed board (Photo 5), using the guide boards to support the ends. Compact the sand with a plate compactor as you did with the gravel.

Step 2: Laying the Stone

Before you begin placing stones in the sand, you must establish a square corner. To do this take a long, straight board and place one end against the house where the patio corner will fall. From the corner point, measure over 6 ft. along the house wall and make a mark. Then, measure 8 ft. along the board from the house and make another mark. Have someone hold the end of a tape measure at the house mark and adjust the board until the 8-ft. mark intersects the 10-ft. mark on the tape (Photo 6). Drive a stake at the end of the board and tie a string from the house corner to this stake. Tighten the string so it is the height of a scrap block of wood above the finished surface of the stone. By aligning the edge of all the end stones with this string, your installation will start square.

With your square starter string in place, slide the corner stone into position. Check its height by sliding your scrap block between the string and the stone (Photo 7). Then check for level from side to side. To make the stone fit properly, lift it up and either add or remove sand from the base using a trowel (Photo 8). Lay the next stone in your pattern. Maintain a joint of about 1⁄2 in. between the stones and check that the matching edges of the stones are flush and that the second is level with the first from side to side (Photo 9). As you add more stones always make the edges meet flush and check the overall slope frequently, especially as you work farther from the house.

A long board with a 4-ft. level on top should keep you in the ballpark (Photo 10). Cutting stone is easy—though very loud and messy—using a circular saw with a masonry blade that has diamond chips bonded to the edge. These blades cost about $15 each and can cleanly cut through 1-in. stone in a single pass (Photo 11). They work much better and last longer than the commonly available aluminum oxide abrasive blades. Once all the stones are laid, align the grout joints using a pry bar or flat bar (Photo 12).

You’ll have to make some compromises on this step because not all the joints will line up perfectly. Just favor the ones that will be more visible and let the others fall where they fall. You can grout the joints with either sand—which we used—or stone dust that’s available from your stone dealer. Spread the material you choose over the patio and use a push broom to fill the joints (Photo 13). Pack the grout into place with either a mason’s trowel or an ice

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