Pouring Concrete Patios: 3-Inch Stone Base Sufficient?

Pouring a concrete patio is not a simple task and requires careful planning, labour, and the right tools and materials. A concrete patio begins with 4 inches of compacted stone, with an additional 4 inches for the concrete slab. This means you need to dig out a space 8 inches deep. A gravel base is also necessary in areas of poor drainage or freezing temperatures. The gravel base should be 4 to 6 inches thick and compacted with a rented compacting machine. The concrete patio should be at least 4 inches thick, with 6 to 8 inches required if the patio will support heavy furniture or structures.

Excavation and site preparation

Mark the Area

Use stakes and string to outline the perimeter of your patio. This visual guide will help keep your excavation accurate and ensure you dig in the right place. Drive 3-foot stakes into the ground, 1 foot outside the corners of the planned patio area.

Excavate the Site

The depth of excavation depends on your local climate and soil conditions. In regions with freezing temperatures and poorly draining soil, excavate to a depth of at least 12 inches. In milder climates with well-draining soil, 6 inches may be sufficient. Use a shovel or a mini-excavator to remove the soil, ensuring you dig down to the required depth evenly. Make sure to remove any sod, grass, weeds, and roots from the area. You can use a tiller for larger areas and a shovel or hoe for cutting vegetation at the base. Spray the area with a non-toxic herbicide to prevent regrowth.

Compacting the Soil

Compacting the soil is critical to preventing future settling and shifting of your patio. Use a tamper to compact the soil after excavation. This step is important for creating a stable foundation for your patio.

Levelling the Surface

Use a rake to smooth the surface and a level tool to check for evenness. Adjust the surface by adding or removing soil until you achieve a level base.

Adding a Base Layer

After excavation, lay a base layer of crushed stone or gravel. This layer typically ranges from 4 to 6 inches in depth and helps with drainage and stability. Spread the crushed stone evenly and use a plate compactor to compact it. If your patio will support heavy furniture or structures like a gazebo or hot tub, consider a thicker base layer of 6 to 8 inches.

Adding a Sand Layer

On top of the compacted base layer, add a 1-inch layer of coarse sand or stone dust. This layer creates a smooth surface for laying your concrete and allows for minor adjustments during levelling.

Creating a Slope for Drainage

To ensure proper drainage, your patio should slope away from your house at a rate of 1/8 to 1/4 inch per foot. Use a builder's level to establish this slope during excavation. This slight grade will prevent water from pooling on your patio or flowing towards your home's foundation.

Assembling Forms

Join 2x6 or 2x8 lumber to form the outer shape of your patio. These forms will hold the wet concrete in place and determine the final shape of your patio. Use deck screws to join the corners and pound stakes into the ground at each corner to hold the forms in place.

Levelling and Straightening the Forms

Use a level to ensure the forms are level and straight. Determine the highest point you want for your patio and adjust the form height accordingly. Drive a deck screw through the stake and into the form to lock it in position. Use a laser level to ensure that the entire perimeter of the form is level.

Adding Reinforcement

Reinforcement is important to minimise cracking in your concrete patio. You can use reinforcing steel mesh or 1/2-inch rebar on a 12-inch by 16-inch grid. Place the reinforcement on the bottom of the form and lift it up during the concrete pour so that it is suspended 2 inches from the bottom.

Calculating Concrete Quantity

Use the following formula to determine the volume of concrete required: (Thickness x width x length of the patio in feet) divided by 27 = Concrete volume required in cubic yards. Order slightly more concrete than you need to account for any errors or unexpected requirements.

Concrete form assembly

• Prepare the job site: Before assembling the forms, ensure you have acquired all necessary permits and identified the location of any underground plumbing or wiring. Mark the four corners of the desired slab area by driving stakes into the ground. Use a tape measure or laser distance measurer to record the lengths of each side, ensuring the angles are correct. Run string lines from the stakes to create an outline. Check if the ground slopes and flatten the site as needed.
• Select appropriate materials: Choose straight, flat lumber with minimal knots for the form framework. For a concrete slab up to 4 inches thick, use 1 x 4-inch wooden sheathing boards. For load-bearing slabs like driveways, use 2 x 6-inch boards for added support. You will also need stakes, deck screws or nails, and a release agent or sealant for the wood.
• Assemble the forms: Join the lumber to form the outer shape of the patio. For a rectangular patio, join the corners with deck screws (three screws per corner). Ensure opposite sides are of equal length. Pound a stake into the ground at each corner to hold the form in place.
• Square and straighten the form: Equalize diagonal measurements taken from corner to corner to ensure the form is square. Use strings along the sides of the form as a reference line to straighten the form, as lumber may not always be straight. Place additional stakes every 32 inches along the outside face of the form to hold the edges straight.
• Level the form: Determine the desired height of the patio at each corner and adjust the form height accordingly. Use a builder's level to ensure the entire perimeter of the form is level. Drive deck screws through the stakes and into the form to lock it in position. Double-check that the top edges of all form boards are level.
• Saw the stakes: Cut the stakes flush with the top edges of the form using a handsaw. This step is necessary for levelling the concrete later in the process.
• Add crushed stone or gravel: Shovel and level crushed stone or gravel within the form to create a uniform base for the concrete. This layer also aids in water drainage. Rake the material to ensure a consistent depth from the top edge of the form boards.
• Reinforce the form: Add reinforcement to minimise patio cracking. You can use reinforcing steel mesh or 1/2-inch rebar on a 12-inch by 16-inch grid. Place the reinforcement on the bottom of the form and lift it up during the concrete pour so that it is suspended two inches from the bottom.
• Final checks: Before pouring the concrete, double-check that the form is securely braced and reinforced. Ensure the form has watertight joints to prevent concrete leakage. Treat the wood with a release agent or sealant to prevent the concrete from sticking to the form.

Concrete pouring

Pouring a concrete patio is not a simple task and requires careful planning, labour and the right tools and materials. Here is a step-by-step guide to pouring concrete for a patio:

Preparation

Before you begin, ensure you have the right tools and materials. These include a pressure washer, safety goggles, a large wheelbarrow, an electric concrete mixer, a bull float, a magnesium float, tools to create the concrete form, a broom, a brush, an edger, a finishing trowel, gloves, buckets, strong heavy-duty tools, 2x4 lumber, 3" screws, compactible gravel, vegetable oil or a commercial release agent, concrete mix or crack-resistant concrete mix, acrylic cure and seal.

Marking and Excavating

Use spray paint, batterboards, a mason's line or strings and stakes to mark out your patio. Make this area 18 inches longer and wider than the finished size of the patio you want. You will need this room for the forms and the space required for working around them.

Excavate the area using a shovel or a rented mini-excavator to dig up the sod and topsoil. Dig 6-8 inches below the current soil level.

Assembling and Levelling Forms

Join 2x6 or 2x8 lumber to form the outer shape of your patio. Screw the corners together using #10 x 3-1/2-inch deck screws. Nail the first form in place with duplex nails and brace it with a 3-pound sledgehammer. Nail the rest of the stakes to the form board in the same way. Cut the stakes so they are flush with or slightly below the tops of the forms.

Adding Stone/Gravel

Shovel and level crushed stone or gravel to make the bottom of the excavation more uniform and to create a space for water drainage. Rake the gravel so it sits consistently 4 inches down from the top edge of the form boards.

Adding Reinforcement

Reinforce the concrete with 6x6-10/10 wire mesh or 1/2-inch rebar on a 12-inch x 16-inch grid. Place the mesh or rebar on the bottom of the form, then lift it up during the pour so the reinforcement is 2 inches off the bottom.

Mixing and Pouring Concrete

Calculate the amount of concrete you will need using the following formula: (Thickness x width and length of patio in feet) divided by 27 = concrete volume required in cubic yards.

If your patio is small, you can buy bags of just-add-water concrete mix. Add this to a portable drum-type concrete mixer with water to create a uniform blend with the consistency of stiff cake batter.

For larger patios, you can save money by buying Portland cement, sand and crushed stone separately and mixing them yourself in a ratio of one part cement, two parts sand and two parts crushed stone.

Alternatively, you can order ready-mixed concrete to be delivered by truck, although this is the most costly option. Concrete reinforcing fibres will make your patio more durable and crack-resistant.

Levelling Concrete

Fill the forms with concrete and use a straight piece of wood drawn across the top edges of the form to smooth and level the concrete. Leave the concrete alone until the surface water dries but it is still soft, then immediately continue smoothing the surface with a trowel.

Finishing

If you want a rough, high-friction surface, use a broom to texture the concrete before it hardens. Keep the surface damp with a garden hose or wet sheets and blankets.

Curing

Let the concrete cure for at least three days, then remove the deck screws, pull the stakes and remove the lumber. Backfill around the patio with topsoil to fill in the space left by the removal of the forms.

Crack Control Cuts

Even with reinforcement, concrete slabs tend to crack over time. Make crack control cuts to direct this cracking along a single line, so the results look better. Let the concrete cure for two weeks, then use a masonry cut-off saw to create cuts 1-1/2-inch to 2 inches down in a grid pattern.

Important Considerations

• The minimum thickness for a concrete patio is 4 inches. If the patio will support a structure such as a gazebo or hot tub, the slab should be 6-8 inches thick.
• Concrete should be poured on a gravel or crushed stone base to prevent erosion and slab settling.
• Concrete must be mixed correctly to avoid issues such as shrinkage and cracking.
• Concrete finishing requires good timing and technique.

Concrete finishing

Prepare the Work Area:

Mark the desired area for your concrete patio, making it 18 inches longer and wider than the finished size. This extra space will be needed for the forms and working area. Excavate the marked area, digging up to six to eight inches below the current soil level. For larger areas, you may want to rent a mini-excavator.

Assemble and Level Forms:

Join wooden boards (2x6 or 2x8 lumber) to create the outer shape of your patio. Secure the corners with deck screws. Ensure the form is square by equalizing diagonal measurements taken from corner to corner. Pound stakes into the ground at each corner to hold the form in place.

Straighten and Level the Form:

Use strings along the sides of the form as a straight reference line and place stakes every 36 inches. Determine the desired height for your patio and level the form accordingly, using a level tool. Saw the stakes flush with the top edges of the form.

Add Crushed Stone:

Shovel and level crushed stone to create a uniform base for the concrete and to facilitate water drainage. The depth of the crushed stone will determine the thickness of your concrete patio.

Add Reinforcement:

Reinforcement is crucial to minimise cracking in your concrete patio. You can use reinforcing steel mesh or 1/2-inch rebar on a 12-inch by 16-inch grid. Place the reinforcement on the bottom of the form and lift it up during the concrete pour so that it sits about two inches from the bottom.

Prepare the Concrete Mix:

Calculate the required amount of concrete using the formula: (Thickness x width x length of the patio in feet) divided by 27 = concrete volume in cubic yards. You can mix the concrete yourself using a portable cement mixer, or order ready-mixed concrete. Remember to add concrete reinforcing fibres to improve durability.

Pour and Level the Concrete:

Fill the forms with concrete, using a straight piece of wood to smooth and level the surface. Let the surface water dry, then continue smoothing with a trowel.

Allow Bleed Water to Evaporate:

As the concrete settles, bleed water will rise to the surface. This is normal, but it must be allowed to evaporate before proceeding. The evaporation time will vary depending on temperature and weather conditions.

Round the Edges:

Use an edging tool to create smooth, rounded edges around the perimeter of the slab. A small ridge should form at the inner edge, ensuring the edge is level with the rest of the slab.

Create Expansion Joints:

Concrete naturally cracks, so it's important to create controlled expansion joints. Place a straight board across the forms and use a grooving tool to create joints that are about a quarter of the slab's thickness. For a 4-inch slab, joints should be 8 to 12 feet apart.

Perform Final Smoothing:

Use a magnesium float to smooth the surface, bringing the "cream" of the concrete to the top. Be careful not to overdo this step, as it can weaken the slab.

Add a Broom Finish (Optional):

For added traction and a slip-resistant surface, use a broom to texture the concrete before it hardens. Make straight, even passes with the broom, without pushing down too hard.

Let the Concrete Cure:

Curing is a critical process that can take up to 30 days. Keep the concrete damp during the first week by spraying it with water a few times a day. After curing, seal the concrete to protect it from stains and other damage.

Remember to wear protective gear, including eye protection, long sleeves, and rubber boots when working with wet concrete.

Concrete curing

Timing

• 24 to 48 hours after the initial set, forms can be removed, and people can walk on the surface.
• 7 days after partial curing, the concrete can withstand traffic from vehicles and equipment.
• 28 days—at this point, the concrete should be fully cured.

Techniques

There are several techniques for curing concrete, which can be categorised into three groups:

Maintaining Moisture:

• Ponding and Immersion: This method involves flooding flat surfaces like pavements and floors with water. It is ideal for preventing moisture loss and maintaining a uniform temperature.
• Spraying and Fogging: Spraying or fogging water through nozzles or sprayers is useful in warm, non-freezing temperatures and low humidity conditions. It helps raise the relative humidity, slowing evaporation.
• Saturated Wet Coverings: Wet coverings such as burlap, cotton mats, or rugs are placed on the concrete and kept constantly wet to maintain a film of water on the surface during the curing process.
• Left-in-place Forms: These provide protection against moisture loss for formed concrete surfaces. Wooden forms should be used, especially in hot and dry weather, to retain moisture.

Reducing Loss of Water:

• Impervious Paper or Plastic Sheets: Lightweight, effective moisture retarders like polyethylene film are applied to the concrete surface to reduce evaporation.
• Membrane-Forming Curing Compounds: These are clear or white-pigmented compounds (for sunny weather) that are sprayed onto the concrete to form a film and slow dehydration.

Accelerating Concrete Strength Gain:

• Steam Curing: Live steam or high-pressure steam is used to accelerate the curing process.
• Heating Coils: Embedded heating elements near the surface of concrete protect it from freezing during cold-weather concreting.
• Electrical Heated Forms or Pads: Precast concrete producers use these to accelerate curing.
• Concrete Insulation Blankets: These are used to cover and insulate concrete surfaces subjected to freezing temperatures during curing.

Additional Tips

• The ideal curing temperature is 20°F or 68°F.
• Concrete should not be allowed to dry too quickly, as this can cause cracks.
• Curing compounds can be used to form a film on the concrete surface, slowing dehydration. These can dissipate on their own or be removed after 28 days.
• Concrete should be protected from freezing temperatures during curing.
• Curing techniques can be combined based on the availability of materials, the size and shape of the concrete, and economic considerations.

Common Mistakes to Avoid

• Not curing concrete: Curing is often overlooked, but it is essential to prevent shrinkage cracks and ensure the concrete gains strength and durability.
• Not maintaining proper temperature and moisture levels: Concrete that is too cold or too hot can lead to hydration issues and cracking.
• Not curing at the right time: Curing must begin as soon as possible after finishing and stamping the concrete.
• Overworking the concrete: Overworking concrete before bleed water has risen to the surface can trap too much water, weakening the slab and causing cracks.
• Not using proper forms: Concrete forms must be strong enough to hold the wet concrete and create a smooth, even pour.
• Not taking weather into account: Extremely hot, cold, or rainy weather can negatively impact the curing process and the strength of the concrete.

By following these instructions and avoiding common mistakes, you can ensure that your concrete patio cures properly, resulting in a durable and long-lasting surface.

Frequently asked questions