How to Lay Flagstone Rock Over Wood Decks

Check the joists for the deck to ensure they can handle the weight of the flagstone. While normal decks utilize a 16-inch span for the weight of the wood and a few bodies, a flagstone deck requires more stability. You need 12-inch joists for flagstone. Cut the joists with the circular saw and install them in place with a hammer and nails every 12 inches under the deck to give it additional support.

Measure for and cut a sheet of plywood to fit on top of the deck. Screw it down onto the old wood decking and cover the entire surface where you plan on installing the flagstone. Paint on a layer of waterproofing to protect all of the wood. Use a paint brush around the edges of the deck and a paint roller for the center areas. Allow it to dry for 24 hours.

Lay down the wire mesh across the installation area with at least 2 inches of overlap between the sheets. Shovel the cement mixture into the mixing machine and mix a batch according to the directions on the container (different types require different amounts of water). Let it mix for a few minutes and then fill the wheelbarrow with the mud.

Pick a corner to start in. Shovel some mud onto the installation area on top of the wire mesh. Lay down your flagstones in a bed of mud, ensuring that there is at least one half to a full inch of mud underneath each piece, minimum. Adjust the amount of mud as necessary to make up for different size pieces and fill the joints with more masonry mud as you work your way back.

Break up larger pieces into smaller chunks and pieces for along the outermost perimeter of the deck by striking the pieces sharply with the chiseled end of the stonemason’s hammer. Work your way out from your initial corner until you are finished.