How to Pour a Concrete Patio

Plan the size, and check the permit

Lay the patio out with string lines and check the corners square with a 3-4-5 triangle, the method Quikrete's patio project guide uses. Keep the dimensions to whole joint panels if you can (see joints below), because a patio that divides evenly into near-square panels cracks less.

Permits vary from town to town more than almost anything else in concrete. Prince George's County, Maryland does not require a permit for an at-grade patio under 500 square feet. Warrenville, Illinois requires one for every patio. Lake Zurich, Illinois charges a permit fee and sets setbacks from property lines. Call yours before you dig.

Size also decides how you buy the concrete. A 12 by 16 foot patio at 4 inches is 64 cubic feet, about 2.4 cubic yards, which is roughly 107 bags of 80 lb mix. That is a ready-mix truck, not a weekend of mixing. The slab calculator does the arithmetic for any size, and what a concrete patio costs prices it.

Slope it away from the house

This is the step that matters most and gets skipped most. The residential code's lot grading section, IRC R401.3, requires impervious surfaces within 10 feet of the foundation to be "sloped not less than 2 percent away from the building," as Seattle's reprint of the code shows. The Department of Energy's Building America guidance gives the same figure in builder's terms: at least 1/4 inch per foot.

Watch for a conflict in the bag maker's own advice. Quikrete's patio guide says "a standard slope of 1/8" per ft," while its sidewalks and small slabs guide recommends 1/4 inch per foot. An eighth of an inch is about 1 percent, half what the code asks for next to a house. Use 1/4 inch per foot.

On a 12 foot deep patio that is 3 inches of fall from the house to the outer edge. Set the forms to that slope before anything else, because you cannot add it later. The slope calculator converts inches per foot to percent and works out the drop for any length.

Dig out and build the base

Strip the sod and topsoil. Quikrete says to take it "at least 6" beyond the mason's strings" so the forms sit on firm ground. Organic soil compresses and rots, so it all comes out.

Then 4 inches of compacted gravel. The residential code's slab-on-ground section, IRC R506.2.2, calls for a 4 inch base of clean graded sand, gravel or crushed stone under building slabs, and the patio handouts from Lake Zurich and Warrenville both require a 4 inch compacted gravel base for patios. Quikrete's method is to spread about 5 inches of compactible gravel and tamp it down to 4. Compact it in a plate compactor if you can rent one, and dampen it before the pour so it does not pull water out of the concrete. The gravel calculator works out the tonnage.

How thick, and what to reinforce it with

Four inches is the standard. Lake Zurich and Warrenville both set a 4 inch minimum for patios, and Quikrete's patio guide uses 3-1/2 inches, which is what a 2x4 form gives you. No model code governs patio thickness directly, and concrete slab thickness covers why 4 inches is convention rather than code. Go thicker under a hot tub or anything heavy; the hot tub slab guide covers that case.

Reinforcement on a patio is mostly about holding cracks tight, not adding strength. Quikrete says that fiber-reinforced crack-resistant concrete "eliminates the need for wire reinforcing mesh." If you use mesh or rebar, it has to sit in the slab, not on the gravel. Slab reinforcement covers the choices.

Where the patio meets the house

• Isolate it. Quikrete's patio guide says to glue "a 1/2"-thick piece of asphalt-impregnated fiber board to the adjoining structure," which creates an isolation joint. NRMCA's CIP 6 calls for isolation joints wherever a slab meets walls and footings, so the patio and the house can move separately.
• Stay below the siding. IRC R317.1 requires wood siding and framing within 2 inches of a concrete patio slab to be treated or protected, as Middleton, Idaho's code determination explains. Set the top of the patio at least 2 inches under untreated siding.
• Mind the door. A patio outside a door is the landing for that door. Under IRC R311.3, the landing at the main exit door can be no more than 1-1/2 inches below the threshold, with an exception for one step down where the door does not swing over it. If the patio would sit lower than that, plan a step: patio steps and single steps covers the rules.

Order the right concrete

NRMCA's CIP 2 on scaling is the guidance for outdoor flatwork: "For exterior slabs, order concrete with specified strength of 4000 psi," where slabs stay moist and see de-icers, and "Concrete should be air-entrained," at 6 percent air for normal 3/4 or 1 inch stone. The residential code's strength table, IRC Table R402.2, does not name patios. Its nearest row is porches, carport slabs and steps exposed to weather, which asks 3,000 to 3,500 psi with air entrainment in cold climates. That is our reading of where a patio fits, not a citation.

In a climate that freezes, order 4,000 psi with air entrainment. For a small patio in bags, what concrete to use for steps covers which bagged mixes are air-entrained. And do not be tempted by the no-mix shortcut on a patio; dry pour concrete explains why.

Pour, finish and joint it

Place the concrete, screed it off the forms, and bull float it. Then the rule most DIY patios break: wait. NRMCA's CIP 14 on finishing says to wait "until the concrete has stopped bleeding" before the next finishing step, and to "NEVER sprinkle water or cement on concrete to facilitate finishing." Working bleed water back into the surface makes a weak top layer that scales off in a winter or two.

Edge the perimeter for "durable rounded edges," then broom it. CIP 14 says to texture the surface "with a coarse or fine push-broom to give a non-slip surface," and CIP 2 says that for most exterior surfaces "a broom finish is adequate." Avoid steel-troweling air-entrained concrete to a slick finish; CIP 14 warns against it, and a slick patio is a slip hazard when wet.

Control joints. CIP 6 sets maximum joint spacing at "24 to 36 times the thickness of the slab," which is 8 to 12 feet for a 4 inch patio, with each panel's length no more than 1.5 times its width. Quikrete's small slabs guide is more cautious at 6 to 8 feet. Cut or tool them a quarter of the slab depth, and if you saw-cut, CIP 6 says to do it "within 4 to 12 hours" of finishing. A 12 by 16 patio jointed at 6 and 8 feet gives four 6 by 8 panels, well inside both rules.

Cure it, then keep the salt off

CIP 14 ends where the job really starts: "Cure the concrete as soon as all finishing is completed." Seven days of curing with plastic, water or a curing compound makes the difference between a patio surface that lasts and one that dusts and scales. Quikrete's patio guide finishes with an acrylic cure and seal once the sheen disappears. How to cure concrete compares the methods, and how long concrete takes to cure covers when you can put furniture and a grill on it.

If you pour in the fall, keep de-icers off the patio for the first winter. The first winter explains why and works out whether your pour date leaves enough drying time.