One of the biggest areas of failure in masonry wall construction is the cap stone or lack of one. This article will deal with the proper installation of a cap stone and leave inferior design (lack of one) for another day!
For centuries the cap stone was the proper termination method for sound masonry construction. With the modern technology available today you can be assured a leak proof termination to your new masonry walls with a cap stone system that protects the masonry wall from unwanted water infiltration.
What are cap stones? – Simply, cap stone are cut stones that 'cap' or terminate the top of a masonry wall such as garden walls, parapet walls, firewalls, etc.
After the masonry has reached it final height the collar joint (the joint between the inner and outer masonry walls or wythes) is carefully filled, smoothed and brushed. Then a sheet of copper flashing is cut to the desired length and width of the wall.
Some contractors lie this directly onto the wall and lay their cap stones on top of it. Others may use a bituminous coating to cover the top of the entire wall prior to laying the copper flashing in place. Butt joints in the flashing are covered with a bituminous coating approx. 12 "- 16" from the joint and a top piece of flashing is lapped 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 holding up to the elements. In this situation any leaking that occurs through the cap stone or joints will drop to the copper flashing and flow out from under the stone and down the wall face, keeping it from the interior of the wall.
You can also crimp the flashing with drip edges and locking seams for the joints if you so desire. Each increase in details adds increased cost to the project but also increases the durability of the wall.
Setting the stones – Now that the flashing is in place and secure you begin to set the cap stones by placing a full bed of mortar on top of the flashing where you will place your first stone.
Smooth and level the mortar bed as needed and set your stone into place. Then level you stone with a mason's level cut your joints beneath the stone making sure all voids are filled and then continue the same process for each additional stone. As you proceed with each additional stone, leave the head joints (the joints between each individual stone which is 3/8 "- ½") open.
Filling the head joints – Now that the stones are in place it's time to come back and tuck-point (fill in with mortar) all the head joints. Some stone will stain very easily from the cements used in mortar so test your stone. If needed, tape the stone on either side of the joint until you have completely filled (tuck-pointed) and 'tooled' that joint, at which time you remove the tape and brush the joint clean.
Your tuck-pointing mortar will be the same mortar that you used to 'bed' the stones with but will be 'stiffer'. This will allow you to compress the mortar into the joint with a tuck-pointing tool assuring a tight and full joint. When you 'tool it' (finish the joint with a concave joint tool) you will compress the joint even further and as you 'slick it' you draw the cements to the surface of the joint giving you a water proof seal between each stone.
Following these few tips and suggestions you can have cap stones that will provide a lifetime of protection from water entry for your new masonry wall.