How do you build a flagstone patio?

Choose the laying method first, because it sets the stone thickness: one supplier directs sand set patios to 1 3/4 in stone and permits 1 1/4 in on a cement base. Then order the tonnage at the low end of the grade's coverage, dig for base, bedding and stone together, compact the base in lifts, screed an even layer of low-fines sand and set each piece so it does not rock.

How many tons of flagstone do I need for a 15x15 patio?

In 1 3/4 in minus stone at 115 to 125 sq ft per ton, with 10% waste, 1.98 to 2.15 tons, and you order the 2.15. In Patio Stone at 70 to 140 sq ft per ton the range is 1.77 to 3.54 tons, because that grade runs from 1 in to 3 in thick.

How deep do I dig for a flagstone patio?

The stone plus the bedding plus the base. For 1 3/4 in stone on 1 in of bedding that is 6 3/4 in over a 4 in base and 8 3/4 in over a 6 in base. This site does not publish a base depth as a requirement, because it depends on your soil, drainage and frost.

What sand goes under flagstone?

A coarse sand with low fines. WSDOT's drainage gradings hold the No. 200 sieve to 3.0 percent maximum, while its mortar grading allows up to 5 percent. Masonry sand is the tempting wrong choice: it screeds smoothly, but its fines hold water and migrate out of the bedding.

How much slope does a flagstone patio need?

Within 10 feet of the house, IRC R401.3 requires impervious surfaces to slope at least 2 percent, about 1/4 in per foot, away from the building where the ground itself cannot fall 6 inches in those 10 feet. Beyond 10 feet none of the documents this site has read sets one. Fall the surface away from the house, set the figure deliberately in the base, and check with your building department which edition and amendments apply.

Is sand set or mortared flagstone better?

They are different structures. Sand set stone carries load itself, so it needs the thicker stone and a well compacted base. A cement base patio is a concrete slab with stone as the finish, so it allows thinner stone but makes the slab the structural job.

How much does a flagstone patio cost?

Lawn Love publishes flagstone material at $13 to $30 per square foot, which is $2,925 to $6,750 of stone on a 15 by 15 ft patio. Base, bedding, delivery and spoil removal are on top, and the base can only be priced once your supplier gives you a weight for it, because LawnStarter prices gravel at $30 to $100 per ton. Last price check: 2026-09-07 (Lawn Love) and 2026-09-08 (LawnStarter).

How to Build a Flagstone Patio

A flagstone patio is usually planned backwards. People choose a stone they like, then work out how to lay it, then discover the stone was the wrong thickness for the method and the order was the wrong weight for the stone. The supplier's own sheet runs the decision the other way round, and so does this guide: method, thickness, tonnage, the hole, and what goes in the hole before any stone does.

By Lucas Pradella, builder and editor. Checked 2026-09-16.

Decide how it is laid before you choose the stone

There are two ways to build the patio, and they want different stone. One stone yard's coverage sheet says it in two sentences:

If you choose to Sand Set your flagstone you would use 1 3/4 in thick stone which as an average coverage of 120 sq. ft. per ton. ... If you choose to install your Flagstone in a Cement Base use 1 1/4 in thick stone which as a coverage of 150 sq. ft. per ton.
Acme Sand & Gravel, Flagstone Thickness, Coverage and Delivery, sheet dated 08-15 (quoted as printed, with the inch marks written out)

Sand set takes the thicker stone. A cement base permits the thinner. The sheet does not say why. A reasonable reading is that it comes down to where the strength lives. On sand, each piece is spanning whatever small hollows the bedding has and taking the load of a foot or a chair leg by itself, so it needs its own thickness. On a concrete base the slab carries the load and the stone is a finish, which is why it can be thinner.

That is a real fork in the project, not a detail. A cement base patio is a concrete slab with stone on top, and the slab is a structural job in its own right; the concrete patio calculator is where that version starts. The rest of this guide follows the sand set route.

Order by weight, lay by area, and expect a range

Flagstone is sold by the ton and laid by the square foot, and the conversion between the two is not one number. The same sheet publishes coverage as a range for each grade, because the thickness written on the grade is a ceiling rather than a measurement: Select Thin, sold as 1 1/4 in minus, is actually 3/4 in to 1 1/4 in, and Patio Stone runs from 1 in to 3 in thick on the same pallet.

Our arithmetic on Acme Sand & Gravel's published coverage per ton, with this site's default 10% added for waste. Each cell is the tonnage at the grade's LOW coverage, the figure to order, with the tonnage at its high coverage after it
Patio	Area	Select Thin, 140 to 160 sq ft per ton	Select Regular, 115 to 125	Select 2 in Heavy, 80 to 90	Patio Stone, 70 to 140
10 x 10 ft	100 sq ft	0.79 t (0.69)	0.96 t (0.88)	1.38 t (1.22)	1.57 t (0.79)
12 x 12 ft	144 sq ft	1.13 t (0.99)	1.38 t (1.27)	1.98 t (1.76)	2.26 t (1.13)
12 x 16 ft	192 sq ft	1.51 t (1.32)	1.84 t (1.69)	2.64 t (2.35)	3.02 t (1.51)
15 x 15 ft	225 sq ft	1.77 t (1.55)	2.15 t (1.98)	3.09 t (2.75)	3.54 t (1.77)
16 x 20 ft	320 sq ft	2.51 t (2.20)	3.06 t (2.82)	4.40 t (3.91)	5.03 t (2.51)
20 x 20 ft	400 sq ft	3.14 t (2.75)	3.83 t (3.52)	5.50 t (4.89)	6.29 t (3.14)

The inversion catches people. The low coverage figure gives the high tonnage, because it describes the thick end of the grade, and that is the end you are protecting yourself against when you order. Going back to the yard for another half ton is a gamble with natural stone, because a later pallet is not guaranteed to match the first.

Look at the Patio Stone column. On a 15 by 15 ft patio the published range is 1.77 to 3.54 tons, exactly two to one, from one product described honestly. The Select grades keep their range within about 15 percent end to end.

And note the weight on the drive. The sheet puts a pallet of the Select grades at 3,500 lb and Patio Stone at 4,000 lb. The 2.15 tons of Select Regular for that 15 by 15 ft patio is more than one pallet, and neither fits through a garden gate on a hand truck. Check delivery access before the order, not on the morning it arrives. The flagstone patio calculator runs the range for your own dimensions.

The hole is several times deeper than the stone

A sand set patio is a stack: compacted base at the bottom, a thin bedding layer on it, and the stone on top, finishing at the level you want. You dig for the whole stack.

Our arithmetic for sand set patios in 1 3/4 in stone on 1 in of bedding. Base depths are inputs to compare, not a specification. Spoil is area times dig depth divided by 324; loose base adds this site's 20% compaction allowance
Patio	Base, finished	Dig depth	Spoil to remove	Base to order, loose	Bedding sand
12 x 12 ft	4 in	6 3/4 in	3.00 cu yd	2.13 cu yd	0.44 cu yd
12 x 12 ft	6 in	8 3/4 in	3.89 cu yd	3.20 cu yd	0.44 cu yd
15 x 15 ft	4 in	6 3/4 in	4.69 cu yd	3.33 cu yd	0.69 cu yd
15 x 15 ft	6 in	8 3/4 in	6.08 cu yd	5.00 cu yd	0.69 cu yd
16 x 20 ft	4 in	6 3/4 in	6.67 cu yd	4.74 cu yd	0.99 cu yd
16 x 20 ft	6 in	8 3/4 in	8.64 cu yd	7.11 cu yd	0.99 cu yd

Three things fall out of that table. The spoil is the biggest volume on the job. A 15 by 15 ft patio on a 6 in base produces about six cubic yards of soil, which is a skip, a truck or a mound on the lawn, and it is almost never in the first estimate.

Two extra inches of base is about 30 percent more dig and spoil, and 50 percent more base to buy. That is why quotes for the same patio differ, and why a quote that does not state its base depth cannot be compared with one that does.

And the bedding is small. Even on the 16 by 20 ft patio it stays under a cubic yard. If your bedding order is starting to look like your base order, the bedding is doing a job that belongs to the base.

This site does not publish a base depth as a requirement. It depends on the soil, the drainage and the frost, none of which a web page can see. The depths above are there to show what the choice costs, and the excavation calculator handles the dig for any depth you settle on.

The base: compact it, and do not trust anybody's tonnage

The base is ordered loose and compacted in place, so the finished 4 in did not arrive as 4 in. This site's allowance for that is 20 percent, which is our figure rather than a standard, and it is included in the table above.

The harder problem is that the yard will quote the base by the ton and you have calculated it in cubic yards. LawnStarter's published range for general gravel is $30 to $100 per ton, and a per ton price is useless on a volume until you know what a cubic yard of that material weighs.

What can be pinned down is the rock itself. WSDOT sizes riprap by weight and volume together, 1 ton against 1/2 cubic yard and 300 lb against 2 cubic feet, and those work out at 148 and 150 lb per cubic foot, about 149 lb per cubic foot for solid stone. A heap of base is that, less the air between the stones, and nobody publishes the air for the heap at your supplier. Ask them what a yard of the material weighs. They run it across a scale every day.

Compact in lifts rather than all at once. A plate compactor densifies the top few inches it sits on, so a deep layer dumped and compacted once has a firm crust over loose material, and that is the profile that settles under a patio later.

The bedding: which sand, and two sources that disagree about its weight

Sand is not one material. WSDOT grades fine aggregate under separate specifications for surface finishing, for masonry mortar, for a sand drainage blanket and for sand drain backfill, and they differ most at the No. 200 sieve, which is the fines. The drainage gradings hold the No. 200 to 3.0 percent maximum. The masonry mortar grading allows up to 5 percent, and it is finer at every sieve than the surface finishing grading.

Under stone that has to drain, the low-fines sand is the one you want. Masonry sand screeds beautifully and is the wrong choice for exactly the reason it screeds well: its fines hold water and migrate. Ask the yard for the sieve analysis, because the name on the delivery ticket is not a specification.

Now the conflict. HomeGuide's sand delivery guide weighs a cubic yard of sand at about 1.35 tons. That is 2,700 lb in 27 cubic feet, or 100 lb per cubic foot. This site's sand page will not print a weight at all, on the grounds that density moves with gradation and moisture and that damp sand bulks.

Both positions are defensible, and the arithmetic shows why they coexist. Set HomeGuide's 100 lb against WSDOT's 149 lb for solid stone and the implied air is about a third of the volume. That is a plausible figure for a loose granular heap, and it is still an assumption about one heap. Use HomeGuide's number to sanity check a quote; do not use it to order a truck. On the 15 by 15 ft patio the whole bedding layer is 0.69 cubic yards, and at HomeGuide's washed paver sand price of $50 to $70 per cubic yard that is $35 to $49 of sand, where the error in a guessed density costs less than the delivery.

Source: HomeGuide. Last price check: 2026-09-15.

Slope: set by the code only within 10 feet of the house

A fall for the patio surface is often given as a fraction of an inch per foot away from the house, and presented as a rule.

The residential code sets one only near the house, and only in one case. IRC Section R401.3 requires the lot to fall not fewer than 6 inches within the first 10 feet from the foundation. Where lot lines, walls or slopes prevent that, its exception requires impervious surfaces within 10 feet of the building foundation to slope not less than 2 percent away from the building. Two percent is about 1/4 in per foot. The section does not say whether a flagstone patio laid with open joints counts as an impervious surface.

Beyond 10 feet from the house, none of the documents this site has read sets a slope for a patio. Not the stone yard's sheet, not the WSDOT specifications, not the cost guides.

Where this site has found a quarter inch per foot written as a code minimum, it is a drain pipe figure. The International Plumbing Code, as adopted by Tennessee, sets 1/4 in per foot for horizontal drainage pipe 2 1/2 in and smaller, and the Uniform Plumbing Code, as adopted by Hawaii, sets it as its general rule for horizontal drainage piping. Those sections govern water inside a pipe and say nothing about a patio. They are cited here only to show that the number exists in code for something else, not as a patio requirement, and this site has not traced where the patio figure itself comes from.

That does not mean a patio should be flat. Water standing against a foundation is a real failure, and falling the surface away from the house is plainly right. It means the specific figure is yours to set, and to set deliberately: decide the fall, put it on the plan, build it into the base with string lines rather than making it up in the bedding, and ask your building department whether your jurisdiction regulates surface drainage near the foundation before you assume nobody does.

Setting the stone

Irregular flagstone has no module, so it is laid by selection rather than by pattern. Every piece is chosen for the gap it is going into, trimmed where necessary, and bedded so it does not rock under a heel.

Screed the bedding to an even depth over the finished base, 1 in in this guide's worked figures. Level and fall belong in the base, compacted, where they stay put. Bedding thick enough to correct a low spot is bedding thick enough to keep settling.
Lay out the large pieces first and spread them, rather than starting in one corner and running out of big stone halfway across.
Fill between them with mid-size pieces, keeping joint widths roughly consistent, and trim to fit rather than forcing a piece into a gap.
Bed each piece and check it for rock by standing on its edges. A stone that rocks now will crack later, and it is far easier to add bedding under it today.
Fill the joints. On irregular flag with wide joints, that joint area is sand rather than stone, which is why the flagstone calculator lets you take it out of the stone order.

Expect some stone left over. The pieces that remain at the end are the ones that did not fit anywhere, and a patio finished from a pallet with nothing spare is one with some forced joints in it.

What it costs

Lawn Love publishes flagstone at $13 to $30 per square foot for material. On the 15 by 15 ft patio that is $2,925 to $6,750 in stone before anything underneath it, which is our multiplication of their rate by 225 sq ft.

The rest is the stack. The bedding layer at HomeGuide's washed paver sand price is tens of dollars, as worked above, plus HomeGuide's $50 to $100 local delivery. The base is priced by LawnStarter at $30 to $100 per ton and can only be turned into a dollar figure once your supplier gives you a weight for it. The spoil has no national figure in these sources at all, and on a 6 in base it is six cubic yards leaving the property.

Source: Lawn Love. Last price check: 2026-09-07. Source: HomeGuide. Last price check: 2026-09-15. Source: LawnStarter. Last price check: 2026-09-08.

So the stone is the line you can price from a web page, and the base and spoil are the lines you cannot. On a patio built properly they are also the lines that decide whether it stays flat.

What this guide does not decide

It does not set your base depth or your frost depth. Both depend on the ground under your patio and the climate above it.

It does not give a mortar or slab design for the cement base method. That is a concrete job with a stone finish, and it starts from the slab.

It does not set a surface slope, for the reason given above: the code sets one only within 10 feet of the house, and your layout decides whether that applies.

And the coverage figures are one yard's sheet. Your supplier's grades and ranges will differ, and their sheet is the one to order against.

Run the numbers

Frequently asked questions

How do you build a flagstone patio?: Choose the laying method first, because it sets the stone thickness: one supplier directs sand set patios to 1 3/4 in stone and permits 1 1/4 in on a cement base. Then order the tonnage at the low end of the grade's coverage, dig for base, bedding and stone together, compact the base in lifts, screed an even layer of low-fines sand and set each piece so it does not rock.
How many tons of flagstone do I need for a 15x15 patio?: In 1 3/4 in minus stone at 115 to 125 sq ft per ton, with 10% waste, 1.98 to 2.15 tons, and you order the 2.15. In Patio Stone at 70 to 140 sq ft per ton the range is 1.77 to 3.54 tons, because that grade runs from 1 in to 3 in thick.
How deep do I dig for a flagstone patio?: The stone plus the bedding plus the base. For 1 3/4 in stone on 1 in of bedding that is 6 3/4 in over a 4 in base and 8 3/4 in over a 6 in base. This site does not publish a base depth as a requirement, because it depends on your soil, drainage and frost.
What sand goes under flagstone?: A coarse sand with low fines. WSDOT's drainage gradings hold the No. 200 sieve to 3.0 percent maximum, while its mortar grading allows up to 5 percent. Masonry sand is the tempting wrong choice: it screeds smoothly, but its fines hold water and migrate out of the bedding.
How much slope does a flagstone patio need?: Within 10 feet of the house, IRC R401.3 requires impervious surfaces to slope at least 2 percent, about 1/4 in per foot, away from the building where the ground itself cannot fall 6 inches in those 10 feet. Beyond 10 feet none of the documents this site has read sets one. Fall the surface away from the house, set the figure deliberately in the base, and check with your building department which edition and amendments apply.
Is sand set or mortared flagstone better?: They are different structures. Sand set stone carries load itself, so it needs the thicker stone and a well compacted base. A cement base patio is a concrete slab with stone as the finish, so it allows thinner stone but makes the slab the structural job.
How much does a flagstone patio cost?: Lawn Love publishes flagstone material at $13 to $30 per square foot, which is $2,925 to $6,750 of stone on a 15 by 15 ft patio. Base, bedding, delivery and spoil removal are on top, and the base can only be priced once your supplier gives you a weight for it, because LawnStarter prices gravel at $30 to $100 per ton. Last price check: 2026-09-07 (Lawn Love) and 2026-09-08 (LawnStarter).

Check these numbers yourself

Flagstone Thickness, Coverage and DeliveryAcme Sand & Gravel, sheet dated 08-15. Published coverage by grade, actual thickness behind each nominal size, pallet weights of 3,500 and 4,000 lb, and the thickness rule for sand set against a cement base. Read 2026-09-07.
2027 Standard Specifications M 41-10, Division 9 MaterialsWashington State Department of Transportation. Section 9-13.1(2) sizes riprap by weight and volume together, giving about 149 lb per cubic foot for solid stone. Sections 9-03.2 and 9-03.13 grade fine aggregate for surface finishing, masonry mortar, a drainage blanket and sand drains, differing most at the No. 200 sieve. Read 2026-09-08.
Lawn Love, how much does a paver patio costWritten by Ayoola Azzan, published 27 December 2025. Flagstone material at $13 to $30 per square foot. Read 2026-09-07.
HomeGuide, how much does sand delivery costWritten by Paul Mazzola, edited by Tom Grupa, published 3 June 2026. Washed paver sand at $50 to $70 per cubic yard, local delivery $50 to $100, and a cubic yard of sand weighed at about 1.35 tons. Read 2026-09-15.
LawnStarter, how much does a gravel driveway costWritten by Michelle Selzer, published 27 December 2025. General gravel at $30 to $100 per ton, priced by weight rather than volume. Read 2026-09-08.
Tennessee Plumbing Code 2021, Section 704.1 and Table 704.1International Plumbing Code as adopted by Tennessee, 2021 edition, read through UpCodes. Sets 1/4 in per foot for horizontal drainage pipe 2 1/2 in and smaller. A pipe requirement, cited only to separate it from patio slope, which it does not govern. Read 2026-09-13.
Hawaii Plumbing Code 2021, Section 708.1 Grade of Horizontal Drainage PipingUniform Plumbing Code as adopted by Hawaii, 2021 edition, read through UpCodes. Sets 1/4 in per foot for all horizontal drainage piping as the general rule. A pipe requirement, not a patio one. Read 2026-09-13.
2018 Seattle Residential Code, Chapter 4 Foundations, Section R401.3Seattle Department of Construction and Inspections. Prints IRC Section R401.3, the 6 inch fall within 10 feet and the exception requiring impervious surfaces within 10 feet of the foundation to slope not less than 2 percent. Read 2026-09-16.

Figures here last checked against the source documents on 2026-09-16. Codes are amended locally; confirm against the edition your jurisdiction enforces.

About the author

Lucas Pradella builds and maintains Nails & Numbers. He is not a tradesperson: every figure on the site is traced to a named, dated document, and the page links it so you can check it yourself. More about how these pages are made.

Every guide on this site: Guides.