Sub Base For Concrete Driveway: High Quality Soil Support System

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Does a Concrete Driveway Need a Base?

Sub Base for Concrete Driveway is the middle layer of the “soil support system”. A good quality sub-base helps in distributing the load evenly, preventing localized soil subsidence, facilitating drainage and reducing risk of frost heave.

A concrete driveway is essentially a slab-on-grade which rests on the “soil support system” below it. The soil support system consists of three layers; subgrade, sub-base and base. A well compacted sub-base using high quality aggregates results in a concrete driveway that lasts much longer and needs much less maintenance.

Soil Support System

Subgrade	This is the existing soil. The soil may have been improved by the addition of fresh soil. The soil layer is compacted.
Sub Base	Sub base is several layers of aggregates on top of the subgrade. The aggregate must be of irregular shape and different sizes. The bigger aggregates go at the bottom and the smaller aggregates stay at the top. The sub base is compacted at each stage.
Base	Base consists of layers of gravel and sand over the sub base. This is smoothed out & compacted.

The quality of the soil support system determines the load bearing capacity of concrete driveway and has a significant effect on the life of the driveway.

How Do You Prepare the Ground for a Concrete Driveway?

Mark out the perimeter of the driveway by driving stakes along the length and breadth of the driveway and tying a string. Remove the topsoil within this marked rectangle to a depth that is equal to the concrete driveway thickness and the base thickness. 

Typically this will be 10 to 12 inches; 4 to 6 inches as concrete thickness and 6 to 8 inches as base thickness.

Fill up any low areas, smooth out and tamp the surface using a hand or mechanical tamper. The soil should be firm enough for you to walk on without leaving a footprint.

This is the subgrade.

Lay a geotextile on the subgrade. This will prevent the aggregates, that form a part of the sub-base, from getting lost in the grade when it moves under hydrostatic or seismic stress. 

RECOMMENDED PRODUCT

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The Best Geotextile Driveway Fabric for Your Driveway & Patio Base

Geotextile driveway fabric is a supportive layer that is laid between the compacted subgrade and the sub-base aggregates to prevent the migration and loss of the aggregates in to the subgrade. It helps stabilize the ground, permit drainage and prevent weed growth.

Here is the Geotextile Driveway Fabric I recommend.

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Next put layers of compactible gravel, crushed stone or recycled concrete for providing the strength to the base. Layer the sub-base material such that the biggest pieces are in the bottom most layer. The layers must be compacted at each stage for a strong and solid sub-base. 

Build up around 4 to 6 inches of this sub-base.

Lastly add a couple of inches of fine gravel & sand and compact it. This layer is the base and will facilitate drainage.

A vapor barrier should be laid over the base before wire mesh or rebars are placed on chairs. The vapor barrier will prevent the concrete slab, above it, from absorbing any moisture from the ground below.

Does Sub Base for Concrete Driveway Need a Permit?

Sub base for concrete driveway may or may not require a permit depending on the local building laws applicable. Each local building authority has it’s own set of rules and specifications regarding residential construction.

In case a permission is required, then the local building authority will also have a list of minimum specifications that must be complied with. The specification for driveways, especially concrete driveways, will almost always have specifications for the base.

Here are a few examples:

Example from OHIO

“The driveway thickness shall be a minimum four inches on a four-inch base of approved tamped fill base or undisturbed earth.”

Specifications from ILLINOIS

• Black dirt shall be dug out a minimum of 6 inches for asphalt driveways and 9 inches for concrete driveways. Stone shall not be placed on black dirt. 
• The site may be inspected before any stone is installed. A second inspection shall be made after a minimum of 4 inches of stone mix is compacted and wire mesh is installed for a concrete driveway or 5 inches of stone mix is compacted for an asphalt driveway. Stone: road mix (CA6) or ¾ / 1 inch stone. 
• Concrete driveways shall be a minimum of 5 inches thick. aAsphalt driveways shall be a minimum of 3 inches thick. 
• Under no circumstances shall a new driveway be laid over an existing driveway. 
• Asphalt aprons are permitted providing that the concrete curb and gutter remain undisturbed. Under no circumstances shall asphalt be laid over an existing curb or gutter to provide a ramp. If the curb or gutter is removed it must be replaced with concrete, per village requirement. 

Example from FLORIDA

• The general building code requirements for residential driveways are:
• For an average family car only; Concrete: five inches Sub- base: six inches.
• For heavier vehicles, such as a delivery truck; Concrete:6 inches Sub-base six inches.

Bottom Line

• To determine whether you need a permit to construct a concrete driveway you need to contact the local building authority. 
• In case permission is required they will have the minimum specifications for a concrete driveway documented.
• If a base is specified, then of course you must comply with the local law. 
• Even if the base is not specified it is better to lay your concrete driveway on a good quality base.
• A good quality base under a concrete driveway will increase its life and reduce  maintenance costs.

Thank you very much for reading this post. I do hope you found it informative and helpful.