How to Install a Capstone on a Retaining Wall

Mix an amount of standard mortar mix appropriate to the number of capstones being installed, following the manufacturer's labelled directions. Mix the mortar in a standard mortar box or wheelbarrow to allow mobility and access to the mix during the cap installation.

Access the top of the block wall and spread a long, narrow mound of mortar to the outside edges of the wall on both sides, using a masonry trowel. The mounds should be approximately 1 inch high. Lay out enough to support a single length of the capstone you are installing.

Hold a capstone piece over the mortar mounds you laid out in Step 2 and lower the capstone onto the mortar. Shift the capstone as needed to center it on the wall to equalize the overhang at each side of the wall. If you are starting at a corner or wall end point, the capstone should overhang at the starting end the same amount as on each side.

Tap the top surface of the capstone gently with a rubber mallet to level the piece. Use a torpedo level to make sure the capstone is straight. Once it's leveled, the mortar joint supporting the piece should be approximately 1/2-inch thick and consistent along both sides of the capstone.

Repeat Step 2 to spread mortar for the next capstone, adding a 3/4-inch layer across the vertical face of the capstone's leading edge. Hold the next capstone over the mortar mounds and lower the capstone onto the mortar. Shift the capstone as needed to center it on the wall to equalize the overhang at each side of the wall, and press it toward the previous capstone to close the mortar joint between them to approximately 1/2 inch. Tap the top surface of the capstone gently with a rubber mallet to level the piece. Use a torpedo level to make sure the stone is straight. As with the previous capstone, the mortar joint supporting the piece should be approximately 1/2-inch thick and consistent along both sides of the capstone.

Repeat the procedure described in Steps 2 through 5 to install each of the remaining capstones along the wall's length.