One of the biggest areas of failure in masonry wall construction is the capstone or lack thereof. This article will cover the proper installation of a capstone and will leave the bottom design (missing one) for another day!

For centuries, capstone was the proper finishing method for solid masonry construction. With the modern technology available today, you can be sure that your new masonry walls will have a leak-proof finish with a stone cap system that protects the masonry wall from unwanted water infiltration.

What are capstones? – Simply capstone are cut stones that ‘cap’ or finish the top of a masonry wall such as garden walls, parapet walls, fire breaks etc.

Once the masonry has reached its final height, the collar joint (the joint between the interior and exterior masonry walls) is carefully filled, smoothed and planed. A sheet of copper flashing is then cut to the desired length and width of the wall.

Some contractors attach this directly to the wall and set their capping stones on top. Others may use a bituminous layer to cover the top of the entire wall before putting the copper flashing in place. Butt joints in the flashing are covered with a bituminous layer of approx. 12″ – 16″ from the joint and overlap a top piece of flashing over the joint. Either way is acceptable, but the bituminous coating offers an additional layer of protection.

Why use copper flashing? – Copper is used because it has a strong history of resisting the elements. In this situation, any leakage that occurs through the cap stone or joints will fall onto the copper flashing and flow from under the stone down the face of the wall, keeping it away from the interior of the wall.

You can also crimp the flashing with drip edges and lock seams for joints if desired. Each increase in detail adds more cost to the project, but also increases the durability of the wall.

laying the stones – Now that the flashing is in place and secure, begin setting the capstones by placing a full layer of mortar on top of the flashing where you will set the first stone.

Smooth and level the mortar bed as needed and set the stone in place. Next, level your stone with a mason’s level, cut joints under the stone making sure all gaps are filled, then continue the same process for each additional stone. As you go with each additional stone, leave the head joints open (the joints between each individual stone being 3/8″ – ½”).

Filling of head joints – Now that the stones are in place, it’s time to go back and aim (fill with mortar) all the head joints. Some stones will stain very easily from the cements used in the mortar, so test your stone. If necessary, tape the stone to each side of the joint until you have completely filled (topped) and “go over” that joint, at which point remove the tape and brush the joint clean.

Your mortar for pointing inwards will be the same mortar you used to ‘sleep’ the stones with, but it will be ‘stiffer’. This will allow you to compress the mortar into the joint with an inward pointing tool ensuring a tight and complete joint. When you ‘use’ it (finish the joint with a coving tool), it will compress the joint further and as you ‘slide’ it will draw the cements to the surface of the joint, giving you a proof seal. of water between each stone.

By following these few tips and suggestions, you can have capstones that will provide lifetime protection against water ingress for your new masonry wall.