Home Outdoors Yard & Garden Structures Retaining Wall

How to Choose Modular Concrete Block

Updated: Jun. 30, 2017

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Overview

Walk around most residential neighborhoods and you’ll find concrete block retaining and garden walls dotting the landscape. No longer just holding back earth, stabilizing slopes and preventing erosion, modern concrete block walls define pathways, create borders for outdoor “rooms” and serve as focal points in featureless yards.

Building a block wall is a significant investment of time (if you do it yourself) and money (whether you do it yourself or pay someone to do it). Shopping for modular block can be confusing because of the many colors, textures, shapes, styles and stacking systems available. This article will help you sort through your options and discuss what you need to know before you shop so you can buy the right block for your garden or retaining wall.

In a nutshell

• The more expensive the block, the more it will look like natural stone.
• Each block system will accommodate curves, steps, corners, caps and setbacks differently. Your wall’s requirements will help narrow your options.
• When you shop, take along a sketch of the wall’s height, length, radius of inside and outside curves, and specific features. Then you’ll have everything you need to choose the material and narrow down prices on one trip.
• Even if you plan to hire a pro, shop around to see actual walls rather than depending on contractors’ pictures to imagine how yours will look.
• Home centers carry some systems, but you’ll find the widest variety of options at landscape suppliers.

What’s the wall’s purpose?

Freestanding

Freestanding wall blocks are widely available at home centers, nurseries and landscape suppliers.

Large retaining walls

Heavy-duty walls require full-size blocks, which are available at landscape suppliers.

A freestanding garden wall 3 ft. or less in height that serves a more decorative function gives you more flexibility with the size, style and type of block you choose. Most manufacturers offer a wide variety of styles in two or three basic sizes. The garden wall size is the most common. These relatively small blocks (about 12 in. long x 4 in. high) are lightweight (less than 25 lbs.) and work best for accent walls around the yard and garden up to about 30 in. high. All four sides are finished and they have special blocks for columns, corners and ends.

Retaining walls, because they’re load-bearing and slope into a hill slightly, require larger blocks and beefier connections. These blocks are 16 to 18 in. long x 6 to 8 in. high and weigh 50 to 75 lbs. Big walls usually entail excavating and moving tons of soil and gravel as well as the heavy block itself. These walls can be daunting to do yourself, so think it through before you unknowingly dedicate a whole summer to the task. Most systems are engineered to handle heights up to 4 ft. Retaining walls above that height must be designed or approved by a licensed engineer.

What kind of look do you want?

Three-way split

The corners are split off the face of this block, leaving a highly textured, rounded surface. These are usually the best choice for tighter curves.

Straight face

When stacked, it has the classic look of chiseled stone. The uniform texture is a good backdrop for a garden or a good visual base to a house. Its muted appearance will look in style for years. Block size varies. In general, use small block for smaller walls.

Tumbled or weathered

The edges are rounded to soften its appearance and make it look more weathered and natural. Different length blocks can be mixed to further vary the look.

Mosaics or ashlars

This combination of tumbled and weathered block in different sizes and colors gives a wall a custom-fitted look. Best used for straight walls; curves require more building experience and block cutting. This type also requires more expert advice for curves, corners and stairs.

Do you want the block to blend with your property’s existing features or to inject something new into the landscape? Although specific shapes, colors and textures vary by region, almost every manufacturer produces four main styles (see photos).

What are your wall’s requirements?

Solid and semi-solid blocks

Solid and semi-solid blocks are heavy (up to 75 lbs.) but are the most versatile. You can simply split them to form 90-degree corners or any other angle, rather than ordering special blocks.

Hollow-core blocks

Hollow-core blocks are about half the weight of their solid counterparts and much easier on the back. After you set each course, you fill the cores with gravel, making the wall every bit as strong as a solid block wall. You have to order special blocks for corners and caps.

Lip systems (interlock or tongue-and-groove)

Lip systems (interlock or tongue-and-groove) are popular with DIYers since they go together fast and easy. However, they aren’t quite as versatile because you can’t vary the setback (the amount each course of wall steps back into the slope). The typical setback is 1-1/4 in. per row.

A pin or a clip system

A pin or a clip system, made from tough plastic or fiberglass, anchors each row of blocks to the ones below. These systems are fussier but will let you slide the block forward (no setback) if you need a more vertical wall (such as where a wall meets the corner of a house).

The details of your project will help you decide on a block system. Different block systems are suited to different project requirements. For example, all block systems have limits as to how tight a curve they can form without being cut. Systems also vary in how they handle corners, setbacks, cap blocks, columns and steps.

What’s your budget?

Add other costs

Although they can be expensive, a well-built wall will last as long as your house.

The cost of block is only a piece of the total project budget. If you’re using a contractor, you’ll obviously pay labor costs. A good rule of thumb for a contractor-built wall is to double the materials price—in other words, half labor, half materials. Make sure to agree on the front end whether your contractor will provide final grading and resodding (or will leave your yard a total disaster). Will the driveway be repaired? Wheel ruts fixed? Topsoil included? These details often account for why one bid is significantly higher or lower than another.

If you plan to build your own wall, beware. There’s a lot of heavy work, including tons of fill and footing material to get delivered and moved. Also consider the tools you’ll need to rent and other additional costs:

• Delivery charges for materials. Don’t wreck your back or your vehicle. Block is heavy, even the garden wall variety. Pay the extra charge to place your materials exactly where you want them.
• Skid steer rental ($200 per day). Keep in mind that using heavy machinery will damage your yard, so you need to add in the costs of grading and resodding as well.
• A block splitter costs $90 per day. You can also have the landscape yard split the blocks for you.
• Plate compactor rental ($50 per day). It’s crucial to compact your footing material or you’ll have a sagging, crooked wall in the future.
• Other materials such as “compactable gravel” for the base, drainage layer material and drain tile.

Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

• Level
• Line level
• Rubber mallet
• Safety glasses
• Spade
• Tape measure
• Wheelbarrow

You’ll also need leather gloves, a block splitter, plate compactor, skid steer, sod cutter, and (for large jobs) a skid steer.

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.

• Gravel
• Modular concrete block
• Sand