How To Set Stone Slabs

Stone slabs offer a durable, attractive surface for patios, walkways, and steps. They’re heavy and hard to cut, so setting them is usually not a DIY project. However, you might still be curious about the process. This Old House expert landscaper Roger Cook and stonemason Roger Hopkins explain how to choose a stone slab and what professionals do to get them in place.

Understanding Stone Slab Installation

Your landscape contractor will help you choose a perfect stone for your project and location. Consider the following factors as you investigate each choice:

• Durability: The slabs should be durable enough to withstand your local climate and intended use. Areas with freezing and thawing cycles can cause some stones to break down over time.
• Size: The slabs should suit your design and be manageable for installation. Large slabs can be impressive, but they’re also heavier and more challenging to position, potentially increasing labor costs.
• Thickness: Thicker slabs are generally more stable, but that comes at a price of a lot more weight. A thickness of at least 2 inches is perfect for most outdoor projects.
• Texture: Consider the stone’s surface for both aesthetics and safety, especially in wet conditions. A rougher texture can prevent slipping but might be harder to clean.

Preparing the Site for Slab Installation

First, the mason will prepare your site for a stable, level slab installation. They’ll remove grass and topsoil, excavating the area down to the depth of the slab plus six inches for a base layer. Then, they’ll tamp the soil and add about 4 inches of compacted crushed stone or gravel. 

The mason should use a long, straight board and level to confirm the site is even. They can add or remove material as necessary to even out the hole at the base layer.

Without proper drainage, pooling water can cause erosion at the site and affect the finished project’s stability. The slabs should have a slight slope away from any buildings, about 1/8 inch per foot, to prevent water from pooling around them. Workers may also connect the edges of the slab installation to lower areas or drainage systems.

Cutting and Shaping Stone Slabs

Landscaping plans often require stone slabs to be cut or shaped to fit a design. Your installation crew will have the right skills and tools to handle this step without damaging the stone.

Making Cuts

Hopkins begins to cut a slab using a tracer chisel and sledgehammer to score a line on the stone. This makes a weak point for controlled splitting, similar to how a glass cutter works. 

Completing the cut is where it becomes more complex. Hopkins drills holes along the scored line with a pneumatic drill, gently inserting feathers and wedges into the holes as he goes. “You don’t try to go that heavy at first,” he says. “That 20-pound hammer is probably giving you about 10,000 pounds of force.” When he gradually taps the wedges with the hammer, he listens for a change in sound as a sign the stone is splitting.

Some professionals use a pneumatic hammer with a carbide-tipped chisel to texture the cut edge of the stone. This process helps the cut edge blend with the slab’s natural surfaces and improves the stone’s grip.

Advanced Cutting Techniques

In some cases, especially for complex designs, advanced cutting techniques may be required. A few other cutting tools that professionals can use include these:

• Wet saw: Wet saws are useful for precise straight cuts.
• Angle grinder: Angle grinders are ideal for small, detailed cuts.
• Circular saw with a diamond blade: These saws can be efficient for larger, straight cuts.

Setting the Stone Slabs

With your site prepared and stones cut to size, the crew can start setting the slabs in place. 

Leveling and Adjusting

Installers will start placing the slabs using a forklift at a fixed point, such as against a structure or at the lowest point for drainage. Once the first slab is level and at the correct height, they’ll keep placing slabs and make sure the gaps between them are consistent.

If you have more than one stone slab installed, the team may use a long level to double-check their work and keep each piece level with the others.

Securing the Slabs

The team may use a bonding agent between the slab and base for added stability, particularly on steps or raised areas. Metal pins and stakes also keep the slabs stable along corners and edges.

Stone Slab Finishing Touches

Once your slabs are set, there are a few final steps to complete the installation.

Filling Gaps

Workers fill the gaps between slabs with one of the following materials:

• Fine gravel: Gravel adds a natural look and promotes drainage.
• Polymeric sand: Polymeric sand hardens when it gets wet, cutting down on weed growth and ant farms.
• Stone dust: Stone dust compacts well and is easy to sweep into gaps.

Sealing

Sealers can protect your stone slabs from stains and weather damage. Choose a sealer according to your type of stone and apply it according to the manufacturer’s instructions. 

Edge Restraints

Edge restraints made from metal, plastic, or stone can stop the stone slabs from shifting over time. Metal edge restraints are durable and adjustable, while plastic ones are more flexible and easy to install. You may also find stone edge restraints that provide a seamless look with the slabs.

Stone Slab Maintenance and Care

Proper maintenance will ensure your stone slab installation remains beautiful and functional for a long time. Follow these tips:

• Clean the slabs: Sweep each slab regularly to remove debris, and wash with mild soap and water as necessary. Don’t use harsh cleaners that could damage the stone.
• Check for shifting: Look for loose and shifting slabs, and weed growth between slabs.
• Refill gaps: It’s not unusual for filler material to settle or wash away. You can refill the gaps if necessary to keep things stable.
• Address cracks promptly: Small cracks can quickly spiral into larger, sometimes avoidable problems if they’re left unaddressed. You can often repair minor damage to the slabs using stone adhesive. Patching compounds can do the trick for moderate to severe damage.
• Reseal the slabs: If you sealed your stone slabs, you’ll likely need to reseal them every 2–3 years to keep the protective barrier.