How to build a retaining wall

Is this a wall you should build yourself?

Most retaining walls homeowners build are short segmental walls: interlocking concrete blocks stacked on a gravel base, with drainage stone behind. That is the wall this guide covers. Before the first shovel, two numbers decide whether it is a weekend project or an engineering project.

• Height, measured from the bottom of the footing. The model building code exempts walls up to 4 feet measured from the bottom of the footing to the top of the wall^[1]. The buried part counts.
• Surcharge. The same exemption disappears if the wall supports a surcharge^[1], any load beyond level ground behind it. Lake Stevens, Washington, defines it as any load in addition to level grade within the area defined by a 45-degree angle from the bottom of the footing^[6]. A slope above the wall, a driveway or a shed near the top all count.

Over 4 feet, or holding up a surcharge, the wall needs a permit, and in places like Snohomish County, Washington, engineered plans^[5]. Saint Paul, Minnesota, also names tiered walls, soil types and surcharge loads from buildings as reasons to require an engineer^[3]. Two short walls stacked close together are not automatically two exempt walls.

The parts of a block wall

Saint Paul's section drawing list doubles as a checklist of what a wall is made of^[3]:

The numbers below come from a published public specification, the Texas State University construction standard for segmental retaining walls up to 5 feet high^[2]. It is written for campus projects with an engineer on call, so use it to understand the method and follow your block maker and your city for the final numbers.

Step by step

Mark the wall and the utilities

Lay out the wall line with stakes and string, and mark where it turns or steps. Verify the location of existing structures and utilities before excavation, as the specification requires^[2]; call 811 to have buried lines marked. Check the distance to the property line too: Saint Paul does not allow any part of a retaining wall on or over the property line without a recorded easement agreement^[3].

Excavate the trench

Dig the trench to the lines and grades of your plan, wide enough for the leveling pad in front of and behind the blocks plus the drainage zone. Excavate carefully so the soil below the base is not disturbed beyond the lines^[2]; soil that does not meet the required strength is removed and replaced with compacted fill in the specification^[2].

Build a compacted leveling pad

The leveling pad is the most important layer. In the Texas State specification, the aggregate base is at least 6 inches thick after compaction and extends at least 6 inches in front of and behind the first course of blocks; it is compacted to a level, hard surface with no gaps under the units^[2]. Where a reinforced footing is required by the local code official, it goes below frost depth^[2].

Set the first course level

Place the first course on the prepared base, in full contact with it, checking every unit for level and alignment and keeping the same top elevation along each section^[2]. Units sit side by side with no gaps, aligned with a string line measured from the back of the block^[2]. Errors here grow with every course above.

Install filter fabric, drainage stone and the drain pipe

Directly behind the blocks, place filter fabric, then at least 12 inches of drainage aggregate between and behind the units, filling the voids in the blocks too; the drainage zone runs up to within 9 inches of final grade and is capped with 9 inches of impervious material^[2]. The drain pipe goes at the lowest possible elevation behind the wall, sloped at least 2 percent, and is daylighted through the face of the wall or to a point away from the wall at each low point or at least every 50 feet^[2].

Stack the courses and compact as you go

Clear excess fill off the top of each course before placing the next, compact the drainage aggregate and backfill before each new course, and check level and alignment every course^[2]. Compact in thin layers: the specification limits lifts to 6 to 8 inches of loose soil with hand-operated compactors, and allows only lightweight hand-operated equipment within 4 feet of the back of the wall^[2].

Add geogrid where the design calls for it

Taller or loaded walls use geosynthetic reinforcement at set heights. Where it is used, the specification places it on compacted backfill level with the top of the blocks, within an inch of the wall face, pulled tight and without wrinkles, continuous with no splices in the strength direction^[2]. Its lengths and levels come from the manufacturer's tables or the engineer, never by eye.

Cap and finish the grade

Finish the top course with cap units per the manufacturer, then grade the soil above so water drains away from the wall, over the impervious cap layer described above^[2].

Before you dig: do you need a permit?

If your wall is over 4 feet from the bottom of the footing, supports a slope, a driveway or a building, or sits near a property line, deal with the permit before the excavator arrives. A complete application in Saint Paul includes a site plan with setbacks and the wall's length, the manufacturer's instructions, an elevation and a section showing the footing, drainage, reinforcement and the slope behind the wall^[3]. Eagle Mountain adds lot dimensions, existing buildings, and the distances from the wall to the property lines with slopes and grades defined^[4].

That file is the part we prepare. Our permit team checks your city's threshold and how it measures height and surcharge, draws the site plan, elevation and section, attaches the manufacturer's details, and tells you up front when an engineer is required. The All-in offer adds our review before you file and every round of city comments until the permit is issued.

Common questions