This DIY article; and the rest of the how-to section of my blog, cover much of the basics of how-to properly build flagstone patios. These articles offer general guidelines, or suggestions at least, that will be of much use to the hobbyist, DIY landscaper/builder, and to professional builders as well. If you need help with site-specific issues for your hardscape project, or if you need guidance with any related type of situation or project, then have no fear, for I also make myself available for…..
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Flagstone: what to use, sand, cement, or gravel?
So getting right to it then, what type of foundation are we going to build for our flagstone patio: sand, cement, or gravel?
Short answer: well it depends. Quarry screenings (if available in your area) are generally best for under the flagstones. Screenings are also one of the best options for in between the stones, but there are other options available which can be utilized to achieve different aesthetic effects. First we will address the structural issue “what to use beneath the flagstone”.
Cement—it will (probably) crack, at some point. May last for quite awhile, but when it does fail it will be much more work to repair it then to repair dry flagstone work.
Sand—ants will dig it up and get it all over the place…also the sand may wash away, causing stones to settle.
Gravel—really no problems here, just use the right type of gravel. Better yet, use modified gravel for the foundation and then use stone dust aka quarry screenings aka grit aka quarry dust as the final leveling agent.
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Okay, let’s get more specific then.
Why is gravel the best choice for your your flagstone patio’s foundation?
Cement will (probably) crack. Especially grade level cement. Especially in a climate with winters like ours in Pennsylvania. Worse way to go about it would be to lay the flagstone upon a bed of gravel and then cement the joints between stones. Horrible idea. The gravel base is flexible and will move ever so slightly during freeze-thaw. Well, if the base was done poorly the movement might be more then slight, but let’s assume the base was done well. A gravel base will definitely move a bit—looking at any of my patios, you’d never know that, but the movement does happen. Cement is rigid—if you put a rigid top on a flexible base then systemic cracking is inevitable.
Cement is of course a fine joint filling material if the flagstone happens to rest upon a concrete foundation. But why on earth would you want to have a concrete foundation anyway? The concrete itself will crack, eventually. In a northern climate it will likely crack within ten years—and the chances of it cracking within the next three years are pretty high too. Environmental impact of concrete production is no small issue either. Anyway, I personally prefer dry stone work. More harmonious, warmer, just better. The feeling you get from a well done dry laid flagstone patio, IMO, is just better than a flagstone patio set in cement. My opinion.
A flagstone patio set in cement can of course look good and last a long time. I’ve built many that look great,–many years later too. But if there is cement between the joints, there just had better be a concrete foundation. I mean it.
Sand….well, if you use a really heavy sand you might get away with it. Most of the sands that are packaged for sale however are way too fine. You can use a course sand for under the flagstones, sure. Back when I used to lay brick patios I would use course sand or quarry screenings inter-changeably and it was fine. Them patios still look good. Those are brick patios however, where the space between the paving units is about a quarter inch wide. The problem with sand is that it washes away by action of water, blows away by action of wind, and gets carried away by action of ants. That’s why stone dust AKA screenings AKA decomposed granite works much better, for underneath flagstones, than sand.
Not as good as my flagstone patios though! Problem with using even course sand beneath flagstone is this: Bricks are of uniform thickness. Thus it’s not too much trouble to just get your gravel foundation pretty close to perfect, then go ahead and screed out one inch of sand for your bricks to rest upon. With flagstone however, the thickness will vary too greatly—you may need half an inch of sand for one stone, but 2 inches for another. If you are using sand, then that variation in thickness will cause problems. Screenings are almost the same thing as modified gravel—they are indeed one of the two ingredients in modified gravel…they are heavy enough that it really is no problem to use 2 inches on one stone and half an inch on another—ten years down the road, that patio will still look sharp.
flagstone patios that are set in sand are vulnerable to ants and wash-out
Occasionally I’ve seen a paver patio messed up by ants. But with flagstone patios that are set in sand ants always attack. I suppose it’s because the joints are inevitably wider with flagstone and/or because the flagstones are varying in thickness meaning you end up with deeper sand in some places. Whatever the exact reason, I can tell you that all of the flagstone patios that I have seen that are set in sand eventually get run amok by ants.
Another reason to use screenings is because screenings also make an excellent joint filler. You do not want to use sand, even course sand between your flagstone joints because it can wash away—unless of course your flagstones are absurdly tight. For pattern-cut flagstone, yes, you can maybe get away with using sand as the joint-filler. Just make sure the base is course sand, not fine. You will need to use fine sand for the joints however because of how tight they are. Again, ants love fine sand—but in this application, pattern-cut stones, tiny joints—fine sand will not be the end of the world—so long as the base is course, of course. That’s for pattern-cut flagstone–or any flagstone where the joints are super tight–in those cases you can maybe get away with sand, so long as you follow the guidelines that I’ve set earlier in this paragraph. For irregular flagstone, or any flagstone with a joint wider than a quarter of an inch, you really really should try and avoid sand, and instead use stone dust.
Can you set flagstones down on your own native soil?
Your own native soil –This one would be great, if your own native subsoil consists of about 20-40 percent clay and the rest mostly sand and gravel. And hadn’t been disturbed in ten years. Then you’d have a good solid base already 🙂 You could most certainly take clay from your subsoil, figure out how much sand and gravel it already contains, and then calculate how much gravel ought to be added, and then fetch some gravel from elsewhere nearby.
What I am talking about here is using the onsite materials, to attempt to mimic the performance characteristics of road base and/or to create a gravel cob-like mixture of soils, well draining, compactable and stable. This type of work is still in the r&d phase for me. More on this, as the research progresses. Suffice it to say that yes, it can be done, but it’s a bit complicated and beyond the scope of this present article.
Okay, so let’s stick with gravel for the foundation, and screenings AKA stone dust as the leveling agent
Back to screenings—when you use screenings for both the leveler and the joint filler for in-between your flagstones you are creating a good scene. If there is ever any minor issue with the screenings underneath the stone it shouldn’t matter too much because the joint filler will settle down and fill the void beneath the flagstone. Having screenings up top and down below, it just works out well.
You can expect to top off the screenings once within the first year—a small bit will settle or wash away. No problem, just sweep in some new material and you’re good. After that, in future years, you’ll be fine. My best recommendation is that clients hire me to do maybe a couple hours maintenance once a year—by no means is this necessary, but I like my work to sparkle.
And it does. Check out what my past clients have to say about my work.
One thing I have not gotten into in this article is polymeric sand. I point you now to another hardscape how-to blog post, if you are curious about poly-sand. If you are poly-curious, that is.
I should maybe also add that using the above system I have never once had a single flagstone patio fail. Okay, maybe one stone with some minor settling—fixable in a few minutes (and rare that that ever happens) but never any major problems. Been doing this a little while too. On my largest flagstone patios I generally would recommend 3 hours of maintenance work should be done, every few years or so. This will keep the patio in optimal shape. I’m picky and want my work to look perfect forever. Often I’ll return to a client’s house years and years later and it will still be in perfect shape. Without any maintenance needed! Usually, within 5 or 10 years, a patio is due for some bit of attention.
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