How thick should a concrete slab be?

The IRC minimum is 3 1/2 in for a slab on ground under Section R506.1. Four inches is common practice and uses 14% more concrete. Vehicle loads, poor subgrade or anything outside the prescriptive path are sent to a design under ACI 332, and expansive soils to Section R403.1.8.

How far apart should control joints be?

NRMCA recommends spacing of 24 to 36 times the slab thickness, so about 10 ft on a 4 in slab, and never more than 15 ft. Keep panels square or nearly square, with length no more than 1.5 times the width, and avoid L-shaped panels.

How deep should a saw cut be, and when do I cut?

At least a quarter of the slab thickness and never less than 1 in. Conventional saw cuts are run within 4 to 12 hours of finishing; early-entry saws cut sooner but still to a minimum of 1 in.

How long does a slab need to cure?

NRMCA gives 3 to 7 days, starting immediately after finishing. Keep moisture in by fogging, wet covering or plastic sheeting that meets ASTM C171 and is at least 4 mils thick, then end curing gradually rather than stripping it all at once.

Is 6 mil plastic enough under a slab?

It satisfies IRC R506.2.3, which asks for a 6-mil vapor retarder with joints lapped at least 6 in. NRMCA recommends more: material meeting ASTM E1745 at no more than 0.1 perms, with a minimum thickness of 10 mils for durability during construction.

Can I add water to the truck on site?

Only within the specified slump and water-cementitious ratio, which is what ASTM C94 permits. Beyond that, NRMCA says added water delays setting, reduces strength, harms durability and increases cracking.

When can I start floating and brooming?

Bull float soon after strike-off, before bleed water appears, then stop. No further finishing until bleeding has ended and the surface water has gone. Broom exterior flatwork after floating, and avoid steel troweling air-entrained concrete.

How to Pour a Concrete Slab

A slab is won or lost before the truck arrives. Almost everything that decides whether it cracks, drains and passes inspection is buried under it or written into the order, and once concrete is moving you cannot change any of it. This is the sequence, the figures that govern each step, and the documents each one comes from.

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

Dig for the whole sandwich, not the slab

Below grade you owe a base course, a sheet and the slab itself. IRC R506.2.2 requires 4 in of clean graded sand, gravel, crushed stone, crushed concrete or crushed blast-furnace slag passing a 2 in sieve, and R506.1 sets the slab at 3 1/2 in minimum. A 4 in slab over that base is 8 in of excavation below finished level before you allow anything for forms.

Most jobs that stall at the last minute stall here. The base is a separate order from a separate supplier, with its own delivery day, and it is discovered after the hole is already dug to slab depth.

The exception is narrow and worth reading before you rely on it: no base course is required on well-drained or sand-gravel soils in Group I of the Unified Soil Classification System, per Table R405.1. That is a classification somebody makes about the soil, not an opinion about whether your garden puddles after rain.

NRMCA describes the same layer from the other direction, as a blotter: a minimum 4 in of compactable, easy-to-trim material that must be dry when concrete is placed so it can take water out of the fresh mix rather than add it.

What the extra half inch costs, in yards and in dollars

Four inches instead of the 3 1/2 in the code asks for is 14% more concrete every time. Here is what that is worth on ordinary slabs, priced at the short-load rate because every one of these is under a full 10-yard load.

Volume at the code minimum against the trade convention, and the delivered cost of the difference
Slab	Area	3 1/2 in, cu yd	4 in, cu yd	Difference	Cost of the difference
10 x 12 ft patio	120 sq ft	1.3	1.48	0.19	$32 to $37
12 x 16 ft patio	192 sq ft	2.07	2.37	0.3	$51 to $59
20 x 20 ft slab	400 sq ft	4.32	4.94	0.62	$106 to $123
24 x 24 ft garage	576 sq ft	6.22	7.11	0.89	$153 to $178
20 x 40 ft shop	800 sq ft	8.64	9.88	1.23	$212 to $247

This is our arithmetic on two published figures: the thicknesses in R506.1 and HomeGuide's delivered rate of $119 to $147 per cubic yard plus $53 per yard under a full load. It is not an argument for pouring thin. It is the number that should be in front of you when somebody says four inches by habit.

Screed the subgrade harder than feels necessary

Here is the figure nobody publishes, and it is ours rather than a document's: a nominal 4 in slab is 4 in only on the drawing. Ground raked by hand has hollows, forms get over-excavated at the edge, and every dip is filled with concrete you bought by the yard.

That is what a waste allowance is for. We default to 10% on the calculators and treat it as a floor on hand-graded ground. The error is not symmetrical either: over-ordering costs you the short-load charge on a fraction of a yard, while under-ordering mid-pour leaves the placed concrete stiffening while you wait, and the joint where the next load meets it is permanent.

NRMCA's finishing sheet makes the same point as a rule for the day: complete all subgrade excavation and compaction, formwork and placement of mesh and embedments before concrete delivery, because delays after the truck arrives reduce the quality of the finished flatwork. Concrete sets on its own schedule, and every minute you spend fixing the base is a minute the slab has already started.

The sheet: the code says 6 mil, the industry says 10

IRC R506.2.3 requires a 6-mil polyethylene vapor retarder between the base course and the slab, with joints lapped not less than 6 in, and lists four exceptions including garages and unheated accessory buildings.

NRMCA's sheet on vapor retarders does not contradict the code so much as leave it behind. It specifies the material by performance, ASTM E1745, permeance no greater than 0.1 perms, and says plainly that low-density polyethylene sheets commonly used in the past have been replaced by stronger, less permeable materials. It recommends a minimum thickness of 10 mils, both for vapor transmission and to survive being walked on during construction.

So the code minimum is legal and the industry recommendation is nearly twice as thick. On a heated building with a floor covering going down later, the extra costs a few tens of dollars on a slab and removes an argument you cannot win afterwards.

Quantity surprises people: the 6 in lap means a 10 ft roll covers 9.5 ft. A 24 ft wide slab takes three runs, not the 2.4 that dividing suggests.

How many runs of poly, once the lap is counted

R506.2.3 laps joints at least 6 in, so a 10 ft roll contributes 9.5 ft after the first run. Dividing the width by 10 gives the wrong answer on almost every slab.

Runs of 10 ft sheet needed across the slab, first run full width and every later run lapped 6 in
Slab width	Runs of 10 ft sheet	Sheet width bought	Extra sheet across the width
10 ft	1	10 ft	0.0 ft
12 ft	2	20 ft	8.0 ft
16 ft	2	20 ft	4.0 ft
20 ft	3	30 ft	10.0 ft
24 ft	3	30 ft	6.0 ft
30 ft	4	40 ft	10.0 ft
40 ft	5	50 ft	10.0 ft

The extra is lap plus the offcut you trim at the last run, not waste you can avoid. The lap runs the length of the slab as well, so order by the run and not by the square foot. And if you are following NRMCA rather than the code minimum, the same table applies to 10 mil material.

Steel belongs in the top half, and chairs are how it gets there

R506.2.4 requires reinforcement to be supported so that it remains between the middle of the slab and its upper third for the entire placement. On a 4 in slab that band is roughly 2 to 2.7 in above the bottom.

Mesh laid on the poly and pulled up with a hook as the pour passes does not meet that, and it is the ordinary reason a reinforced slab behaves like an unreinforced one. Chairs cost less than the concrete covering them and they hold position while people walk on the mat.

Order by the quarter yard, and know where bags stop making sense

Ready-mix is sold in quarter-yard steps, so an order rounds up rather than to the nearest. Below a full load there is a short-load charge: HomeGuide puts ready-mix at $119 to $147 per cubic yard delivered, with $53 per yard added under a 10-yard load.

Bags are the alternative and the crossover arrives early. Forty-five 80 lb bags make one cubic yard, because an 80 lb bag of Quikrete Concrete Mix yields 0.60 cubic feet and a yard is 27. That is 3,600 lb to carry and mix. A 10 by 12 ft patio at 4 in is about 74 bags once an allowance is added, which is a full day of mixing before any finishing happens.

Bags still win in three situations: a pour small enough that the truck's minimum load costs more than the bags, a site a truck cannot reach, and a job you have to place alone at your own pace.

Strength and air come from the table, not the yard

IRC Table R402.2 sets the strength by element and weathering potential: porches, carport slabs, exterior steps and garage floor slabs at 2,500 psi where weathering potential is negligible, 3,000 psi moderate and 3,500 psi severe. Footnote d requires air entrainment of 5 to 7 percent for that group in the moderate and severe columns.

Weathering potential itself is a map, IRC Table R301.2(1), adopted jurisdiction by jurisdiction. Your building department knows the value for your address. A web page that tells you your county's figure is guessing, which is why this site does not.

Mixing from bags? Strength is not the constraint: Quikrete Concrete Mix and Sakrete High-Strength Concrete Mix both publish 4,000 psi at 28 days, above every cell in that table. Air entrainment is the constraint, because those bags are not air-entrained and nothing you do in a mixer adds it.

Air has a second consequence on the day. NRMCA's finishing sheet warns against steel troweling air-entrained concrete, because closing the surface traps bleed water and air under a dense skin and the skin later separates. Exterior flatwork is broomed for that reason as much as for grip.

The water is the one thing you can ruin on the day

Bag sheets state the water and mean it: Quikrete 6 pints per 80 lb bag with 9 pints the maximum expected, Sakrete 3.5 quarts, which is 7 pints. Those figures are the water-cement ratio behind the strengths they publish.

With ready-mix the same argument has a standard behind it. NRMCA's sheet on jobsite water says adding water is permitted by ASTM C94 only while the specified slump and water-cementitious ratio are not exceeded, and that water beyond the design mixing water delays setting, reduces strength, harms durability and increases cracking.

On a hot afternoon the mix stiffens and the natural move is another splash from the hose. It makes placing easier and the slab weaker, and it is the only step here that trades one directly for the other. If placement is genuinely hard, the answers are ordering a higher slump with an admixture, or pouring earlier in the day.

Place, level, then wait for the bleed water

NRMCA sets the order of operations, and the waiting is the part people skip.

Place the concrete close to its final position and avoid segregation. Do not drag it long distances with a rake.
Strike off to level without sealing the surface.
Bull float or darby soon after strike-off, before bleed water appears, keeping the tool flat so the surface is not sealed prematurely.
Wait. No further finishing until bleeding has stopped and the water has left the surface.
Texture after floating. For exterior flatwork, a coarse or fine push broom gives the non-slip finish.
Cure it.

Finishing while bleed water is on the slab works that water back into the surface, which is how a slab ends up dusting or scaling later. The wait is not laziness, it is the difference between a surface that lasts and one that does not.

Joint it before it cracks itself

Concrete shrinks as it dries and cracks where it likes unless you decide for it. NRMCA is blunt about what joints are: pre-planned cracks.

The rules that matter for a residential slab:

Spacing: 24 to 36 times the slab thickness, which puts a 4 in slab at about 10 ft, and in no case more than 15 ft.
Panels square or nearly so, length no more than 1.5 times width, and no L-shaped panels.
Depth: at least a quarter of the slab thickness, and never less than 1 in.
Timing: conventional saw cuts within 4 to 12 hours of finishing; early-entry saws go sooner but still cut a minimum of 1 in; tooled joints are run early in the finishing process.

Isolation joints are the other half, separating the slab from walls, columns, footings, steps and posts so the two can move independently.

A 20 by 24 ft garage slab at 4 in therefore wants joints at roughly 10 ft, which is a cut down the middle each way. Skipping them does not avoid cracks; it only means the slab chooses where they go.

Joint spacing worked out for the usual thicknesses

NRMCA's rule is 24 to 36 times the slab thickness, capped at 15 ft. Applied to the thicknesses people actually pour:

Contraction joint spacing, from NRMCA CIP 6 applied to each thickness
Slab thickness	At 24 times	At 36 times	Use no more than
3.5 in	7.0 ft	10.5 ft	10.5 ft
4 in	8.0 ft	12.0 ft	12.0 ft
5 in	10.0 ft	15.0 ft	15.0 ft
6 in	12.0 ft	18.0 ft	15.0 ft

A 24 by 24 ft garage slab at 4 in therefore needs a cut each way to land inside 12 ft panels, and the panels stay square, which is the second half of the rule. Joint depth follows the same table: a quarter of the thickness, so 1 in on a 4 in slab because the 1 in floor governs, and 1.5 in on a 6 in slab.

Curing is three to seven days, and it starts immediately

NRMCA's curing sheet is short and specific: curing begins immediately after placing and finishing and continues for 3 to 7 days, and its purpose is to keep moisture in and temperature favorable so hydration carries on.

Methods split into applying water, such as fogging, sprinkling or wet coverings, and retaining it, such as plastic sheets that conform to ASTM C171 and are at least 4 mils thick. Sheeting is easier to manage on a driveway; it can also mottle the color, which matters on anything decorative.

End curing gradually rather than pulling everything off at once, and let cover materials dry before removal.

Cold weather has a definition, and it is not a feeling

NRMCA defines cold weather concreting as a period when, for more than three consecutive days, the average daily temperature is below 40°F and the air temperature is not above 50°F for more than half of any 24-hour period.

That matters because the precautions are a package, not a preference: protecting the concrete, heating materials where needed, and keeping it warm long enough to gain strength. If the forecast meets that definition, either plan for it properly or move the pour. A slab that freezes in its first days never recovers the strength it lost.

What this guide does not decide

It does not size anything structural. Slab thickness for vehicle loads, reinforcement schedules, footings and frost depth are engineering questions, and frost depth in particular is set locally and cannot be published as one number.

It does not cover expansive soils, which R506.1 sends to Section R403.1.8, nor anything outside the prescriptive path, which the same section sends to a design under ACI 332.

And it does not tell you your weathering potential or your frost line. Those come from your building department, and that call takes less time than reading this page.

Run the numbers

Frequently asked questions

How thick should a concrete slab be?: The IRC minimum is 3 1/2 in for a slab on ground under Section R506.1. Four inches is common practice and uses 14% more concrete. Vehicle loads, poor subgrade or anything outside the prescriptive path are sent to a design under ACI 332, and expansive soils to Section R403.1.8.
How far apart should control joints be?: NRMCA recommends spacing of 24 to 36 times the slab thickness, so about 10 ft on a 4 in slab, and never more than 15 ft. Keep panels square or nearly square, with length no more than 1.5 times the width, and avoid L-shaped panels.
How deep should a saw cut be, and when do I cut?: At least a quarter of the slab thickness and never less than 1 in. Conventional saw cuts are run within 4 to 12 hours of finishing; early-entry saws cut sooner but still to a minimum of 1 in.
How long does a slab need to cure?: NRMCA gives 3 to 7 days, starting immediately after finishing. Keep moisture in by fogging, wet covering or plastic sheeting that meets ASTM C171 and is at least 4 mils thick, then end curing gradually rather than stripping it all at once.
Is 6 mil plastic enough under a slab?: It satisfies IRC R506.2.3, which asks for a 6-mil vapor retarder with joints lapped at least 6 in. NRMCA recommends more: material meeting ASTM E1745 at no more than 0.1 perms, with a minimum thickness of 10 mils for durability during construction.
Can I add water to the truck on site?: Only within the specified slump and water-cementitious ratio, which is what ASTM C94 permits. Beyond that, NRMCA says added water delays setting, reduces strength, harms durability and increases cracking.
When can I start floating and brooming?: Bull float soon after strike-off, before bleed water appears, then stop. No further finishing until bleeding has ended and the surface water has gone. Broom exterior flatwork after floating, and avoid steel troweling air-entrained concrete.

Check these numbers yourself

2015 Seattle Residential Code, Chapter 5 Floors, Section R506Seattle Department of Construction and Inspections, page 168. Carries IRC Section R506 in full: minimum thickness, base course, vapor retarder and reinforcement support. Read 2026-09-16.
2018 Seattle Residential Code, Chapter 4 FoundationsSeattle Department of Construction and Inspections, page 124. Carries IRC Table R402.2 and footnote d on air entrainment. Read 2026-09-16.
QUIKRETE Concrete Mix, product no. 1101, technical data sheetQuikrete data sheet 1101, CSI 03 31 00. Yield 0.60 cubic feet per 80 lb bag, water 6 pints starting and 9 pints maximum, 4,000 psi at 28 days. Read 2026-09-16.
SAKRETE High-Strength Concrete Mix, technical data sheetSakrete TDS, Division 03 31 00, ©2023, file dated January 2024. 3.5 quarts of water per 80 lb bag, 4,000 psi at 28 days. Read 2026-09-16.
HomeGuide, how much does concrete cost per yardWritten by Tom Grupa, 6 January 2026. Ready-mix $119 to $147 per cubic yard delivered, plus $53 per yard under a full 10-yard load. Read 2026-09-15.
NRMCA CIP 6, Joints in Concrete Slabs on GradeNational Ready Mixed Concrete Association. Joint spacing of 24 to 36 times slab thickness, 15 ft maximum, depth one quarter of thickness and not less than 1 in, saw cutting within 4 to 12 hours. Read 2026-09-16.
NRMCA CIP 11, Curing In-Place ConcreteNational Ready Mixed Concrete Association. Curing for 3 to 7 days, methods that apply or retain water, plastic sheeting to ASTM C171 at least 4 mils. Read 2026-09-16.
NRMCA CIP 14, Finishing Concrete FlatworkNational Ready Mixed Concrete Association. Bull float before bleed water appears, wait for bleeding to stop before finishing, broom texture for exterior flatwork, caution on steel troweling air-entrained concrete. Read 2026-09-16.
NRMCA CIP 26, Jobsite Addition of WaterNational Ready Mixed Concrete Association. Water may be added within the specified slump and water-cementitious ratio under ASTM C94; beyond that it delays setting, reduces strength and increases cracking. Read 2026-09-16.
NRMCA CIP 27, Cold Weather ConcretingNational Ready Mixed Concrete Association. Cold weather defined as more than three consecutive days with average daily temperature below 40°F and air temperature not above 50°F for more than half of any 24-hour period. Read 2026-09-16.
NRMCA CIP 29, Vapor Retarders Under Slabs on GradeNational Ready Mixed Concrete Association. ASTM E1745 at 0.1 perms or less, minimum 10 mils recommended, granular blotter layer at least 4 in and dry before placement. 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.