Q:
The stones on my flagstone patio are fastened securely, and the concrete base seems to be in good condition. However, the mortar between the stones has chipped out due to harsh winters. Do I need to rip out the stones and start from scratch, or can the joints be fixed somehow?
Ed Gregg, Westminster, MD
A:
Richard T. Kreh, a mason in Frederick, Maryland, replies: The joints in flagstone walks or patios definitely can be refilled, or repointed, with mortar, provided that the concrete base is in good condition and the stones are not loose. To make sure all the stones are well adhered, tap the surface of each one with the wooden handle of a hammer. A loose stone will sound hollow when tapped. If you find any, remove them, clean out the old mortar under and around them, and rebed them in fresh mortar.
Next, carefully chisel out the old mortar joints to a 1-in. depth. The best chisel to use is a plugging, or joint, chisel. Available at any mason’s supply, it has a flat, tapered cutting edge that allows you to cut out the mortar joints without chipping the edges of the stones. Brush or vacuum all the loosened mortar out of the joints.
The most important part of repointing flagstone is using the correct mortar mix. A common misconception is that the harder or richer the mortar is, the better it will hold up in the joints. In fact, the exact opposite is true. For repointing, you need a softer mortar that allows the joint to give or stretch a little bit without cracking or shrinking when it is pointed against a harder material, such as flagstone.
The repointing mix I recommend is 1 part type-1 portland cement, which is the most common, 2 parts mason’s hydrated finish lime type SA or S, and 8 parts washed masonry sand. Mix up only a small amount at a time because repointing is slow work. I find that a large coffee can works great as a measuring cup. Add enough water to blend the ingredients together and mix them a little on the stiff side. You’ll get the hang of what works best for you after a while. The mortar can be repointed with a flatblade slicker tool and a small pointing trowel.
Before you repoint any mortar in the cutout joints, dampen them by applying a little water with an old paintbrush. It helps to keep the mortar from drying out too quickly and gives it a chance to cure properly. After the mortar has cured for a week, the stones should be washed down with a regular masonry-cleaning agent, which also is available at a masonry-supply store.
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Good job