How to Build a Retaining Wall

Our sample project shows the construction of a very simple four-course retaining wall, such as you might build to flatten a slope to create a small patio or walkway. Your own retaining wall might be shorter or taller, longer or narrower, depending on your needs. Adjust your material quantities as necessary.

1. Level the Ground

   • Measure and lay out the position of your retaining wall on the building site.
   • With a flat-blade shovel, remove soil and level the area where the retaining wall will be built. In some cases, this will involve major excavation as you cut back into the hillside to establish a flat space, while in other cases you will create just a narrow flat trench where the bottom row of blocks will sit. Strive for a flat, level, and compact surface for the bottom row of retaining wall blocks to rest.
   • Use a short 2x4 as a screed, scraping off soil using a sawing motion to create a level surface.
   • Use a long 2x4 with a level on it to check a wide surface to make sure it is level.

2. Lay the Base

   A retaining wall will be most stable if it is built over a porous base. The porous layer will help with drainage on the completed wall and will also make it easier to level the first row of blocks.

   • Spread a 2-inch layer of sand or gravel over the area where the first row of blocks will be laid.
   • Use a long 2x4 and level to smooth and even out the base layer.
   • Retaining walls taller than four blocks high will generally require the base to be compacted with a hand tamper before proceeding.

3. Set the First Block

   • Beginning at one end of the wall, set the first block into the base layer.
   • Press it firmly down, though not so hard that you squeeze the base layer away.
   • Use a level to check this first block for the level in both directions: from side to side and from front to back.

4. Complete the First Course of Blocks

   • Lay the next adjoining block, and again use a 2x4 and level to make sure it is flat and perfectly aligned with the first block. Aligning from block to block in this first row is very important; as you continue to add courses of blocks upward, any differences between lower blocks will be transmitted to upper courses, often in disastrous ways. 
   • Continue with subsequent blocks to complete the first course of the wall.
   • Check after each block to make sure the row remains level and aligned with your desired layout. Retaining wall blocks have angled sides that allow for gently curved walls if you desire.

5. Set the Next Course

   Subsequent rows of blocks are installed using a "running bond" brickwork pattern, in which the seams on adjoining courses are offset by one-half block. This type of pattern will be considerably stronger and more stable than vertically aligned seams (known in the trades as "stack bond").

   Install all full blocks for the second course before cutting and fitting the half-width end blocks on the ends.

6. Cut the Half Blocks

   The half blocks at the end of alternating rows can be cut using a masonry chisel and hammer. Retaining wall blocks have a V-notch groove on the backside to allow the blocks to easily cleave in half.

   • Stand the block upright, face down, then position the tip of a broad masonry chisel into the groove.
   • Strike sharply onto the end of the chisel to split the block into two pieces.
   • You may need to strike several times for the block to break.
   • A hand maul is the best tool for hitting the chisel, but a framing hammer will also work.

   Tip

   If your chisel does not have a cushioned end, it's a good idea to wear hearing protectors, as the sound of a hammer striking a bare metal chisel can be quite loud.

7. Set the Half Blocks

   At the end of each alternating course, use half blocks to complete the row. Where possible, face the rough cleaved surface outward, so it faces into the adjoining soil.

8. Check Each Course for Level

   As each row is completed, double-check for level.

   • For a short wall, it's sufficient to use a 2- or 4-foot carpenter's level to check two or three adjacent blocks at a time.
   • For long walls, the best way to check is to drive stakes at the end of each wall, then use a long string with a string level attached to create a reference line as you install each course of blocks.

9. Check the Back Side of the Wall

   • When building a retaining wall, you need to provide ample space between the back of the blocks and the earth behind the wall. This space is also necessary to provide an area for backfilling with porous sand or gravel. As each course is completed, check this space.
   • You may need to excavate additional soil to accommodate the increased setback of the blocks as the wall grows higher.

   Tip

   In areas that experience frequent heavy rains, you can run a porous drain tube along the bottom of the backfilled trench, routing it to an exit point somewhere to the side of the retaining wall. This allows excessive water behind the wall to safely drain away through the perforated drain tube without putting dangerous stress on the wall. In regions with frequent heavy rain, this may be a feature required by building codes.

10. Backfill Behind the Wall

    • When the wall reaches its completed height, carefully fill in the space behind the wall using sand or gravel. It is very important to use a porous material for backfill, as it will allow water to easily drain away, lessening the weight and force placed on the wall during heavy rains. Ordinary soil can be far too dense, creating an enormous weight that puts dangerous stress on the wall.
    • The wider the backfill area with gravel or sand, the better your wall will drain. If you wish, you can leave the drainage backfill slightly lower than the top of the wall, then fill the remaining space with topsoil for planting grass or other plants.
    • Portion out the backfill material in small amounts to let the material settle. If you shovel in too much at once, you may create hollow spaces in the backfill that can compromise the wall's stability.
    • Occasionally rake or shift the sand with the shovel to coax the backfill material into settling. The more compact the backfill, the more stable your wall will be.

    Tip

    Landscape professionals sometimes cover the back surface of the wall with landscape fabric before beginning to backfill. This fabric will prevent sand from seeping through the cracks between the blocks.

    
    

When to Call a Professional

A concrete block retaining wall under 3 feet tall can be a good DIY project, but any taller wall requires the services of retainer wall contractors. A professional has the knowledge, experience, and equipment to handle the enormous labor necessary to complete the job. A professional will also understand local building codes and if your area is especially prone to erosion and runoff problems. If you have an existing retaining wall that needs repair because it is leaning or caving in, a professional will be better equipped to understand why the fail happened and how to best solve the issue.