How to Mortar Stone
Using mortar to build a stone wall holds the wall together and prevents the stones from shifting. Without mortar, the stones will move imperceptibly over time until they become unbalanced, and the wall falls apart. If you are looking for a simple method of building a vertical-sided stone wall, the steps outlined below will make the job simple enough for even a novice stone layer.
Step 1: Plan Ahead
If you are going to be using mortar, it is a good idea to have everything planned out ahead of time. Obviously, your forms should be in place, but other considerations should be met as well, such as having the stones placed where they can be reached easily, and a clear path along one or both sides of the wall for you move through as you work. If you have a specific pattern of stones to be laid, place them in the order you plan to use them.
Step 2: Using Wall Forms
For a vertical wall, stone mortar will hold the rock in place after it sets, but you need forms to maintain the shape while that is happening. Build the forms out of 3/4-inch plywood with 2x4 stakes spaced no more than 2 feet apart. To plumb the form walls, tamp the ground around the stake to move the wall in the correct direction. Splice seams with a 24-inch strip of plywood and 1 horizontal 2x4 brace for each 12 inches of wall height. Install a 2x4 stake in line with the joint.
Step 3: Mix the Mortar
Mix the stone wall mortar as directed on the bag. You can increase the mortar's ability to bond to the stone by adding 8 ounces of bonding agent for every bag of mortar mix. Do not add too much bonding agent, or the mortar will be brittle when it dries.
Step 4: The Mortar Base
Add 1 inch of mortar in a line along the bottom of the wall. If you are using bonding agent, allow the mortar to set for 5 to 10 minutes before placing the first stones.
Step 5: Laying Stones
Fit the stones into the wall one at a time, shifting each one back and forth to seat it into the mortar. This is where the form walls come in handy, as they allow you to press the stones down for a tighter fit into the mortar, which in turn spreads the mortar to fill spaces and gaps in the stones. Build the wall horizontally, adding a layer of mortar to the wall, and then placing the next row of stones.
Step 6: Remove the Forms
Allow the wall to set for 4 to 8 hours before removing the forms. To remove them, pull the stakes, and then carefully pry the plywood away from the wall. Use a mason's sponge to apply mortar to spots that need more fill. Wash the sponge out thoroughly and use it to clean any excess mortar from the stone faces.