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The boulders should be about the same size. Using a rock dolly, place the boulders on a level area about 6 feet apart. Make sure the flattest side of the boulders is on the bottom to prevent shifting.
Mark the boulders where the seat will sit. The "shelf" should be at least a few inches wide to receive the bench.
Measure the distance between the two rocks to determine the length of the bench.
Stack the 6-foot-long pieces of 2x12s on top of each other to create the bench seat.
Line up the boards flush and screw them together with 2 1/2" stainless or galvanized screws. Measure and cut off the corners evenly to soften the look of the bench.
Dry-fit the plank seat to the boulders. Because no two rocks are alike, it may be necessary to notch out an area in the rock for the seat to fit securely. If so, carve out a notch with a masonry drill and a masonry chisel. Always wear safety glasses and ear protection when drilling into rock.
Once the seat fits on the rock (Image 1), set the seat aside and use a masonry drill to drill a hole at an angle under the seat's position (Image 2). This hole should be about 1/2-inch wide and 8 inches deep. Drill a similar hole in the other boulder. It's important to drill the holes at an angle to keep from splitting off a large piece of the boulder.
Drill holes through the seat that match up with the holes in the boulders. When you're finished drilling, dry fit the seat with the bolts to guarantee that it sits level between the boulders.
Use an epoxy gun to fill the holes with epoxy. Before the epoxy hardens, position the boards onto the boulders and hammer 12-inch threaded bolts with washers through the seat holes and into the epoxy-filled holes in the boulders.