How to build a flagstone patio
First of all I do not really like one-size-fits-all answers. The one size fits all mentality is one that tends to produce mediocrity—and the world has enough of that already, thank you very much. With that said, please do consider the following as general guidelines. Soil conditions, stone type, availability of materials and climate conditions in your area such as freeze-thaw—-these are all variables that may impact how you build your flagstone patio.
DIY gravel foundation for flagstone patio
Having said that, I’ve been doing this for some years now, my system for dry laid flagstone works well and I will share it with you presently. Dig down about 8 or 9 inches. Compact the sub-grade. Lay down approximately 6 inches of 2a modified gravel, AKA road base–and compact that.
Make sure this gravel base is not level, but is pitched, so water can flow. Direct the water away from the house. I usually go with a pitch of about 2.5″ of fall, for every ten feet of run. Conveniently, that equals 1 inch of pitch (or “fall”, if you prefer) over 4 feet of run. Convenient, because I keep a 4′ level by my side when leveling flagstone.
More information about how to build flagstone patio foundations: gravel, sand, cement or what?
How to lay the flagstone in stone dust
Lay out the flagstone on your compacted gravel base. Use stone dust as a leveling agent, lifting up each stone individually and raising it to the correct height with the stone dust. Do this for every stone. One they are all brought up to the correct height and with the correct pitch then you can fill the gaps between stones with stone dust.
As a general rule: do not set flagstones in sand. Especially not a fine sand, such as play sand nor bar sand. A thick, chunky sand comprised of jagged bits should work fine though. You want a good portion of the sand to be comprised of pieces about 1/8th of an inch in size. Ants love fine sand. They will build their anthills all over the place if you use too fine of a material. You want a heavier, denser stuff that will not wash away nor attract ants. I live in Pennsylvania—screenings, AKA quarry dust is widely available from masonry suppliers, landscape suppliers and quarries–usually going by the name “screenings”. But I have completed projects like this across the country, and across the country…
Stone dust may also be sold as:
- decomposed granite (stone dust made from crushed granite, usually costs more than other types)
- 1/4″ minus (meaning crushed stone that contains particles as large as 1/4″, all the way down to flour-like fines)
- 3/8″ minus ( interchangeable with 1/4″ minus…)
- pathway fines
- quarry dust
- quarry fines
- screenings (so called, because these are the particles that fall through the smallest screen that the quarry uses, when they crush stone into gravel. For 1″ gravel, they use a 1″ screen…and so for all sizes of gravel, all the way down to fines)
- you may need to call around and ask suppliers, describing the product you need, but pretty much any quarry creating gravel will also produce pathway fines
More information: polymeric sand versus stone dust for in-between your flagstones?
Hopefully this answers some very basic questions. If you still need more help, please know that I also offer DIY flagstone phone consultations.
DIY Flagstone and Hardscape help
My rates for DIY consultations are as follows:
$80.00 for a half hour consultation
$115.00 for a full hour
Monies are payable via paypal.
devin@devineescapes.com
Thank you
Hi Devin,
Great website, and thank you so much for all the useful information!
What is the greatest depth of screenings you recommend? Would three inches of screenings on top of the base result in a very stable patio?
Thanks!
Usually I do not go deeper then about 2″ with screenings. If a stone needs to be raised a full 3 inches, I’ll put down some gravel, tamp that, then use like an inch or so of screenings.
If you do not have gravel, but still need to raise a flagstone 3 inches, then I’d lay a couple inches, lightly dampen the material if it’s very dry, then tamp it, then use an inch or so of loose screenings to bring it all the way up to the right level. Honestly, I’d do that for 2″ or 2.5″, but for a full 3″ raise, then I’d find some gravel.
And you’re welcome, I hope this helps! More info here
Hi Devin
I just found your website and I admire your work. And would have liked to consider working with you however I am in San Francisco.
I have a 400 sq ft flagstone project at hand .and I’m torn between going concrete base vs. gravel bed install.. contractors are recommending concrete w rebar, as it allows working with thinner easier to cut quartzite flag. My yard is sloping with highly sandy soil..and the current slab in cracked in numerous place, hence my worry which you write about. And then earthquakes…
I need to go with a quartzite stone as my yard is mold prone due to lack of direct sunlight and damp, salt fog typical of my area. My design also has curved sides – making the install complicated using harder to cut quartzite. Plus my design calls for larger flags mixed with smaller..
I would appreciate any quick sugggestions you may re. This dry vs. wet install approach – and how best to deal with using quartzite. Easier to cut Bluestone is too porous for my yard and mold situation. and I don’t want to deal with controlling using sealers.
Sorry, I just noticed that I never responded to this. By now I hope you have an awesome patio–if you still have questions or you still need to find a contractor let me know. I would definitely, go with dry laid, for your quartzite patio, myself.
Love your site. I just found it. Great work. I was on a crew that did dry flagstone in screenings back in my 20s.
I just got about 200sqft if flagstone for a project at my house. It’s been wonderful revisiting the craft. However, I screwed up my math. I have my pitch munch much more shallow than 1”/4’. I’m at about 1/2” over 8’. My base is packed and based on the surrounds, it would be very difficult to redo it. I hope I’m not in too bad of shape.
I guess I’m missing something, or I don’t know what you mean by “based on the surrounds it would be difficult to redo”. Surrounding objects, boulders, a porch, or some objects are making it difficult? If there were no obstacles, it would take me 10 minutes of raking to get the correct pitch, then 20 minutes to hand tamp it, or less if there’s a vibratory plate compactor available.
1/2″ fall over 8′ is basically zero. Most flagstone has irregularities to it’s flat surface….as such, you need to pitch it more than just a tiny bit, if you want surface water to flow, rather than puddling or possibly even flowing in the wrong direction.
But there’s too much info you’re not giving me–is there a door, or other bench mark on the high side that you need to meet? I use stone dust as my leveling agent–I’ll get the gravel base pretty much right on, then I’ll lay out my flagstones and then I’ll use stone dust to level them out. perhaps you can just use more stone dust, up to inch and half or even 2 inches on the high side, while using almost no stone dust on the lower side–and this way get it a bit closer to 1″ over 4′.
Sorry, that’s about the best I can offer with what I got. If you want to do a phone consultation and you can send me photos and measurements, and payment, then I can do some design work with you over phone or email and find a solution. Other than that, the best answer I have is this: you’re not in great shape, currently. Do the difficult work and get more pitch.
Hello Devin. This website is great. I’ve been combing through the different pages, blogs and replies. I’ve found a lot of great information that has answered a lot of my questions. I am curious about two things that you may be able to help me out with? The first is in regards to the DG screenings. I see in the blogs you recommend no more than 2″ deep. My plan is to color concrete and make my flagstone from molds (don’t hate me). My “stones” will all be a uniform thickness this way. Because of this would I not need as much DG? maybe only 1″ deep on top of my base layer?
My second questions is in regards to edging. My plan is to essentially have 2 patios. 1 upper and 1 lower. This way I don’t need to grade my back yard as much. Doing it this way I envision the smaller upper patio with a step, maybe two steps, down to the main patio. At this point I am planning on using some patio blocks for the steps. This step or two down would mean I would have a small wall separating the two patios. Again at this point, I’d likely use patio blocks. You have a post on one of your blogs (the DIY flagstone help blog) that shows one of your students patios that he made. It has the flagstone but ends with a patio block wall at the edges. So to finally get to my question. Does anything different need to be done when you get to the edge where the wall would be? Thanks for the help.
I copied the picture from the blog and pasted it below. Not sure if it will work though.
1. If your flagstones are uniform thickness, then you may screed out a 3/4″ or 1″ thick bed of stone dust. To do this:
1. get your gravel foundation nice and flat, and pitched in the right direction.
2.. Set out some 1″ pipes. Steel is good, pvc works (but may be floppy and less forgiving. Set the pipes maybe 3′ apart from each other.
3. Check the pipes with a level, that they are level with each other, and pitched the same. 4. Rake out some stone dust in between the pipes, and then use a 2×4 or level to draw back the stone dust to being even with the pipes.
5. Remove pipes, replace the bit of stone dust from where the pipes were, smoothing out with a square trowel.
This method will work well, if your molded pavers/faux stones are in shapes that easily fit together without to much puzzling.
Your photo did not work but that’s okay, I remember the student-built patio which you are referencing. Pinkish or red flagstone, if I remember correctly.
Block walls use the same sort of foundation as I use for flagstone work. I’d do the foundation for the lower patio and the block wall at the same time and set them to the same height. I’d use a 6″ thick block…the lower patio stones, or “stones” will conceal the lower couple of inches of the block–this is good, as you want your blocks partially buried, so they do not become undermines by movements of soil and water.
Cap the block wall either with the same “stone” or paver, or else with wall block caps of about 2″ thick–and then you’ll end up maintaining a good 6″ step, which is a good height for landscape steps.
If you think you’ll need more site-specific guidance, feel free to email me some photos and we can set up a consult. Otherwise, I’m glad you’ve found this website helpful and wish you good luck and much enjoyment with your patio project 🙂
Great Devin. Thanks for the reply. My last question or maybe its just confirming if I understand what I’ve read on your site. When I apply the DG on top of the base I do not compact that correct? Looks like you place the stones and then wet things down with a shower to set everything in place?
After I have put in the base, compacted it and pitched it in the correct direction: then, I set out all of the flagstones, cutting/trimming the stones as needed. After that, once all of the stones are puzzled together–then I level the stones up, using the DG. The DG does not get compacted.
I lift a couple stones out of position, then place however much DG I think is needed to get it to level, the put the stone back into position, setting the stone into place with a couple smacks from the mallets. After all of the stones have been leveled up, then I sweep stone dust/dg into the joints. The hose, on a shower setting, is used to set the dg into place.
I do have a recent article HERE, that describes the stone-leveling process in a bit more detail.
Thank you for providing this resource! All of it has been extremely helpful as I prepare to lay a 170 sq foot bluestone patio in my SE PA backyard. One question: I love the look of moss or creeping thyme or succulents climbing out of the spaced between pavers. Is it possible to encourage this while maintaining the stability of the patio? Ie could I use some mixture of soil and screening in the cracks and plant thyme seeds in that?
A joint that has moss growing in it will not wash away. But you need enough shade for moss….I do not have much experience with installing thyme into flagstone joints, but from what I’ve seen, it like-wise helps to stabilize the joints.
To answer your question: yes, this is the method that my customers have used. I’ve seen a 1:1 soil and screenings mix work well (or 2:1, even 3:1 in favor of screenings. Or straight screenings, if you’re going to plant moss . So it may depend on what sort of plant you’re using, but in my experience you can use less soil into the mix than what you might guess)
Hi Devin – fantastic content and thank you for what you do. I am working on a 20 foot diameter flagstone patio. I want it to be approx. 6″ above the prevailing grade as the yard slopes. I am building a round boulder back drop and smaller boulder perimeter around the front so I’m trying to determine how deep to dig down. There is built in pitch to the yard so I’m wondering if I go down an even 6-8 inches or if I can go deeper on high side of the slope and lower on the front to avoid heavy excavation. Thanks for your advice.
Chris,
You should be able to go deeper on the high side and less on the lower side….so long as you’re at least removing the top 3 inches or so of grass/roots/organic top soil, and so long as you are able to retaining the excavated uphill side of the yard, with your round builder back drop. Best of luck–and spend some time in the how-to section of this blog, as there are many more articles that will be helpful in building your patio.
Devin – greatly appreciate it. I’m going to use 16-24″ Delaware round boulders for a 2 layer backdrop and perimeter. Question – Would you use 3/4 clean stone, QP or just the earth (clay/rock) to batter and back up the short wall? Perhaps a combination of the dirt and clean-stone? Any opinion on this would be great! Thanks again!
I would backfill with earth, clay/rock. If you backfill with clean gravel, then I’m concerned about gravel migrating into the center of the wall. Gravel sized bits of stone add nothing the structural strength of a wall, and can potentially act to destabilize larger stones within the wall. Like in-filling your wall with ball bearings. Also, if you backfill with clean gravel…then you’re giving water a place to sit, but where’s it going to go?
So I’d just build your wall sturdy, and backfill with soil. If you did use gravel behind your wall, then I’d want you to protect the wall from the gravel, and protect the clean-ness of your clean gravel–with filter fabric. And I’d want you to pipe away that water, using 4″ perforated pvc. But this only makes sense in situations where there’s expected to be a lot of storm water, a lot of ground water, coming down that hill….and you mentioned your yard being a slope.But is it a hill, going way up, and bringing down tons of water? Pipes/fabric/gravel is something I rarely, rarely have been convinced to do, as it is rarely necessary—at least for a well built dry stone wall.
Here’s an article about how I build dry stone walls.
Devin – you are a highly knowledgeable “saint”. If i could just find 2 workers to help me I’d get this done! The hill is very stable. I’m thinking if I just swale it behind the wall and run any runoff out that way,I’d be fine since it’s a heavy clay sediment. Also, it is 20 foot diameter so I’m sloping approximately 2 1/2″ from hill side to low side. That works correct?
Also, If i’m using the 6″ of roadbase for the full circumference, should I start my first course of boulders in the roadbase or just on the solid virgin clay/dirt? Was also toying with using smaller boulders or maybe belgium block as a circumferential edging around the full perimeter as I’ll be starting high at the apex of the circle with the boulders and tapering them down into the front. Any thoughts on this would be invaluable. Your work and creativity is remarkable. Thank you again!
Sorry Devin – I’m reading the post on how you build your walls. Apologize for asking when you’ve already answered this.
Can you re-post this comment, here instead:
https://www.devineescapes.com/how-to-build-a-dry-stone-wall/
I’ll respond when I get a minute–and I’ll delete the now-redundant original comment.
One question I’m pondering. If I am using round/oval delaware boulders, should I double wall the base? My plan was to go approx. 2 courses high or approx. 20-24″. It might make sense to go with a double boulder base and stack the second tier on th 2 below. Your opinion would be great. I’ve not seen much on building with round boulders but I like the softness of how they look. Thanks again.
This one, too if you don’t mind.
Hi Devin – I moved my wall questions to the other post.
Appreciated. Will read and respond tomorrow or probably tonight.
What can we use to repair the filling between the flagstone? The patio was built about 15 +years ago, with flagstone that is soft and flaking. Spaces between the flags need to be redone with something and the whole patio needs to be sealed with something. Suggestions please?
Sealing stone that’s already deteriorating the way you describe won’t save it, won’t preserve it, and may even hasten the deterioration.
Joints should be filled with mortar if the patio is on a concrete foundation.
Or with stone dust, if the patio is on gravel.
Hi Devin,
Your articles are very informational and helpful. I am wondering if this product would work as a filler between the pavers. We first laid down gravel and then laid down a layer of 3/4” minus crushed limestone is what our quarry calls it in Iowa. I have tried to call around and ask for stone dust but not getting anywhere. I’m wondering if this product would also work as filler? The quarry also had 3/4” crushed limestone. Any suggestions?
You say “pavers”. So, this is a concrete paver, or faux flagstone sort of material? I did pavers a long time ago, remember how they were installed, and have watched slight changes in the industry norms and products available, over the years–but I’ve been doing only natural stone since 07. You say you laid down gravel and then 3/4″ minus…..well, 3/4″ minus is exactly what I recommend for flagstone and paver foundations. But what do you mean by gravel? Is there clean stone (gravel without the fines) beneath the 3/4″ minus that you laid on top?
My procedure is to excavate, tamp the sub-soil, lay down 3/4″ minus gravel, and tamp that. If you laid clean gravel beneath the 3/4″ minus, then hopefully you laid some sort of separatory fabric material between the two, lest the fines from the 3/4″ minus settle into the voids within the other gravel? AANd hopefully, it’s a non-woven felt-like filter-fabric LIKE THIS
And finally, the 3/4″ minus crushed limestone. So, where are you located? I’m wondering how much rain you get, and how cold your winters are, because we heard some things about using crushed limestone as hardscape foundations. It can hold a lot of moisture, and then expand, especially in places with cold winters. Now, I’ve been okay with people using 1/4″ minus crushed lime stone as joint material….for example, when that was all that was available, or when that off-white/cream/tan color was being saught after. But that’s just the leveling material and joint material, so maybe an inch thick…..not a full 6″ hardscape foundation. In the case of a 6″ foundation of crushed limestone, in a northern climate–I’d have concern.
To your question then: I recommend using a quarter inch minus type crushed stone, or similar size, 3/8″ minus or 1/8″ minus, or DG (decomposed granite) or “pahway fines”–name vary place to place, but if they’re making crushed stone, then they should be making crushed stone in a quarter inch minush or comparable size. But I wouldn’t use limestone, in this case–especially in combination with the limestone you already have down.
Hey Devin! Thanks again for the phone consultation on my San Francisco, CA back patio. I picked out a nice pile of “Blue Ridge Flagstone” (looks like PA bluestone to me??), from American Stone & Soil in Richmond as you recommended. They had probably a dozen pallets of it, selling for $650 a ton, and was also the least expensive flagstone on their lot – they were getting $1k/ton of most of their stone. They’re pretty nice looking IMO, just about 1.5″ thick and pretty large and uniform with some rust staining, which I happen to think looks nice.
So that’s my west coast intel for you, as requested! We just got the stone delivered today, after the huge rain we got last week I was glad that the dirt patch of a back yard didn’t end up down the hillside. Do you reckon I should let it dry out more before starting on the base? It’s quite a bit softer (and easier to level out, I must say) having got some water.
I’m looking forward to sweating this weekend, and we’ll let you know how it’s coming! Can’t recommend an hour long chat with Devin enough, he’s cheaper than a psychotherapist and made me feel way less anxious about the project.
Thanks for the intel re: Bay area stone prices and availability 🙂
Yes, let it dry a bit–you don’t ever want to compact wet clay–if it’s too wet it will get spongy/kinda like a moon bounce, or stepping on a water bed. If that happens…it can be a pain, causing you to have to dig deeper, and have deeper gravel foundation.
Dig when it’s easy though, and let it dry out a couple days, if possible. Then tamp, lay gravel, tamp etc.
Hi Devin!
I’m building my first patio on a budget and have found your blog extremely helpful and funny! I’m using white sandstone that ranges in thickness from 2″ – 1/8″ for my patio in Utah. What are the thickest sandstone pavers you’d use?
As well, would you have varying depths of compacted road base to lay the thinner slabs, of raise them using decomposed granite?
Thank you so much for this great information!
The thickest sand stone pavers I’ve used were maybe 4″. Usually ranging from inch and a half to three inches. If I’m leveling up the flagstones and have one that is a lot thinner–then it depends how much I need to raise the stone. If the stone only needs to be raise an inch or less–then I just use loose decomposed granite. If it needs to be raised an inch and a half–then I might add an inch of DG, then tamp it, then level it up using a half inch of loose DG. Or, if gravel happens to be handy, then i might use a shavel or two of gravel, instead of the one inch of tamped DG. If the stone needs to be raised two inches or more–than I’m using gravel, and tamping it, then using dg for the top half inch or so.