Lava Rocks For Patio Concrete Base: Pros And Cons

Lava rock, also known as volcanic rock, is a popular material for patio base projects. It is lightweight, porous, and comes in a variety of colours, making it an aesthetically pleasing choice for landscaping projects. It is also durable, cost-effective, and easy to install. However, some people have raised concerns about its strength and stability for construction purposes. While lava rocks are used in construction in some places, they are not suitable for certain structural applications. One alternative is to use lava concrete, which has been used since Roman times and offers superior thermal and sound insulation.

Lava rock as a base for concrete slabs

Lava rock, or volcanic rock, is a lightweight, porous material formed from the cooling and solidification of lava. It is a popular choice for landscaping projects due to its durability and aesthetic appeal. When considering its use as a base for concrete slabs, several benefits and considerations come into play.

Benefits of Using Lava Rock as a Base

Lava rock provides a solid and stable foundation for concrete slabs. Its porous nature also aids in effective drainage, preventing water from pooling on patios or other surfaces. Additionally, lava rock is a sustainable and cost-effective option. As a natural and renewable resource, it does not require mining or processing, making it environmentally friendly. It is also generally less expensive than other base materials such as crushed stone or gravel, and its lightweight properties can reduce labour and transportation costs.

Types of Lava Rock

Basalt, pumice, and scoria are the primary types of lava rock used in landscaping. Basalt is a dense and durable rock, often chosen for its strength. Pumice, on the other hand, is light and porous, making it ideal for water drainage. Scoria, also known as lava rock, stands out for its vibrant red or black colour, making it a popular choice for enhancing the look of outdoor spaces. Each type of lava rock has unique properties that should be considered based on the specific requirements of your project.

Preparation and Installation

When using lava rock as a base for concrete slabs, proper preparation is essential. First, a layer of landscape fabric should be laid down to prevent weeds from growing through the rocks. The lava rock should then be evenly spread and compacted to create a firm and stable base. For larger projects, a power compactor can be used to achieve adequate compaction. Additionally, filling in the voids in the lava rock with stone dust can improve stability and reduce shifting.

Considerations and Limitations

While lava rock is a durable material, it is essential to consider its limitations. Lava rock can be brittle and prone to breakage, so it may not be suitable for structural applications. For concrete slabs, ensuring proper compaction and adding a layer of sand on top of the lava rock base can enhance its performance. Additionally, the depth of the base is crucial. It should extend below the frost line to prevent heaving and potential damage to the concrete during freezing temperatures.

In conclusion, lava rock can be a viable option for a base under concrete slabs, offering benefits such as durability, drainage, sustainability, and cost-effectiveness. However, proper preparation, compaction, and consideration of the specific type of lava rock and project requirements are necessary to ensure a successful outcome.

Pros and cons of using lava rock for patios

Lava rock is a unique and aesthetically pleasing option for patios, but is it the right choice for your outdoor space? Here are some pros and cons to help you decide.

Pros

One of the biggest advantages of using lava rock for your patio is its durability. As a natural stone, lava rock can withstand harsh weather conditions and will not degrade or decompose over time, making it a long-lasting solution for your outdoor space.

Lava rock is also a sustainable and cost-effective option. It is a natural, renewable resource that does not require mining or processing, making it eco-friendly and generally less expensive than other patio base materials such as crushed stone or gravel. Plus, its lightweight and porous nature makes it easier to work with, saving you time and labour costs during installation.

The insulating properties of lava rock are another benefit. Its rough, porous surface enables it to quickly absorb and retain both moisture and heat. This helps to maintain consistent soil and plant temperatures, benefiting any surrounding plant life.

Cons

One of the main drawbacks of lava rock is its tendency to float and blow away, particularly if it is not properly secured. This can create a mess and result in an uneven patio surface over time. To avoid this, it is recommended to lay the lava rock over a layer of landscape fabric and secure it with an edging material to avoid displacement, especially in areas with heavy foot traffic.

Another consideration is the removal process. While lava rock can be a more permanent solution than mulch, it may be difficult and time-consuming to remove or replace in the future. This is something to keep in mind if you plan on redoing your patio or landscaping in the future.

Lastly, while lava rock can provide a unique, natural look to your patio, it may not be to everyone's taste. Some people prefer the more natural appearance of mulch or other landscaping materials. It is important to consider the overall aesthetic you wish to achieve and whether lava rock aligns with your vision.

In conclusion, lava rock can be a durable, cost-effective, and visually appealing option for patios, but it also has some potential drawbacks. Be sure to weigh the pros and cons before making a decision, and remember to consider the specific needs and requirements of your project.

Types of volcanic rock for landscaping

Volcanic rock, also known as lava rock, is a lightweight, porous, and aesthetically pleasing landscaping material. It is formed from the rapid cooling of magma or lava, resulting in a porous surface due to escaping gases. This process also leads to the inclusion of elements like silica or calcium carbonate, making it more porous than common stone types like granite.

Lava rocks are durable, attractive, and easy to use. They are often black or dark grey due to iron oxide but can also be vibrant reds, oranges, and yellows. The two main types of lava rock are:

Basalt

Basalt is a dense, hard, and dark-coloured rock with high compressive strength. It is suitable for heavy-duty landscaping projects such as pathways or retaining walls. Basalt is also known for its strength and durability, making it ideal for building patios, walkways, and flower beds.

Pumice

Pumice is a light and porous rock that is ideal for water drainage. It is commonly used in aquascaping due to its non-reactive properties. Pumice is also used to create a vibrant, eye-catching element in landscapes, particularly when paired with green plants.

Another type of volcanic rock, scoria, is also mentioned as a variety of lava rock. It is characterised by its vibrant red or black colour and is often chosen for its aesthetic appeal.

Lava rocks are available in a range of shapes, from rounded to jagged, and can be used alone or in combination with other materials to create unique landscapes. They are a great alternative to organic mulches, providing excellent drainage and weed prevention without the need for yearly replacement.

How to prepare the ground for a patio

Preparing the ground for a patio is a relatively simple process, but it requires careful planning and execution to ensure a sturdy and durable foundation. Here is a step-by-step guide on how to prepare the ground for a patio:

Planning

Before beginning any physical work, it is essential to plan the location and design of your patio. Draw a detailed plan to scale on graph paper, including relevant measurements and nearby features such as your house and fencing. Decide on the placement of your slabs, whether you prefer uniform rows or a pattern of different-sized slabs. Consider the type of paving slab material you want to use, such as porcelain, sandstone, limestone, or concrete, taking into account factors like budget, maintenance, and durability.

Clearing the Area

Once you have decided on the location of your patio, remove any existing structures or vegetation, such as turf or weeds. Cut the turf into strips and remove them, saving some to fill in gaps between the lawn and the patio later if desired.

Marking the Area

Use wooden pegs and a builder's line or string to mark off the area for your patio. Ensure the corners are square using a set square. As a final check, measure the diagonals, which should be the same length. Adjust the edges as necessary until the marked area is perfectly square.

Calculating the Fall

Calculate the necessary fall for your patio, which is a gentle gradient in the sub-base that allows for proper drainage and ensures water runs away from nearby structures, such as your house. The ideal gradient for a non-textured slab is 1:60 (a 16mm drop in level per metre), while for a textured slab, it is 1:80 (around a 12.5mm drop per metre). Multiply the ideal fall by the length of the patio's sloping side to determine the total difference in height between the highest and lowest points.

Digging

Now, you need to calculate the depth to dig. The depth will depend on the thickness of your patio slabs and the construction of your sub-layer. As a reference, a total depth of 175mm is recommended to accommodate a 100mm sub-base layer and 50mm thick paving slabs. Remember to include the necessary fall in your calculations.

Pegging the Sub-Base

Hammer wooden pegs into the ground, marking the finished depth of the hardcore layer from the top. Place these pegs 75-100cm apart across the highest side of your site, ensuring the marked line is level with the earth. Repeat this process for subsequent rows, maintaining the necessary fall.

Adding the Hardcore Mix

Add your chosen hardcore mix (e.g. MOT Type 1 or Hardcore) to reach the top of the pegs. For added stability, it is recommended to add the hardcore in two 50mm layers. Add the first 50mm layer and rake it over to level it, then compact it using a wacker plate or a tamper. Repeat this process for the second layer.

Adding the Slablayer

Apply a 25mm layer of Slablayer on top of the compacted hardcore mix and rake it out. Use a spray gun to dampen the Slablayer, and then rake it again to level the surface.

Laying the Patio Slabs

The first paving slab, also known as the 'key slab', is the most crucial one as it provides a guide for the rest. Position it correctly, starting at the corner of the highest point of the patio. Dampen the underside of the slab and gently tap it down with a rubber mallet to bed it into place. You can use a wood block to protect the slab if needed.

Securing the Slabs

After positioning the key slab, lay the remaining slabs following a sequence of your choice. Remember to leave joint gaps between the slabs, using spacers to maintain consistent spacing. Check the levels regularly with a spirit level to ensure the slabs are sitting flat and that the fall is accurate.

Filling the Joints

Finally, fill the gaps between the paving slabs with a concrete mix or mortar. For small joints (less than 5mm wide), use a soft-bristle brush to sweep kiln-dried sand into the gaps. For medium joints (5-10mm wide), use a dry mortar mix of one part sand to one part cement. For large joints (more than 10mm wide), use a wet mortar mix of four parts sand to one part cement, smoothing it with a trowel.

Allow your patio to set for at least 24 hours before use, and consider applying a sealant to protect against staining and algae buildup, especially if you've used concrete slabs. Regular maintenance, such as brushing and washing, will also help prevent algae buildup and keep your patio looking its best.

The history of lava concrete

Volcanic or lava concrete has been around since Roman times and has been used in many parts of the world. The Romans used a mixture of volcanic ash, lime, tiny volcanic rocks, and water to create concrete structures that have lasted for millennia. This type of concrete is distinct from traditional concrete in terms of workability, performance, and applications.

Lava concrete is a lightweight semi-fluid that can support itself vertically and can be efficiently cast in place. It is created using the same three core ingredients as regular concrete: aggregate, water, and cement. However, it is blended to become highly aerated and "fluffy", sacrificing strength and durability for insulation and sound absorption. The key advantage of lava concrete is its superior thermal and sound insulation performance, which is inherent to the material without the need for additional insulation or structural additives.

The use of lava concrete allows for structural innovation and integration, such as retaining and buttressed walls, curves, arches, and transitional shapes. It is also cost-effective for creating non-traditional details, such as integrated cast fireplaces, niches, light sconces, and window/door frame details.

The colour of lava concrete varies depending on the source of the volcanic aggregate. Natural scoria aggregates range from red to black, softening to plums or greys in the concrete material, while natural pumice ranges from medium to light grey. The texture of lava concrete can also be varied based on the size of the aggregate used. Fine-grained lava concrete can be milled to a smooth finish, while gravel-sized lava concrete can be left as-is for a pebble texture.

Overall, the history of lava concrete is deeply rooted in Roman times and has evolved to become a versatile and innovative building material that offers superior performance and aesthetic qualities.

Frequently asked questions