Q: We have a flagstone patio with the stones mortared in place. The mortar is crumbling, leaving the stones loose and making a couple of steps particularly dangerous. To repair this, do I need to remove the stones and start all over?
A: Stone patios are often built on a flexible base of crushed rock and sand to avoid the problem of deteriorating mortar. But with features such as steps, a rigid, mortared base is the only way to build. The problem is that the water in the mortar mix causes the concrete to shrink as it cures. Particularly in colder climates up north, the freeze/thaw cycles will cause the concrete to expand, which creates voids between the pavers and eventually leads to crumbling mortar. To fix this, first use a hammer and small chisel and chip away the old, loose mortar. Sweep the joints clean. Next, mix a batch of mortar consisting of three parts Portland cement and five parts sand. Use only enough water to make a semi-dry mix; it should not slosh around like regular cement. Trowel the mortar between the pavers and tamp down the pavers and mortar using a mallet and blocks of wood.