Replacing Mortar Joints in a Stone Patio

Q: DEAR TIM: I have a patio repair job at my house. The irregular pieces of limestone are fine, but the mortar joints between them are failing. In some places, the stones have lifted up. The patio is about 20 years old. Is repairing the mortar something I can handle? What is a good way to ensure the repair lasts? Is it possible the stones are set on sand? -- Albert J., San Antonio

A: DEAR ALBERT: You have a great patio there. I would not consider taking up the stone. From your photos, I can see that a few days of repairs will have that space looking like new.

It's possible the stones were laid on a sand base. Because you rarely have extended periods of freezing weather in your area, I see no issues with a sand base. What would concern me more is movement beneath the sand caused by expansive clay soils. Parts of Texas have extensive amounts of clay soil that shrink and swell as the moisture content in the clay goes up and down. This movement is enough to crack foundations, so it can easily move your thin patio stones up and down.

I recommend that you chisel out all the old mortar between the stones. You can do this by hand, or you can rent a demolition hammer from a tool-rental business. These electric tools are miniature jackhammers that will quickly pulverize the mortar. Be careful not to crack the limestone.

As you're removing the mortar, save it in a pile. This will help you estimate how much new sand and cement you need to buy for the patio repair. You want to buy plenty of sand from the same batch and plenty of Portland cement from the same manufacturer. This will ensure that the color of the new mortar joints is the same. If you use different batches of sand or buy different types of cement, the mortar color might vary. Be sure to buy coarse sand for this job.

After all the old mortar is removed from the cracks, you need to adjust the height of the stones so they are reasonably flat. If this means lifting them up and taking sand out from underneath some stones or adding sand under others, do so. You can use the old mortar you removed as a base under stones that are low.

To prevent future movement caused by the soil, try this. Get some scrap 1/2 -inch diameter hard-copper tubing from a plumber. Cut it into short lengths that are 1/2 inch longer than the thickness of the patio stones. Tap these little pipes somewhat randomly in the spaces between the stones. Try to have at least one pipe section for every 6 square feet of patio area. The top of the pipe should be flush with or slightly lower than the top of the stones.

Carefully install the new mortar. Mix it using three measures of clean sand, 1 1/2 measures of Portland cement, and enough water to make the mixture similar to bricklaying mortar. This means that the mortar will be stiff enough so it can stand up about 1 1/2 inches on a trowel. Try to work on overcast days or when the patio is in the shade. Cool, damp weather is the best. Sunny, hot and windy days are the worst, as the new mortar will dry too rapidly.

The joints between the stones must be dust-free before you add the new mortar. Use a spray bottle filled with clean water to mist the sides of the stone and the sand base just before you add the mortar. This small trick will help the mortar make a good bond between the stones.

Only mix as much mortar as you can install in one hour. Never add additional water to mortar that starts to harden. Be careful not to put mortar inside the open copper pipes that are sticking up. Once the patio-repair job is complete, these pipes will allow rainwater to get under the patio to keep the soil moist. If the weather gets bone-dry, use your hose or sprinkler to get water into the little pipes so the soil stays moist all year long. If you do this, the soil will not shrink and swell, and your patio repair should last for a very long time.

Tim Carter can be contacted via his Web site, http://www.askthebuilder.com/printer_Submit_Question.shtml.

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