How to Attach Flagstone to Wooden Steps

Flagstone is most commonly used as an exterior paving material for patios and walkways. As a natural stone, it offers a durability and natural aesthetic that is rivaled by only a few materials. Wood is often used in exterior stairs, as it is a versatile building material. While wood is usually long-lasting, stair treads (or steps) are vulnerable to damage when under constant use. Applying flagstone to the surface of wood steps can greatly improve both the aesthetic and the life of the stairs, and can be completed through a generally straightforward process.

Things You'll Need

• 1/4-inch cement-based backer board
• Tape measure
• Lumber pencil
• Angle grinder with masonry blade
• 1-inch deck screws
• Electric screwdriver
• Mortar mix
• Large bucket
• Electric drill with mortar paddle
• Brick trowel
• 2-foot level
• Grout bag
• Large sponge

Instructions

1. Measure the width and length of the wood step tread (the horizontal portion of the step) using the tape measure. Mark these dimensions on the cement-based backer board with the lumber pencil.

2. Cut the 1/4-inch cement-based backer board at the same dimensions as the wood step tread, using the angle grinder and masonry blade; refer to the angle grinder owner's manual for safety and operation directions.

3. Set the cut backer board on the step (with the grid side of the board facing up), lining up the edges of the backer board with the edges of the step.

4. Drive the 1-inch deck screws through the backer board and into the wood step, using the electric screwdriver. Insert the screws into the designated circles marked across the backer board grid pattern, no less than 1/2 inch from the edge of the backer board. Repeat this process, cutting and fastening backer board to the wood stairs, working your way from the top step to the bottom.

5. Lay out the flagstone pieces over the wood steps, in any desired pattern, leaving about 1/2-inch gaps between all stones and allowing the stone to hang about 1/2 inch over the front edge of the steps. Trim any stones to fit, using the angle grinder when necessary.

6. Prepare the mortar mix as directed on the packaging, using the electric drill and mortar paddle in the large bucket.

7. Lift a flagstone piece from a step and apply a thick layer (about 3/4 inch) of mortar to the backside of the flagstone, using the brick trowel. Press the flagstone to its original place, hard enough so the mortar joint between the stone and the backer board is only about 1/2 inch. Work your way across the wood step using this process, and set the 2-foot level over the surface of the set flagstone to ensure an even and level surface. Repeat this process on remaining steps, working your way from the top step to the bottom.

8. Allow the mortar to fully cure as indicated on its packaging before moving forward.

9. Prepare the grout mix as directed on the packaging, using the electric drill and mortar paddle in the large bucket.

10. Evenly apply the grout to the joints between all pieces of flagstone, using the grout bag. Remove all excess grout from the flagstone surface and joints, using the sponge. Rinse the sponge in clean water frequently.

References

• "Stonework: Techniques and Projects"; Charles McRaven; 1997
• "Fundamentals of Building Construction: Materials and Methods"; Edward Allen and Joseph Iano; 2003