How Thick Should A Concrete Patio Be?

How thick should a concrete patio be?

The answer is that certain factors will influence the thickness of your concrete patio. To determine the depth of your slab, you will need to figure out what kind of load you will put on the slab.

Concrete Patio Thickness

Minimum standards for the construction of concrete patios will generally apply to ensure that safety is maintained. The minimum thickness of any concrete slab is 4 inches. However, if the concrete patio will have heavy features added upon it, you may want to thicken it to 6 or 8 inches. The concrete is typically laid on a 2-3 inch base of gravel, sand, limestone, or other aggregates.

The strong gravel base will prevent the concrete from shifting and cracking. Gravel allows water to drain and, when packed tightly, doesn’t shift beneath the concrete. A hand tamper or a plate compactor should be used to pack the subbase.

In most areas, large concrete slabs require approval from the city’s building or zoning department. Slabs are permanent structures, and as such, are subject to zoning restrictions. Local building codes may dictate the design’s elements, including the thicknesses of the gravel base and the slab, the type of concrete and its internal reinforcement, and the need for a moisture barrier under the slab.  

Your concrete slab can be the standard gray color or a highly decorative focal point of your backyard. You could opt for exposed aggregates or even try etching with an acid stain. You may also wish to use the concrete slab as a subbase for tiles or pavers. Be clear what you intend to do regarding the desired finish of your concrete patio.

Benefits Of Concrete

Concrete is one of the most economical and longest-lasting building materials. It is known for its high strength, durability, and low maintenance and provides a perfect patio base. Ready-mix is routinely used for poured concrete patio slabs. It is a crack-resistant cement that has a pre-blended mix of cement, sand, gravel, and additives. 

Generally, there is very little maintenance required for a concrete slab patio, and your patio can look renewed with a simple wash with a high-pressure water cleaner. Concrete patios are best suited to even grounds.

Bear in mind that laying a concrete patio slab as a DIY project can be challenging, with consideration needed to be given to level finishes that allow water to run off the surface.  

Ideally, concrete should be poured and finished during the early morning, before air temperatures and wind speed rise and the air humidity drops.  

What You Will Need

Equipment/ tools

  • Tape Measure
  • Hammer
  • A watering device with sprayer
  • Form boards
  • Wooden stakes
  • Line level
  • Carpenter’s level
  • A concrete mixer (if mixing yourself)
  • Plate compactor or hand tamper
  • Bull float
  • Shovel
  • Hose 
  • Broom – medium bristle
  • Bucket
  • Edging trowel

Materials You Will Need

  • Bags of concrete mix 
  • Lengths of timber for formwork
  • Timber stakes
  • Reinforcing wire mesh
  • Compatible Subbase material
  • Vegetable oil

How To Prepare The Patio Slab Site

Before you start, contact your local building department to see whether a permit is required and how close to lot lines you can build. In most cases, you’ll measure from the lot line to position the slab parallel to it. Make sure there are no pipes or services below the ground you are proposing to put your patio slab. The national call-before-you-dig number is 811. If there are any buried utilities, these can be marked with paint or flags so that you don’t accidentally dig into an underground utility line.

Measure The Slope

The slope from the house to the outside of the patio should be approximately 2 inches to allow for runoff. You may need to regrade the site with or without the help of an excavator if the ground isn’t level. 

Lay Out A Rough Project Outline

Drive stakes into the ground around the project area, attaching mason rings between each one. Using a level, make sure the strings run at a consistent height. Measure from the string to the ground with each stake and adjust the slope with some extra grading if necessary.

Excavate The Site

Dig a test hole to make sure the soil is suitable for your purpose. Make sure any soft, questionable, or organic material is removed and replaced by structural fill soil. Excavate the soil to a depth of 8 inches, allowing 4 inches for the subbase and 4 inches for the concrete slab. The land should be graded or sloped, so the excavation site’s center is slightly higher than the sides, which encourages water to drain from the future patio after the water flows through the gravel. Spray the soil with water just enough to settle the dust. Compact the soil tightly with a plate compactor or hand tamper.  

Set form boards in place along the perimeter. These can be two by 4-inch wood boards or flexible hardboard secured with wooden stakes. This boxing will hold the wet concrete in place until it cures.

Add A Subbase

By using something like ¾ inch rock as a base that has good draining qualities, you can be sure that the concrete will not crack. This subbase is then compacted. Spread a level, 2-inch layer across the soil, and check for levelness with a carpenter’s level. A gravel base with irregular size pieces locks together better than a base with round pieces. Gravel dust binds the stones further. Spread another 2-inch layer of grit base over the first 2-inch layer. Pack the second layer tightly. Check again for levelness and add extra gravel to low spots.

Place Reinforcing Mesh

If reinforcing mesh is required, cut it to fit inside the formwork allowing 2-inch clearance between the formwork and end of the bars so the reinforcing will be completely covered in concrete. Do not allow the reinforcement to sag between the supports or sit directly on the compacted base.

How To Pour A Concrete Slab

Be aware that cement-based products are alkaline and can cause burns to exposed skin or eyes, so wear protective gear.

Mixing

In a non-porous vessel, such as a wheelbarrow, add the concrete mix (2 bags at a time). Add water as per instructions, but remember that excess water ruins good concrete. Buying concrete in dry, premixed bags makes sense for relatively small patio slabs. A 50 square foot slab at 4 inches thick needs about 28 80-pound bags. For large slabs, consider ordering ready-mix concrete delivered by a concrete truck. Ready-mix is more expensive than bagged cement, but it’s far more convenient, and you don’t have the concern of getting the mix right.

Pour The Concrete Slab

Coat the interior surfaces of the form with oil so that the concrete won’t stick to them. Dump the mixed concrete inside the formwork in piles. Distribute the concrete with a shovel to fill the form.

Screed The Concrete

Use a long, straight 2×4 board to screed the top of the concrete. Rest the board on top of both sides of the form, and move the board back and forth in a sawing motion while pulling it backward. Use a shovel to remove excess concrete or to add concrete to fill in low spots. If the slab is larger than 8 feet in either dimension, cut control joints at 8-foot intervals, using a 2×4 board and Mason’s trowel. Position the board across the form, then follow the edge of the board with the trowel, slicing down into the concrete.

Finish The Concrete

It may be necessary to release any entrapped air bubbles though you could use a release agent that is added to the concrete during the mixing stage to help reduce air pockets from forming.

Let the concrete cure until the “bleed water” (moisture that rises to the top after screeding) disappears, then smooth the surface with a darby wood float. Allow the bleed water to disappear again.

Cure The Patio Slab

Keep the concrete moist while curing by misting the concrete with water each day for the next week or two.

Final Tasks

The wood form can be removed. Some backfilling around the patio slab may be required. The concrete can be sealed with a specific concrete sealant according to the manufacturer’s directions.