How to Pour a Small Concrete Patio Slab or Pad (Phoenix Homeowner Guide)

Step-by-step guide to pouring a small concrete patio slab or pad in Phoenix: permits and Arizona 811, slope away from the house, subgrade and gravel base, forms, the right concrete strength, joints and curing in the heat, with the code behind every step.

By Gabriel · Licensed Phoenix, Arizona General Contractor ROC 367058 (R-62) · 2026-11-13

A small concrete slab is one of the most useful things you can add to a Phoenix yard: a patio for chairs and a grill, a pad for a shed, trash cans or a water softener, or a landing outside a back door. It is also a job where small mistakes show up for years. A slab poured on soft dirt settles, a slab that tilts toward the house sends monsoon water against the wall, and a slab without joints cracks wherever it wants.

Below is the order I follow when I pour a small patio slab or pad. Every step says why it matters and which rule or guide is behind it: the 2024 International Residential Code (IRC) as adopted by Phoenix, Arizona's underground-utility law (A.R.S. 40-360.22), Maricopa County's dust rule (Rule 310), the American Concrete Institute (ACI) guides on joints and hot-weather concrete, and the OSHA silica rule for saw-cutting. This guide is for a ground-level slab that is not part of the house foundation. If you already have a slab with cracks, see my guide on how to repair concrete cracks instead.

Tools and parts

• Waterproof gloves, long sleeves, safety glasses, rubber boots, knee boards, and a respirator for any cutting
• Tape measure, stakes, string line, line level or laser, and a 4-foot level
• Shovel, square spade, wheelbarrow and a hand tamper or rented plate compactor
• Clean graded gravel or crushed rock for the base (2-inch minus), and a tarp to cover the pile
• Straight 2x4 or 2x6 form boards, wood stakes and screws, and isolation-joint (expansion) strip where the slab meets the house or another slab
• Rebar or welded wire with chairs or dobies to hold it up, tie wire, if you are reinforcing the slab
• Concrete: bagged mix and a mixer for a very small pad, or ready-mix ordered by strength for anything bigger
• Straight screed board, magnesium bull float or darby, edger, groover, steel or mag hand float and a broom
• Plastic sheeting or curing compound, a garden hose with a mist nozzle, and a wet-cutting saw if you saw-cut the joints

Step-by-step

1. Plan the size, thickness and permits

   Mark out the slab with stakes and string and decide what it will carry: chairs and a grill, a shed, or an air conditioner or water heater pad. For a patio I plan at least 4 inches of concrete. Measure the area and multiply length x width x thickness (in feet) to get cubic feet, then divide by 27 for cubic yards. Before you dig, ask the city and your HOA whether your project needs a permit or approval, especially if the slab is near a property line, under a future patio cover, or connected to the street or sidewalk.

   Why: The IRC says concrete floors on the ground must be at least 3 1/2 inches thick (or designed to ACI 332) and must use the concrete strength from Section R402.2, so 3 1/2 inches is a floor, not a target. The model IRC lists sidewalks and driveways as exempt from a building permit, but Phoenix uses its own administrative rules for permits instead of IRC Section R105, and work in the street right-of-way or under a roof structure is a separate question. Asking first avoids tearing out new concrete.

   Code: 2024 IRC R506.1 (slabs on ground at least 3 1/2 in. thick, or per ACI 332; strength per R402.2) · Model 2024 IRC R105.2 item 5 (sidewalks and driveways exempt from building permit) · Phoenix amendment: IRC R105 reserved (Phoenix administrative code governs permits)

2. Call Arizona 811 before you dig

   Contact Arizona 811 (dial 811 or use their website) before you dig even a few inches. Wait until every utility has marked its lines with paint or flags, or has told you there is nothing to mark. Also look for your own private lines that 811 does not mark: irrigation, landscape lighting, a gas line to a grill or a line to a pool or detached garage.

   Why: Arizona law says a person may not begin excavating in a public right-of-way or a utility easement without first finding out where the underground facilities are, and that utilities must mark their lines within two working days of the request. You may not dig before the marking is complete. Back and side yards often have easements for electric, gas, cable or water lines, and a shovel or stake through a line is dangerous and expensive.

   Code: A.R.S. 40-360.22(A) (locate underground facilities before excavating in a right-of-way or easement) · A.R.S. 40-360.22(B) (operators mark within two working days; no excavating before marking is complete)

3. Set the height and slope so water runs away from the house

   Set your string lines at the finished height of the slab. Where the slab meets the house, keep the concrete surface low enough to stay at least 2 inches below the stucco weep screed (the metal or plastic edge at the bottom of the stucco), and never cover it. Slope the string down away from the house about 1/4 inch per foot (2 percent), so a 10-foot patio drops about 2 1/2 inches. Make sure the low edge has somewhere for water to go, not into your neighbor's yard or back toward a wall.

   Why: The model IRC requires impervious surfaces such as concrete within 10 feet of the foundation to slope at least 2 percent away from the building. Phoenix replaced that section with the drainage rules in Chapter 32A of the City Code, so check the local requirement, but the goal is the same. The IRC also requires the stucco weep screed to sit at least 2 inches above paved areas so trapped water can drain out of the wall; a patio poured up against or over the screed traps water in the wall.

   Code: Model 2024 IRC R401.3 (impervious surfaces within 10 ft of the foundation sloped at least 2% away) · Phoenix amendment to IRC R401.3 (drainage per Phoenix City Code Chapter 32A) · 2024 IRC R703.7.2.1 (weep screed at least 2 in. above paved areas)

4. Dig out grass, roots and topsoil without making a dust cloud

   Excavate the area a few inches wider than the slab, deep enough for the base and the concrete. Remove all grass, roots, mulch, old pavers and loose topsoil down to firm, undisturbed soil. Lightly water the area before and while you dig so dust does not blow onto the street or the neighbors, and keep the dirt pile damp or covered until it is hauled away.

   Why: The IRC requires all vegetation, topsoil and foreign material to be removed from the area under a slab, because organic material rots and leaves voids that the slab later settles into. Maricopa County Rule 310 requires anyone creating disturbed surface areas to use dust control measures such as pre-watering or phasing the work, before, during and after the dust-generating work, and requires a Dust Control permit once a project disturbs 0.10 acre (4,356 square feet) or more.

   Code: 2024 IRC R506.3 (remove vegetation, topsoil and foreign material under the slab) · Maricopa County Rule 310 Section 305 and 305.11 (dust control measures for disturbed surface areas) · Maricopa County Rule 310 Section 302.1 (Dust Control permit at 0.10 acre or more)

5. Compact the subgrade and any fill

   Tamp or run a plate compactor over the bottom of the excavation until it is firm and does not move under your boots. If you have to fill low spots, use clean fill in thin layers and compact each one; do not leave loose dirt from the dig under the slab. If the soil is soft, swells when wet, or you find old trash or debris, stop and get advice.

   Why: The IRC requires fill under a slab to be free of vegetation and foreign material and compacted to give the slab uniform support, and it limits fill depth to 24 inches of clean sand or gravel or 8 inches of earth unless approved. Uneven support is the main reason small slabs crack and tilt. Where expansive or shifting soils are likely, the IRC sends foundations and floor slabs to engineered design, which is beyond a DIY patio.

   Code: 2024 IRC R506.3.1 (fill free of vegetation, compacted, depth limits) · 2024 IRC R401.4 (soil tests where expansive, compressible or shifting soils are likely) · 2024 IRC R403.1.8 (foundations and floor slabs on expansive soils)

6. Spread and compact a gravel base

   Spread about 4 inches of clean graded gravel or crushed rock over the compacted soil, rake it to the same slope as your strings, and compact it. Check the depth from the string in several places so the concrete will be the same thickness everywhere. Keep the delivered gravel pile covered with a tarp or damp when you are not working with it.

   Why: For slabs below grade, the IRC calls for a 4-inch base course of clean graded sand, gravel, crushed stone or similar material passing a 2-inch sieve, with an exception for well-drained sand-gravel soils. A compacted base gives even support and lets water drain from under the slab. Maricopa County Rule 310 requires open storage piles to be covered or kept moist, and bulk material to be watered when it is loaded or unloaded.

   Code: 2024 IRC R506.3.2 (4-in. base course; exception for Group I soils) · Maricopa County Rule 310 Section 305.4 (water bulk material when stacking, loading and unloading) · Maricopa County Rule 310 Section 305.5 (cover or moisten open storage piles)

7. Build straight, strong forms and isolate the slab from the house

   Set 2x4 or 2x6 boards on edge along the string lines, with stakes every 2 to 3 feet on the outside and screws through the stakes into the boards. Check the top of the forms with a level so they follow the slope you set. Where the new slab touches the house, a wall, a step or an existing slab, glue an isolation-joint strip to the existing surface for the full depth of the new slab.

   Why: The forms set the finished height and slope, so if they bow or sink, the slab will hold water. An isolation joint lets the new slab and the house, wall or old slab move separately so the new concrete does not crack against them or push on the wall. ACI's guide to joints in concrete construction (ACI 224.3R) explains the different joint types (isolation, contraction and construction joints) and where each one belongs.

   Code: ACI 224.3R-95 Joints in Concrete Construction (isolation joints separate the slab from walls and other slabs) · Model 2024 IRC R401.3 / Phoenix City Code Chapter 32A (finished slope away from the building)

8. Decide on vapor retarder and reinforcement

   A plain outdoor patio usually does not need plastic under it. If you might enclose it later as a room, talk to the city first, because then it is treated like a house floor. If you use rebar or welded wire, set it on chairs or concrete blocks so it sits near the middle of the slab, and keep it at least a couple of inches in from the edges. Do not lay mesh on the ground and plan to 'pull it up' during the pour.

   Why: The IRC requires a vapor retarder under concrete floor slabs, but it exempts driveways, walks, patios and other flatwork not likely to be enclosed and heated later. Where reinforcement is used, the IRC requires it to be supported so it stays between the center and the upper one-third of the slab for the whole pour. Steel lying on the ground does nothing to hold cracks tight.

   Code: 2024 IRC R506.3.3 Exception (vapor retarder not required for patios and flatwork not likely to be enclosed and heated) · 2024 IRC R506.3.4 (reinforcement supported in the center to upper one-third of the slab)

9. Order the right concrete and do not add extra water

   For a patio or pad in Phoenix, order or buy concrete rated at least 2,500 psi; a stronger exterior mix is fine. Tell the ready-mix supplier what you are pouring, how thick, and that you will be pouring early on a hot day. Mix bagged concrete with the amount of water printed on the bag. Do not keep adding water to make it easier to spread.

   Why: IRC Table R402.2 sets the minimum strength for concrete exposed to the weather (porches, carport slabs and steps) by weathering region: 2,500 psi where weathering is negligible, more where it is moderate or severe. Phoenix's Table R301.2 lists weathering as negligible, so 2,500 psi is the code minimum here, and air entrainment, which is required in moderate and severe regions, is not required by the code in Phoenix. Extra water weakens concrete and increases shrinkage cracking.

   Code: 2024 IRC R402.2 and Table R402.2 (minimum compressive strength by weathering potential; air entrainment for moderate/severe) · Phoenix 2024 IRC Table R301.2 (weathering: Negligible; frost line depth: 0)

10. Pour early, screed and float before the heat sets it

    Start at first light in the warm months. Lightly dampen the base (no puddles), place the concrete, and spread it with a shovel or come-along, not a rake that separates the stones. Strike it off with a straight board resting on the forms, sawing back and forth, then bull float or darby the surface once. Wait until the water sheen on top disappears before edging and final finishing, and do not sprinkle water or dry cement on the surface to 'help' it.

    Why: ACI's hot-weather concreting guide explains that high temperature, low humidity and wind speed up moisture loss and setting, which leads to cold joints, plastic shrinkage cracks and lower strength; all are normal in a Phoenix summer. Planning the pour for the coolest part of the day and having enough helpers is the main defense. Mixers, vibrators and saws plugged in outdoors at a house must be on GFCI-protected receptacles.

    Code: ACI 305R-20 Chapter 1 (high temperature, low humidity and wind accelerate moisture loss and setting) · 2023 NEC 210.8(A)(3) (GFCI protection for outdoor dwelling-unit receptacles)

11. Edge the slab and cut control joints

    Run an edger along the forms to round the edges. Cut control joints with a groover while the concrete is still workable, or saw-cut them with a wet saw as soon as the slab is hard enough that the blade does not ravel the edges (often the same day or the next morning). Space the joints so the panels are close to square, about 8 to 12 feet apart on a 4-inch slab, and make them about one-quarter of the slab thickness deep (1 inch on a 4-inch slab). Finish with a broom for a non-slip surface.

    Why: Concrete shrinks as it dries, so it will crack; control joints decide where. The ACI joint guidance summarized by the U.S. Bureau of Reclamation recommends contraction joints spaced 24 to 36 times the slab thickness, with panels close to square, and a joint depth of about one-quarter of the thickness. If you saw-cut, OSHA's silica rule (written for contractors, and good practice for anyone) lists a handheld or walk-behind saw with water fed continuously to the blade as the dust control, and does not allow dry sweeping where wet methods or HEPA vacuums can be used.

    Code: ACI 224.3R-95 (contraction joint spacing about 24 to 36 times slab thickness; depth about 1/4 of thickness) · OSHA 29 CFR 1926.1153 Table 1 items (ii) and (iv) (handheld and walk-behind saws: integrated water delivery) · OSHA 29 CFR 1926.1153(f)(1) (no dry sweeping of silica dust where safer methods are feasible)

12. 

13. Cure it slowly and keep weight off

    As soon as the surface can be touched without marking it, keep the slab moist: mist it and cover it with plastic sheeting weighted at the edges, or spray a curing compound, and keep it covered for about a week if you can. Keep foot traffic off for at least a day and heavy loads, vehicles and sheds off for longer, following the supplier's advice. Strip the forms after a few days, backfill the edges, and seal the joints and edges with a flexible sealant once the slab has cured.

    Why: ACI's hot-weather guide has a section on curing and protection because concrete that dries out early loses strength and surface durability and cracks more. In dry desert air a bare slab can lose its moisture in hours. Sealing the joints later keeps water, dirt and weeds out of them; the method is the same as in my crack-repair guide.

    Code: ACI 305R-20 Section 6.4 (curing and protection in hot weather)

When to stop and call a licensed pro

• The slab is large, carries a vehicle, or is a driveway or sidewalk connected to the street; heavier loads, the right-of-way and city rules make it a different job.
• The slab will be under a patio cover, attached to the house foundation, or might be enclosed as a room later, which brings building-permit and floor-slab rules into play.
• You find soft, wet, swelling or filled soil, old debris, or utility lines where the slab goes.
• The site does not drain, the slab would have to sit above the stucco weep screed, or water would have to go toward a wall or a neighbor's property.
• You need more than a small batch of concrete. Ready-mix has to be placed and finished quickly, and in Phoenix heat a pour can get away from a small crew.
• You need to saw-cut, break out or remove existing concrete and cannot do it with water-fed tools and a respirator.

Common questions