Q: Each time it rains, a puddle develops on the outer edge of my circular patio. The patio is made of older concrete and it rests on a low retaining wall. The puddle appears to be related to a crack in the patio. Can I fill in the patio where the puddle is? I am worried about the appearance of the patch. I only want to fix this one time. What are my options?

— Amy P., Warrenton, Va.

A: I see puddles frequently, even at my own home where there are depressions in my asphalt driveway and where my front sidewalk meets my drive. (I didn’t build the house.)

The patio repair is not that hard to do, but you threw in a wild card with respect to what the repair will look like. You want your patio to look nice and not like a hodgepodge of different colors and textures. Judging from the excellent photos you sent, I can tell you that it would be next to impossible to install a patch that blended perfectly.

There's another issue in play. You mentioned there's a crack in the patio and the low curved retaining wall. And you also want to make the repair once.

When you add all these pieces of the puzzle together, you only come up with one repair option in my book. You need to do a thin — perhaps just two inches — concrete overlay.

Concrete overlays can come in a variety of flavors. You can do one as thin as a coat of stucco. This means the overlay is really just fine sand and cement. It’s possible to get the overlay as thin as an eighth an inch, but a fourth an inch would be an easier project to manage for the DIYer inside you.

If you hired me to write the simple set of specifications for your job so you could get accurate bids and be sure the job would be done right, here’s what I’d go with:

I’d want the concrete overlay to be two inches thick where it starts at the outer edge of the concrete patio where it passes over the low retaining wall. Before I’d even start to do the work, I’d cut a scrap piece of wood two inches thick and place it on the outer edge of the patio. Then I’d rest a straight edge on the small piece of wood.

I’d want to check to see how thick the overlay would be when it got back to the house, making sure the patio had a consistent slope of an eighth an inch per foot. This way rainwater would drain off the patio with ease. You can get by with a sixteenth-inch per foot, but it takes an expert concrete finisher to maintain this slope and not create a new puddle in the middle of the overlay.

The concrete would have small pencil reinforcing rods in it. These rods are three-eighths an inch in diameter. I’d prime and paint them with a rust-resistant paint first, as I don’t want the rods to start to deteriorate in case you put de-icing salt on the patio in the winter months. These rods must have at least a half-inch of concrete under them when the overlay is poured.

I'd make sure the spacing of the rods was two feet on center in both directions. Imagine a giant piece of graph paper with the steel rods as the lines. This steel ensures the overlay will stay together as one piece, so part of the slab won’t settle lower than an adjacent piece. That is the problem now; it is dropping over the retaining wall.

The size of the stones in this concrete overlay is critical. I’d not want any stone to be larger than three-eighths an inch in diameter. Some gravel pits sell pea gravel, round stones about the size of small grapes. You can also use small crushed gravel that is a similar size.

I’d specify a seven-bag mix, which makes the concrete stronger than the minimum 4,000 pounds per square inch strength recommended for outdoor slabs in a cold climate. My specifications would also state pressure-washing the existing concrete patio to get a great bond between the old and new concrete.

I'd list as an option the application of a thin layer of cement paint to the old, damp concrete before the pour. The steel rods make this problematic. If there are enough helpers present, they can brush the cement paint on the damp concrete just before it's covered with the new overlay concrete. You make cement paint mixing pure Portland cement with water until it's the consistency of normal paint.

Keep in mind you can add color to the concrete. You can buy dry-shake pigments and make your new patio look like any surface.

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