Boulder Retaining Wall: Everything You Should Know!

A retaining wall composed of boulders can furnish a framework and clearness to your landscape architecture. Differing from a retaining wall comprised of concrete blocks, a boulder retaining wall preserves an innate appearance that integrates rather than confronts other open-air attributes.

The durability of a boulder or other rock retaining wall, if properly constructed, should surpass one century or more. These walls are among the most enduring structures in landscaping, persisting long after the destruction of homes and barns. Various of these time-honored, large rock retaining walls continue to be in service today. One of the oldest conserved stacked rock retaining walls is Hadrian’s Wall, constructed in 122 C.E.

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Boulder Retaining Wall

Retaining walls made of boulders or large rocks may be a viable option for landscaping at your domicile. This comprehensive article will enlighten you on all the essential aspects of boulder or large rock retaining walls.

What is Boulder Retaining Wall?

To begin with, a Boulder Retaining Wall entails the use of large rocks or boulders that are distinct from other stones. These boulders are defined as rocks larger than 10 inches ( 25.6 cm) in’ diameter by experts. Boulders make one of the finest building materials due to their solidity and resistance to weather erosion. Additionally, they can endure substantial pressure from the soil  behind the wall and minor earth movements.

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Boulder Retaining Wall
Boulder Retaining Wall

Boulder Retaining Walls: Advantages and Disadvantages

Utilizing boulders or massive rocks as building materials for retaining walls possess both positive and negative aspects. However, various factors hinder some homeowners from opting for this type of material.

Boulder Retaining Wall


Visual allure – Boulders have an intrinsic beauty and innate elegance that result in visually pleasing retaining walls. The organic design style of these walls allows them to blend harmoniously with the surrounding environment.

Durability – The size and composition of boulders make them invulnerable to erosion caused by natural elements such as wind and rain.

Versatility – No two boulders are identical, so you can fashion distinctive and unparalleled retaining walls that stand out from the rest.

Maintenance – A retaining wall made of rocks necessitates minimal maintenance if properly constructed in the beginning.

Cost – Depending on your location, building a large rock retaining wall can be an economical alternative to other styles of retaining walls built with boulders. The cost of boulders varies according to location and may be less expensive in places where natural boulders are readily available.

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Space – The massive size of retaining wall boulders may be unsuitable for small yards as they require ample space. Construction of boulder retaining walls often involves the use of heavy-duty equipment that necessitates wide entrances to the yard.

Height – The construction type of boulder retaining walls limits the maximum height of these walls. For tall retaining walls, a concrete retaining wall is a better choice.

Construction – Building retaining walls using boulders of substantial size may not be a feasible DIY project as it requires lifting heavy rocks and building the wall with expertise and specialized equipment.

Cost – Living in areas where boulders are scarce increases the cost of acquiring them. Using large boulders necessitates professional expertise and specialized equipment, thereby raising the cost of building the retaining wall.

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Construction Method for Boulder Retaining Walls

Boulder Retaining Wall

Boulder retaining walls can be constructed in two main ways: dry-stacking or wet-laid with mortar, with an option to combine the two methods. However, some contractors may be unfamiliar with one or the other. Therefore, it is crucial to discuss this matter with them beforehand to ensure that they are on the same page as you in terms of which method they prefer and why.

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Dry Stack (Gravity) Method

The dry stack or gravity method is a widely used and easy way to construct a rock retaining wall, relying on the weight of the boulders to retain the wall’s shape. This method necessitates rocks stacked in an interlocking pattern, achieved’ by utilizing specific rock contours, rock weight, and frictional interaction. If you’re using heavy boulders and the soil is moist, this method is a good option.

The cracks between the rocks serve as an innate drainage system that allows water to seep through and relieve the pressure that would build up behind the wall. This innate drainage system will maintain the integrity of the wall structure and increase its lifespan. However, make sure to pack small rocks rather than soil behind the wall for better drainage, especially if you live in an area with heavy rainfall or wet soil. Using the dry stack method can be challenging as it requires time, patience, and expertise to find rocks that fit together in a seamless and sturdy manner.

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It isreally essential to discuss the construction method with your contractor before beginning the project to ensure that both parties are on the same page. The contractor’s familiarity with the method will determine the success &durability of the boulder retaining wall.

Wet Laid Mortar Method

When constructing a retaining wall with boulders, contractors have the option of utilizing the Wet-Laid (Mortar) Method. This technique involves the use of mortar or cement to provide extra strength to the wall. The Wet-Laid method is ideal for those residing in dry environments, as there is little concern regarding moisture buildup in the soil. However, if one dwells in a moist environment, they must incorporate drainage to the wall.

In order to prevent heavy moist soil from putting pressure on the wall, retaining walls require drainage stone behind them. Depending on the amount of water present in the environment, drainage pipes or holes may also be necessary to release the pressure building up behind the wall. In this method, the type of mortar utilized is crucial. Type “M” or “S” mortar should be utilized for this project, with type “M” being the strongest due to its high concrete content. As it mimics the strength of the stone itself, type “M” is best suited for use with stone. Type “S” is appropriate for smaller boulder walls and can withstand excessive moisture better than type “M”.

Combination Method

Another method of constructing a retaining wall with boulders is the Combination Method. This technique utilizes a combination of both methods to achieve a natural look with increased strength. Mortar is used along the rear of the wall in this method, creating the illusion of a dry-stacked wall in the front. Furthermore, pigmented mortar can be employed to match the stones and reduce the visibility of mortar lines throughout the wall.

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Rocks for Boulder Retaining Walls

When it comes to creating retaining walls using boulders, there are various types of rocks available to choose from, such as granite, sandstone, quartz, limestone, basalt, and bluestone. Landscapers commonly use three types of boulders for this purpose: fieldstone boulders, quarry boulders, and water-washed boulders. Both fieldstone and quarry boulders possess sharp edges, flat planes, and non-symmetrical divots.

Boulder Retaining Wall

To construct boulder retaining walls, landscapers generally use the following types of boulders:

Fieldstone Boulders – These boulders can be found on the surface of the earth and possess innate characteristics and age. They often display the presence of moss and dirt, as they have been exposed to the elements of nature.

Quarry Boulders – These rocks are excavated from the ground and are comparatively younger than fieldstone boulders. These boulders are easier to manipulate due to their fewer angles, making them ideal for those inexperienced in DIY projects. They also create more stable walls.

River Rock – These rocks are found in rivers and have been subjected to water erosion over time. The continuous water flow smooths their edges and gives them a round shape. They come in various sizes, with some experts classifying certain ones as boulders. If you’re aiming to create a wall with rounded stones, river boulders are the best choice.

Please note that these aforementioned rocks are widely used for constructing retaining walls, and choosing the right one depends on several factors, including the wall’s intended use, location, and the designer’s preference.

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Additional Tips while constructing Boulder Retaining Walls

Boulder Retaining Wall

While considering’ the construction of a boulder retaining wall, it is important to keep certain factors in mind. To ensure that you are well-informed before investing in such a wall, take into account the following tips:

Wall Hue – The natural coloring of boulder retaining walls can vary greatly, spanning hues such as off-white, pink, orange, gray, black, and reddish. The wall’s final appearance will be significantly influenced by its coloration.

Wall Configuration – Large rock retaining walls typically come in two main types of configurations. The first is a modular configuration, in which similarly-sized and -shaped boulders are utilized. The second is a random configuration, which employs boulders of diverse sizes and shapes. The latter type of wall requires more time, planning, and skill to execute.

Permits – Most retaining walls that exceed 4 feet in height require a permit.

Professional Assistance – Given the size &weight of the boulders involved, constructing a large boulder retaining wall can be quite difficult for the average do-it-yourselfer. It may also be necessary to enlist the aid of a licensed contractor for the project’s completion.

The Expense of a Boulder Retaining Wall

Boulder Retaining Wall

As with all things, the expenditure of a boulder retaining wall fluctuates depending on numerous factors. According to HomeGuide, the range for the price of a rock wall is between $2,000-$10,000. On average, the cost for a rock retaining wall is roughly $6,000. Boulders are valued at around $50-$200 per linear foot (including labor and materials) or $100-$350 per ton for only the boulders. The price for a large boulder retaining wall will place a heavier strain on your budget, totaling $600 per ton.

Elements that Influence the Expense/Cost of a Boulder Retaining Wall

Remember to take into account the following additional factors that will impact the cost of the finished retaining wall.

Magnitude and Blueprint – The size and height of the wall will have an effect on the cost as more materials and labor will be needed. The intricacy of the wall’s blueprint will also impact the cost. A plain wall will be cheaper than one with bends, columns, and steps.

Footings and Drainage – Higher walls require deeper footings. Most mortared walls will require some type of drainage, which will increase the cost.

Wage Expenses – Count on paying a minimum of $40-$80 per hour for labor or $10-$30 per square foot.

Engineered Plans – Expect to employ a structural engineer to draft plans for your wall if it is over 4 feet high.

Intense Construction – Depending on your location, you may need to clear the ground, remove old walls, or level the site. This will necessitate the use of heavy machinery and operators.

Geogrid – Geogrid is a pliable mesh that builders put behind rock walls. This mesh aids in stabilizing the soil behind the wall. It is beneficial if the soil is sandy and fine, but it is not always necessary to use Geogrid. Its use can raise labor costs up to 50%.

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Boulder Retaining Wall Ideas

Here are some inspirational ideas for boulders/rock retaining walls:

Variegated Boulder Retaining Wall:

Boulder Retaining Wall

This design incorporates boulders of different colors and sizes to create a wall that blends seamlessly with its surroundings. The addition of plants and flowers softens the overall look of the wall.

Dry Stacked Flat Boulder Retaining Wall:

Boulder Retaining Wall

This design uses the dry-stack method to create a wall that features a random pattern of boulders in varying sizes.The boulders used in this wall have deep orange hues that are common’ in fieldstone.

Mortared Stone Retaining Wall:

Boulder Retaining Wall

This design features smaller, round boulders mixed with small stones to create a varied and interesting stone’ wall.

Boulder Retaining Wall for Seating:

Boulder Retaining Wall

This design adds extra seating to your outdoor space by creating a curved boulder retaining wall around a fire pit.Use boulders that are of similar size and flat on the top for comfortable seating.

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Mixed Material Retaining Wall:-

Boulder Retaining Wall
Boulder Retaining Wall

Boulders can work well with other materials. This design combines flat, dry-stacked boulders with custom weathered steel to create a unique & visually interesting wall.

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