How to Apply Decorative Rock Over Fiber Cement Board

Decorative rock is a popular exterior material that is highly resistant to water damage and impervious to rot. Most decorative rock exteriors start with a carefully installed layer of fiber cement board. Then the decorative rock is applied to the surface of the fiber board.

When purchasing decorative rock, carefully calculate the exterior surface of your home, subtracting area for doors, windows, and other areas that will not be covered with stone. Then purchase enough decorative rock to cover that area plus 10 percent. If you have a particularly complicated pattern that you plan on installing, you may require additional decorative rock for matching purposes.

Step 1: Clean the fiber cement board with a brush and water to remove any remaining dirt from the surface of the board. If the fiber cement board is newly installed, you can skip this step. Allow the fiber cement board to dry thoroughly before proceeding.

TIP: If you have a complicated pattern that requires exact centering and placement, use a carpenter's level and pencil to dry guidelines onto the fiber cement board.

Step 2: Place a tarp down where you will be working. On top of the tarp, place about 25 square feet of stones on the top. If there is a pattern you want to try, now is the time to arrange the stones properly.

Step 3: Apply thin set mortar with a trowel to the side of the fiber cement board. Try to cover a small area that is only 3x3 feet in size with about ½ inch of mortar. Never start from the middle unless your pattern demands it: a corner is a good bet if you don't have a preference.

TIP: Don't mix more mortar than you'll be able to use in 20 minutes. Also, pay particular attention to the ratio of mortar mix to water. Appropriately mixed mortar will be easy to apply, too wet or too dry mortar will be difficult to apply and may crumble prematurely.

Step 4: Press rocks into the mortar on the fiber cement board. If your decorative stones have definite rows, use a carpenter's level to make sure that they don't get too far off course.

Step 5: Continue applying mortar, then decorative rock to the surface of the fiber cement board until the exterior of your home is completely covered.

Step 6: If necessary, pipe mortar into the spaces between the decorative rock, smoothing the mortar with a trowel as necessary. Whether you complete this step will depend on the pattern you chose: some stone will be installed without spaces for mortar.