What Is the Best Mortar for Flagstone Joints?

Flagstone is a popular choice for creating beautiful and durable outdoor spaces. Whether used for patios, pathways, or retaining walls, the success of any flagstone project relies on the mortar used to hold the stones together. When it comes to choosing the best mortar for flagstone joints, Type M mortar is often considered the top choice. Known for it’s exceptional strength, Type M mortar is made by combining Portland cement and sand at a ratio of 3 parts sand to 1 part cement. This mixture, when mixed with water, produces a cement with a remarkable compressive strength of 2500 pounds per square inch (PSI).

What Kind of Sand Do You Put Between Flagstone?

When it comes to choosing the best mortar for flagstone joints, there are two main options to consider: polymeric sand or stone dust. Polymeric sand is a specially formulated sand with an acrylic binder added. It first gained popularity around fifteen years ago and has since become a common landscape product. The acrylic binder in polymeric sand helps to bind the sand particles together, creating a more stable and durable joint filler between the flagstones.

On the other hand, stone dust, also known as screenings or rock dust, is a fine gravel that’s often used as a base material for flagstone installations. It’s made up of finely crushed rock particles and is an affordable and readily available option for filling joints between flagstones. Stone dust can be swept into the joints and compacted to create a solid base, which helps to prevent shifting and settling of the flagstones over time.

Both polymeric sand and stone dust have their own benefits and drawbacks. Polymeric sand offers a stronger and more stable joint, as the acrylic binder helps to lock the sand particles together, reducing weed growth and erosion. It also has a more polished and finished appearance.

Stone dust, on the other hand, is a more affordable option and can be easily installed by homeowners. It allows for more natural drainage and can be easily refreshed or topped up if needed.

Consulting with a professional landscaper or contractor can also help in determining the best mortar for your specific flagstone project.

When it comes to filling the joints between flagstones, there are various options available depending on the type of base. While sand or groundcover can be used for joints over a dry bed, mortaring is often the preferred choice for a seamless and sturdy surface. Similarly, joints over a concrete base are typically mortared as well.

Can You Mortar Flagstone Joints?

When it comes to filling the joints between flagstones, there are various options to consider. One popular option is to use sand, which can be a quick and easy solution for creating a natural look. By simply sweeping sand into the joints, you can achieve a rustic appearance while still allowing for some water permeability. Another option is to plant groundcover between the flagstones, which provides a lush and green contrast to the stone surface. This can help create a more natural and organic look for your outdoor space.

One crucial factor to consider is the climate in which the flagstone will be installed. If you live in an area with freeze-thaw cycles, it’s important to choose a mortar that can withstand these conditions. Look for a mortar that’s specifically designed for outdoor use and is resistant to cracking and deterioration due to fluctuating temperatures.

By choosing a high-quality mortar that’s suitable for your specific project needs, you can ensure a long-lasting and visually appealing result. Whether you opt for sand, groundcover, or mortar, the key is to select the method that best suits your aesthetic preferences and functional requirements.


It’s exceptional strength, achieved through a precise mixture of Portland cement, sand, and water, ensures a robust foundation for flagstone and building stone in retaining walls. With it’s ability to withstand significant pressure, Type M mortar offers durability and longevity. Thus, it becomes the ideal choice for those seeking to securely set flagstone and achieve a sturdy and reliable construction for years to come.

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