Retaining Wall F.A.Q.

How tall can I build an Allan Block wall?
Unreinforced Allan Block walls or gravity walls, can be built up to 5 ft high (1.5 m) in sandy soils and up to 3.5 (1.0 m) ft in clay soils. Follow the Allan Block Installation Guide for easy installation instructions. Masonry reinforcement works well for walls up to 15 feet high and geogrid reinforcement can handle walls in excess of 40 feet high. Always consult a qualified Allan Block engineer on jobs of this scale.
Do I need a concrete base for my retaining wall?
No. Allan Block walls are designed to sit on a compacted granular base. This saves time and money, while providing a solid base for the blocks. See the Allan Block Landscape Walls Guide for the recommended depth and width of base for your wall.
How long will my retaining wall last?
Standard concrete building blocks made in the 1950s using a similar process as Allan Block are still performing in building structures around the world. With today’s technological improvements, we believe Allan Block will last a lifetime. Check out our tips on caring for your retaining wall
Is Allan Block safe for the enviornment?
Yes. Allan Block is made using natural, raw materials. No harmful chemicals, such as those found in treated timbers or railroad ties, are used in manufacturing Allan Block. We are an environmentally friendly concrete product.
Can I cut a block?
Yes. Although a better question to ask is “how do I avoid cutting the blocks?” Good planning and design, and the use of compatible block sizes during construction not only save time, but also reduce the need to cut block. However, if necessary, Allan Block is just about as easy to cut as timber, with the right tools.

We recommend a masonry cut-off saw with a diamond-tipped blade. These saws operate like chain saws and the blades last. To split blocks yielding a rough cut, you can use a hammer or masonry saw and a chisel. Score the block with a shallow cut on the front and back and snap in two with light blows on the chisel. The break will be a little rougher than making a complete saw cut, but you can still get a good fit. A diamond blade power saw is typically used to give a clean cut.

How do Allan Block walls stack up?
Allan Block walls are dry-stacked, locking together without pins, clips or mortar. The unique front lip creates a built-in setback, allowing the blocks to automatically interlock as they are stacked. The benefits of the hollow core include built-in drainage and easy to handle and to stack blocks.
Why does Allan Block set back when stacked?
Allan Block sets back as it is stacked to improve wall stability and performance. It is designed with an automatic setback of 6 or 12 degrees of “lean” into the hillside. The Allan Block top lip and bottom notch combine to form the automatic setback – no measuring is necessary. Block setback is noted on all Allan Block publications.
What do I need to know when designing my wall?
Consider style and function as you initiate your landscape wall design. Review the Residential Retaining Wall Guide as you consider stairs, terraces, and finishing details.
How much does an Allan Block wall cost?
Though Allan Block costs vary depending on which block you chose, material costs for Allan Block are competitive with treated wood and stack rock, are less than lannon stone, boulders and other stackable masonry systems, and cost up to 30 percent less than walls requiring concrete footings. The 12-degree setback of an Allan Block wall means less reinforcement is needed compared to more vertical walls. Because Allan Block is so easy to handle and install, it is the most cost-effective retaining wall system available.

Contact your local AB Dealer for a price quote.

What should I know before starting my retaining wall?
Review information Allan Block Residential Retaining Wall as you consider soils, slopes, surcharges, block style and wall heights in the planning of your small retaining wall project.
How strong is an Allan Block wall?
Allan Block walls can be adapted to meet the structural requirements of any retaining wall site. Walls up to 5 ft high (1.5 m) in sandy soils and up to 3.5 ft (1.0 m) in clay soils typically can be installed without any pre-engineered reinforcement. Taller walls can be strengthened using conventional masonry techniques or the AB Reinforcement Grid method. Site and soil conditions determine the maximum height on any project.
What if my base has steps in it?
Try to start each wall at the lowest point and work up, stepping the blocks up as you build into the slope. It is better to step up the base row than to step it down.
Do I need a permit to do a retaining wall?
Contact your local building code department to see if a building permit for landscape retaining walls is required in your area.
Can I build an Allan Block wall in water?
Yes. The design flexibility of Allan Block makes it the perfect choice for waterfront applications, even in areas where freezing is a concern. Consult your Allan Block dealer or a landscape architect for special considerations when building in water.
Can I plant a garden or grass behind my wall?
Yes. Planting or landscaping in the block cores or behind the blocks is easy and environmentally safe. Allan Block does not leach any harmful chemicals that could affect your plants or shrubs. A drip irrigation system can even be built into the wall during construction.
Can I build a fence above my wall?
Yes. Fences can be built into the blocks hollow core or behind the wall. The wall may have to be reinforced to support the wind load from the fence. Consult your Allan Block dealer or a landscape architect for special considerations when building a fence.
How many block do I need for my wall?
The number of blocks needed depends on the unit size of the Allan Block you have chosen and the size of your project. Typically, you can determine this number by first multiplying wall height by wall length to get a square foot area. Next, divide the square foot area by the Allan Block unit size to get a rough estimate of the amount of blocks you will need. Quick estimating guides can be found in Allan Block Landscape Walls Guide, or you can contact your local Allan Block dealer. When estimating, be sure to include outside corners and caps, if they are required for your project. We also have an estimating tool and smartphone apps to help you!
How much do the block weigh?
Allan Block is available in a variety of different weights – from 30 pounds (14kg) to more than 75 pounds (34kg) per unit. Heavier blocks are typically designed for taller walls. Unique to Allan Block is the hollow core making it lighter and easier to lift and stack than traditional concrete block.
What tools do I need to build a wall?
Tools required to install Allan Block depend on the size of the job. Small walls may only require basic tools such as a shovel, rake, wheelbarrow, small sledgehammer, 3-inch chisel, level and hand tamper. Larger or more detailed walls may require additional equipment, such as a plate compactor and a diamond blade saw for cutting block. Discuss your project with your local Allan Block dealer for advice on tools and equipment or consult the tool list in the Allan Block Landscape Walls Guide.
What are my color and style choices?
From the natural antique appeal of AB Europa, to the marbled color or clean look of the AB Collection, Allan Block has colors and styles that complement any landscape. Check out our retaining wall products and choose the look that’s right for your project. Call your local AB Dealer to confirm colors and styles available in your area.
Can I build stairs into my wall?
es. Stairs can be designed and built using curves or corners in a number of different styles. Check out the Residential Retaining Wall Guide for detailed design and construction details.
Can I build a radius with Allan Block?
Minimum curve radiuses are based on the block size. find your radius size on the Radius Chart.
What is a terraced wall?
Terrace walls tame a slope by building two or more low walls rather than one tall wall. Once the lower wall is built, the next wall can be constructed, leaving an area in between the walls for landscaping or planting. Ask Your AB Dealer for design assistance and see Terrace Walls for more design and construction details.
How do I finish a wall?
Unique to Allan Block is the raised front lip on the block that provides a perfect edge for landscape mulch, decorative rock or AB capstones. Learn more about finishing options.
What is a gravity wall?
A gravity wall is a wall that relies on its own weight to hold up the soil behind it. The weight of the blocks and the granular fill in the block cores, combine to become the weight of the wall.
What is a reinforced wall?
A reinforced wall is a wall that uses layers of reinforcement grid to tie the blocks and soil behind the blocks together to form a larger and heavier wall.
When do I need to reinforce a wall?
The need for reinforcement depends on the weight the wall is holding back. See the pre-engineered “Soil Reinforcement Chart” to see if your wall needs reinforcement.
What is AB reinforcement grid?
AB Reinforcement Grid is a high strength polyester mesh that is coated with a protective black film and is conveniently available from AB Dealers. See the AB Grid for more information about AB Reinforcement Grid and its use.
Can I use lighting in my wall?
Yes. Lights built into the wall can provide a beautiful ambience to patios or add safety lighting along stairways. Use lighting fixtures designed for outdoor use and follow manufacturers instructions.
How do I find a certified Allan Block installer?
Us at Oregon Block and your local AB Dealer will be able to recommend a Certified Allan Block Contractor.
How do I get started?
After a quick review of your plan, the Residential Retaining Wall Guide, you should be ready to start building your wall. Here is a short checklist: Order materials, gather tools, prepare the site and layout the wall locations and dig the base trench (starting from the lowest point). Follow the step-by-step installation guide and don’t hesitate to call your Allan Block Dealer should you have any questions or need some additional help.
What base material should be used on my wall?
A good base material is a crushed stone ¼” to ¾” in size. This material compacts well and makes it easy to level the base block.
Is compacting the base important?
Yes. A compacted base provides a solid foundation to build on and will not allow water to saturate the base material leading to a potential weakening of the base.
Is leveling the base important?
Yes. The base blocks need to be leveled back to front and side-to-side. If the base isn’t level it will continue to get worse the higher you go.
Why should I bury part of the base course?
Partial burying the base block reduces the risk of erosion and potential undermining of the wall due the base being exposed. Bury a minimum of 1 inch per foot of wall height.
Do I need to stack running bond?
No. “Running bond” refers to a pattern where the seam between any two blocks falls exactly on the center of the block below it. Perfect running bond is not essential for Allan Block walls. We recommend a minimum average offset of 3 inches. You can insert half blocks as needed to help maintain some staggering.
What should I use to fill behind the wall?
We recommend 1/4″ to 1-1/2″ clean gravel. This allows for optimum drainage to reduce pressures on the wall. If a gravel has to many “fines” it can slow or stop drainage creating a potential for wall failure. Ask your local dealer what types of clean gravel they offer.
How high can I go before backfilling?
Backfill every row of block you stack – and check your wall for level at the same time. Backfilling more than one row at a time may cause the wall to lean forward. Make sure to compact every layer of block as well with a walk behind vibratory plate compactor.
How do I use AB reinforcement grid?
AB Reinforcement Grid simply rolls out along the wall. Follow the installation instructions printed on every roll of grid.
How do I build corners?
Outside corners use the Allan Block 90 degree corner units. AB corners come in a pairs of a left and a right hand unit. As the left and right hand units are alternately stacked they form a strong interlock.

nside corners are formed using standard Allan Block units. Part of the top lip on the blocks is removed so the blocks can alternately over lap on each other to form a strong interlock. See the Residential Retaining Wall Guide for details.

How do I make a corner cap?
Corner caps are typically miter cut with a saw on the job. Some Allan Block Dealers may offer cutting services or pre-cut corner caps for the convenience of their customers. (Allan Block does not manufacture a “corner cap unit”) learn more on how to cut AB capstones.
Do I need an engineer?
Generally, Allan Block recommends the use of a licensed engineer on projects with taller walls or unusual site conditions. Local building codes may require the use of a licensed engineer for walls above a certain height. Our design charts and tables provide generalized engineering solutions. Each retaining wall site is unique, however, and specific evaluation will provide the most accurate solution.