Flagstone Patio on Crushed Stone

DEAR TIM: I intend to install a flagstone patio in the very near future and have been told I can place it on compacted crushed limestone that contains screenings. What are screenings and will this method work? The person giving the advice said the crushed limestone beneath the flagstones would eventually harden like concrete. What steps should I take to install the limestone in this fashion? I want to know if this will work as I don't want to have to tear everything out and start over. Tim G., Geneva, IL

DEAR TIM: Flagstone is one of the most popular patio materials I know of. The reasons are many. It is extremely durable, it looks magnificent and is naturally slip-resistant. It is also heavy so I can see why you only want to install each flagstone just one time.

Here are three piles of crushed stone. The one on the left is just larger pieces of crushed rock. The middle pile are the fines. The pile on the right is the crushed rock mixed with the fines. The larger coin on the left is a quarter and the smaller coin is a dime. PHOTO BY: Tim Carter

Screenings are another name for fines. These fines that range in size from dust-sized particles to small, angular rock chips no bigger than a pencil eraser. They are a by-product of the rock-crushing process and are a very important component for the base material under your proposed patio.

If you were to use just crushed limestone with no fines, the flagstone would have a good base, but not the best. The introduction of these fines is what allows the crushed limestone to mimic poured concrete.

Without the fines there would be considerable void space between each of the larger pieces of limestone. But when you add the fines into the mix, the void spaces disappear. The fines lock each piece of crushed limestone together and make it nearly impossible for them to move. This is what happens when the sand and cement harden in concrete. It is not much different than you being in a tightly packed elevator. In that situation you can barely move your arms much less walk around inside the elevator.

The first step for installing this crushed limestone is to ensure the soil is compacted just beneath the place where the patio will be. Use a mechanical vibrating plate compactor or an old-fashioned hand tamper for this job. You can then install a geotextile fabric or traditional asphalt felt paper over the soil before the crushed limestone is installed. The fabric or felt paper stops soil particles from migrating up into the crushed limestone during wet weather.

I would plan to install no less than 6 inches of limestone for the patio base under the flagstone. But the limestone is not installed all at once. It needs to be installed in two lifts of 3 inches each. Install 3 inches of the loose limestone with the fines and spread it out evenly. Use a mechanical vibrating plate compactor to compact the limestone. A hand tamper is not the proper tool to use for this part of the job. Run the compacting machine back and forth in parallel lines across the limestone as you might cut your grass. Then do the exact same thing but at 90 degrees to the initial direction you ran the machine.

After the first lift is compacted, it is time for the final lift. This layer is very critical as it must conform to the final shape and surface of the patio. If you want your patio to be perfectly level, then this final layer of crushed limestone must be level or nearly so when it is installed and compacted. Remember, the final surface of the patio is just going to be 1 inch higher or so than this layer of crushed limestone. The flagstones are going to sit on this layer of crushed limestone. Imagine how hard it is going to be to remove compacted crushed limestone if you discover that it is too high.

The biggest concern with installing flagstone in this manner is the material used to fill the gaps between the flagstones. You can use just pure fines for this aspect of the job so long as they are more than rock dust. It is imperative to have small rock chips in the fines.

Even after the fines are compacted between the edges of the flagstones, there can be problems once the patio is finished. In periods of heavy rain, it is possible for the fines to wash out from between the flagstones. This creates an ongoing maintenance issue.

You can mortar between the flagstones with a mixture of Portland cement and sand, but I would only do this a year after the patio was installed. This will give the limestone base plenty of time to compact with a little help from Mother Nature. If you do not want to wait, you can lightly sprinkle the limestone base with water for several hours after the final layer has been installed and compacted. Then come back on the following day and run the vibrating plate compactor over the limestone base one final time. Fill any low spots that may develop.

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