How to Install Flagstone with Open-Graded Base

February 19, 2025 Andrew Strautman

How to Install Flagstone Using an Open-Graded Base

Flagstone is a popular choice for patios, walkways, and outdoor living areas due to its natural beauty and durability. Using an open-graded base for installation provides excellent drainage and stability, making it an ideal method for Sonoma County’s climate. In this guide, we’ll walk you through the step-by-step process of installing flagstone using an open-graded base.

Why Choose an Open-Graded Base?

An open-graded base consists of crushed stone with little to no fine particles, allowing water to drain efficiently through the layers. This method reduces the risk of heaving from freeze-thaw cycles and prevents water from pooling under the flagstone. It also provides a strong foundation without compacting over time.

Materials & Tools Needed

Flagstone
3/4" clean crushed stone (for base)
3/8" clean crushed stone (for leveling layer)
Landscape fabric
Polymeric sand or gravel for joints
Plate compactor
Rake
Shovel
Wheelbarrow
Level
Rubber mallet
Broom

Step-by-Step Installation

1. Plan and Mark the Area

Determine the area where you want to install the flagstone. Use spray paint, string lines, or stakes to outline the space. Consider factors like drainage, slope, and functionality.

2. Excavate the Site

Dig to a depth of about 5-7 inches below the final surface level. The depth may vary depending on the thickness of your flagstone, but the goal is to ensure a solid foundation.

3. Install Landscape Fabric

Lay down high-quality landscape fabric to prevent soil migration while allowing water to drain through. This step is crucial for maintaining the integrity of the open-graded base.

4. Add the Open-Graded Base Layer

Pour 3/4" clean crushed stone into the excavated area to create the base layer. Spread it evenly to a depth of 3-4 inches, then compact it using a plate compactor. This layer provides structural support while allowing water to drain freely.

5. Apply the Leveling Layer

Spread 3/8" clean crushed stone over the compacted base to create a leveling layer. This layer should be about 1-2 inches thick and act as the setting bed for the flagstone. Rake it smooth to create a stable, even surface.

6. Lay the Flagstone

Place the flagstones carefully on the leveling layer, ensuring each stone is stable and sits flush with adjacent pieces. Adjust stones as needed using a rubber mallet to ensure an even surface.

7. Fill the Joints

For a natural look, use polymeric sand to fill the joints between the flagstones. Sweep the material into the gaps and lightly mist with water if using polymeric sand to activate the binding agent.

8. Final Compaction and Finishing Touches

Gently compact the stones by walking over them or using a plate compactor with a rubber pad. Sweep the surface clean and mist the area lightly to settle the joint material.

Maintaining Your Flagstone Installation

Regularly sweep debris to prevent dirt buildup.
Replenish joint material as needed to maintain stability.
Check for shifting stones and adjust them if necessary.
Keep drainage pathways clear to prevent water from pooling.

Why Choose Sonoma Outdoor Escapes for Flagstone Installation?

At Sonoma Outdoor Escapes, we specialize in professional flagstone installations using open-graded bases for long-lasting durability and proper drainage. Whether you're creating a flagstone patio, walkway, or garden path, our team ensures a seamless and visually appealing result that complements your outdoor space.