When building or renovating a driveway, one of the most important decisions is choosing the right sub-base material. The sub-base is the layer of material that sits underneath the top asphalt, concrete or paver surface of the driveway. A proper sub-base provides structural support, prevents shifting and settling, and allows for drainage. Selecting the right sub-base material for your specific driveway project and soil conditions is key to creating a durable, long-lasting driveway.
What are the different sub-base options?
There are several different types of aggregate materials commonly used for driveway sub-bases. The most popular options include:
- Crushed Stone – This refers to angular crushed rock, typically 3/4-inch or 1 1/2-inch size. Crushed stone has excellent compaction and drainage properties.
- Crushed Gravel – Rounder in shape than crushed stone, crushed gravel packs well and provides good drainage.
- Crushed Concrete – Recycled concrete that has been crushed and screened. It contains both angular and round particles for drainage and compaction.
- Washed Gravel – Clean gravel free of fines, this smooth gravel option compacts tightly.
- Pit Run Gravel – Also known as bank run gravel, this is machine crushed gravel with fines left in place for compaction.
- Road Base – A mix of crushed stone and fines used as a base for road paving, this packs tightly for stability.
- Sand – While not ideal, clean coarse sand can work for lightly used driveways over firm soils.
Many of these crushed stone materials are available in different sizes, such as 3/4-inch minus, 1 1/2-inch minus and 3-inch minus. The “minus” refers to the maximum particle size in the gravel. The smaller the minus size, the more finely graded the material is.
What factors determine the best sub-base?
Choosing the best sub-base type and depth for your specific driveway requires considering several important factors:
- Traffic Load – The heavier the vehicles using the driveway, the thicker the sub-base needs to be. Multiple heavy vehicles require a 6 to 8 inch sub-base depth.
- Soil Conditions – Weak, wet or clay soils demand a thicker sub-base for stability. Well-draining sandy or gravel soils may only need a 4 inch depth.
- Climate – In cold climates, the sub-base needs to extend below the frost line to prevent heaving. In hot climates, the base should deter vegetation growth.
- Drainage – Sub-base materials like crushed stone, gravel and road base promote drainage. This is critical for wet soils and low-lying areas.
- Cost – Materials like crushed concrete and road base tend to be the most economical sub-base options.
Additionally, the type of driveway surface material also impacts the sub-base requirements:
- Asphalt driveways require 4 to 8 inches of a crushed stone sub-base.
- Concrete driveways need a minimum of 4 inches of gravel, road base or other compactable fill.
- Paver driveways demand a 6 inch sub-base depth for stability, plus a 1 inch bedding layer.
What are the best sub-base materials for driveways?
Based on the key factors above, these are generally the best sub-base materials for driveways:
Angular crushed stone #57 or #67 is an excellent sub-base material. The angular rocks lock together when compacted to provide a stable base. Crushed stone is relatively affordable and provides reliable performance for most home driveway projects. Common sizes used are 3/4-inch minus and 1 1/2-inch minus.
Like crushed stone, crushed gravel provides excellent compaction and drainage properties. The rounded shape allows crushed gravel to pack together tightly. It can be a more economical alternative to crushed stone, with 1 inch minus being a common size.
This combination of crushed stone and fines is especially useful for clay soils. Road base packs firmly to resist settling and shifting. The fines help fill voids to achieve maximum density. Road base is often more affordable than other crushed stone. A modified road base with polymer additives can further improve stability.
Using recycled concrete rubble provides significant cost savings as a driveway sub-base. Most crushed concrete contains a blend of sizes from fines up to 1 1/2 inches. With angular and round particles, crushed concrete effectively resists movement when compacted. Just be aware that demolished concrete can sometimes contain debris.
For gravel driveways, a washed 1/2-inch or 3/4-inch gravel makes an excellent sub-base. Clean, smooth gravel compacts tightly for a firm base layer. This is a smart choice when using a gravel driving surface. Be sure to use a geotextile fabric between the sub-base and gravel surface.
What sub-base depth do I need?
The recommended sub-base depth depends on the factors discussed earlier, such as vehicle loads, soil type, climate and drainage. Here are typical sub-base depth guidelines:
- 4 inches for light-duty driveways over stable soils
- 6 inches for moderate traffic driveways
- 8+ inches for heavy vehicles and weak soils
- Extend base below the frost line in cold climates
When constructing the layers, it’s better to exceed minimum depths. A thinner sub-base is more prone to failure. An extra 2 inches of depth provides a factor of safety.
Asphalt driveways need a minimum 4-6 inch sub-base, preferably 8 inches for heavy vehicles. Use 6-12 inches over problematic soils. Extend 8 inches below frost in cold climates.
For concrete driveways, install at least 4 inches of crushed stone. Thicker layers up to 8 inches provide longer service life. Concrete driveways should have 6 inch depth over clay soils.
Paver manufacturers recommend installing pavers over a 6 inch sub-base depth. Use 8-12 inches for poorer drainage or heavier vehicles. Plus, pavers require a 1 inch bedding layer of sand or stone dust above sub-base.
Sub-base installation tips
Proper installation of the sub-base layer is just as important as material selection. Here are best practices for installing an effective driveway sub-base:
- Excavate soil to allow for thickness needed based on vehicles, soil and climate.
- Compact subgrade soil in 6 to 8 inch lifts to 95% standard proctor density.
- Use geotextile fabric to stabilize soft soils and separate sub-base.
- Install sub-base in 4 to 6 inch layers. Compact each layer to 98% density before adding more.
- Crown the sub-base to allow water runoff. Use a 1% cross slope.
- Extended the sub-base 2 feet wider than the driveway surface to provide support at the edges.
A plate compactor is needed to thoroughly compact each lift of sub-base material. Proper compaction prevents future settling issues. A smooth, uniform sub-base provides the ideal foundation for driveways.
Sub-base vs. base course
Sub-base materials are often confused with base course materials. What’s the difference?
The sub-base provides foundational support and drainage beneath the surface layer. Base course refers to the material directly below the driveway paving. It’s often the same aggregates as sub-base, but graded specifically to offer a smooth, compacted surface for paving.
For asphalt driveways, the common base course materials are:
- Crushed stone base (DTD) – 0 to 1 inch crushed stone
- Asphalt base – 0.5 inch maximum crushed RAP
Concrete driveways use a 4 inch aggregate base course. This usually consists of 3/4 inch minus crushed stone compacted over the sub-base.
Pavers require a thin 1 inch bedding layer of sand, stone dust or chips over the sub-base before installing the pavers. This helps level and fill voids beneath the pavers.
So while sub-base and base course often use the same type of aggregates, their gradation and purpose differs. The sub-base provides deep foundational support. Base course serves as the smooth, stable surface on which to pave.
Do I need geotextile fabric under sub-base?
Installing geotextile fabric beneath the sub-base is highly recommended for most driveway projects. Geotextile provides valuable functions:
- Separates and prevents sub-base from mixing with soft subgrade soils
- Stabilizes weak soils to prevent rutting under loads
- Allows water to drain while filtering sediment
- Prevents vegetation from growing up into the base
Without geotextile, fines from the subgrade can infiltrate the sub-base over time. This results in reduced stability. Geotextile acts as barrier to keep the layers intact. Look for Type I geotextile fabric with strength rated for roadway use.
When is geotextile fabric required?
Geotextile is strongly recommended for driveways under these conditions:
- Soft, wet, swampy or organic subgrade soils
- Fine soils like silt, clay or loess that are prone to mixing
- Any evidence of frost heave or soil movement
- Presence of a high groundwater table
- Use of sand or other unbound granular sub-base
Without fabric, sub-base can sink into weak soils. Geotextile provides shear strength to help reinforce the foundation beneath the driveway.
When is geotextile fabric not necessary?
Geotextile may not be required if:
- Constructing over very stable, free-draining soils
- A bound sub-base material like cement-treated base is used
- An asphalt or concrete edge restraint surrounds sub-base
For example, a gravel driveway over sandy gravel soils likely won’t benefit from fabric. Or if using rigid concrete or plastic edging, the restraint helps contain the base.
Sub-base vs. base: Which is more important?
For driveway construction, both the sub-base and base course layers are critically important to long-term performance. However, the sub-base provides some foundational benefits that make it the most crucial component.
Benefits of a high quality sub-base include:
- Distributes vehicle loads over a wider area, reducing pressure on subgrade
- Prevents frost heaving and pumping of fines up from soil
- Provides reliable drainage and reduces water impact on subgrade
- Helps compensate for weak or variable soil conditions
- Less prone to differential settlement or washout failure
In contrast, the base course primarily functions as the smooth, stable platform on which to pave. While the base course does provide some load distribution benefit, this is relatively minor compared to the sub-base depth.
In summary, an inadequate sub-base is much more likely to lead to major problems down the road. A sufficient sub-base can help compensate for minor deficiencies in the base course. Prioritizing sub-base depth and materials is wise for the long run.
Choosing the optimal driveway sub-base involves evaluating load demands, soil type, climate factors and costs. Crushed stone, gravel, road base and crushed concrete provide excellent stability when properly compacted. A sub-base depth of 6 to 8 inches suits most residential driveways. Use geotextile fabric to improve subgrade stability. A quality sub-base prevents shifting, extends pavement life and reduces repairs. Focusing on sub-base best practices will pay off for years to come.