Retaining Wall Cinder Blocks: Which Type, and How High You Can Go

Can you use cinder blocks for a retaining wall?

Yes, under about 4 feet, if the cinder block retaining wall is built as a reinforced masonry wall: a footing below frost depth, rebar through the cores, the cores grouted solid, and drainage behind it. A standard cinder block dry stacked on soil is not a retaining wall, and no amount of stacking makes it one.

The reason is what resists the soil. A concrete masonry unit is designed to carry vertical load in a mortared wall. It has nothing built into it to resist a sideways push. Rebar, grout and a footing are what turn the stack into reinforced concrete that can take that push.

A segmental retaining wall unit, usually shortened to SRW, is a different product entirely. It is solid rather than hollow, much heavier per unit, and tapered so that a rear lip or a pin sets each course back slightly. The finished wall leans into the hill, and its own weight does the work.

For a garden border or a divider that is not holding back soil, either block is fine. For anything retaining soil, use cement retaining wall blocks made for the job, or build the CMU wall as a reinforced, grouted, footed wall.

Unit anatomy and density classes are covered on what is a CMU block. Because a gravity wall holds soil with mass, concrete block weight matters far more for an SRW than it does for a mortared wall.

Cinder block vs retaining wall block: what actually resists the soil

Both systems share a trench, a level first course and gravel behind the wall. Past that they are different construction types, and the method for one does not transfer to the other.

Schematic section, not to scale.

| | Mortared CMU wall | Dry stack SRW wall | |---|---|---| | Unit | Hollow concrete masonry unit, 8 by 8 by 16 in nominal | Solid interlocking wall block with a rear flange or pin holes | | What resists the soil | Rebar, grout and a concrete footing | Unit weight, back batter and interlock | | Mortar | Yes, laid in Type S mortar | None | | Base | Concrete footing below the local frost line | Compacted aggregate leveling pad | | Reinforcement above the limit | Engineered design | Geogrid layers run back into the fill | | Practical height | 3 to 4 ft, then engineering | 24 to 36 in for retail flange block, 3 to 4 ft for a full depth gravity unit |

Note that the table says what each system relies on, not how to build it. Footing depth, rebar size and spacing, lift heights and cure times are design decisions that vary with soil, climate and load, and they belong to your local code and your engineer rather than to a national guide.

Gravity, pin and interlocking lip systems

Retail systems come in three forms.

A plain stacking block relies on weight alone and builds a gravity wall. A pin system block locks each course with plastic or fiberglass pins. An interlocking lip block, also called a flange block, sets each course back automatically with its rear flange, and this is the dry stack cinder block retaining wall that most DIY guides describe.

Manufacturers commonly rate retail flange block to somewhere between 24 and 36 inches, so the height printed on the pallet wrapper governs, not a figure from any website including this one. Above that, a stack block retaining wall becomes geogrid reinforced, sometimes called mechanically stabilized, with layers of geosynthetic running back into the soil from between courses.

How high can a retaining wall be before it needs a permit?

Over 4 feet total height. IBC Section 105.2, as summarized by CMHA, exempts earth retaining structures from a building permit only up to 4 feet. Above that a permit is required, and a shorter wall can still need one when it carries a surcharge such as a driveway, a slope or a structure above it. Municipalities add their own rules, and some require permits at lower heights.

How the height is measured matters as much as the number. Retaining wall height runs from the top of the leveling pad to the top of the uppermost unit, excluding the cap and including the buried portion, so a half buried first course still counts.

How high each system goes:

• Gravity walls, which hold soil with unit weight alone, typically top out at 3 to 4 feet, and lower in poor soil or under a surcharge. The real maximum depends on unit depth, unit weight, face batter, soil type and loading, which is why the same block is rated differently in different soils.
• Geosynthetic reinforced walls have no theoretical maximum. CMHA notes that walls over 50 feet are increasingly common. Once a wall passes the gravity ceiling, an engineer designs it.
• Reinforced CMU walls have a practical do it yourself ceiling of 3 to 4 feet. Above that, engineering input.
• Terraced walls behave as separate walls only when the spacing between them is greater than twice the lower wall's height. Closer than that, the design treats both as one tall wall.

The best retaining wall blocks for 4 feet high are full depth SRW units, 12 inches deep or more, rated by their manufacturer for 4 feet without geogrid in your soil type. At exactly 4 feet you are at the gravity limit and at the permit line at the same time, which is the point to stop and get the wall designed. A 4 inch tall retail block is a garden border unit and should never be stacked to 4 feet.

Retaining wall block sizes and shapes

Retaining wall blocks are sold by face height, and the face height tells you what the block is for.

| Face height | Typical length | What it builds | |---|---|---| | 3 to 4 in | 8 to 12 in | Garden borders, tree rings, bed edging. Mini retaining wall blocks. Not for retaining | | 6 in | 12 to 18 in | Seat walls, planters, low retaining. The 16 inch retaining wall blocks and 12 x 16 retaining wall blocks class | | 8 in | 12 to 18 in | Structural walls to the gravity ceiling and beyond with geogrid. The 18 inch retaining wall blocks, 8x18 retaining wall block and 18 x 8 retaining wall blocks most searches mean | | 2 ft | 3 to 6 ft | Bin blocks. A concrete bin block retaining wall, plus commercial and engineered big block. The extra large retaining wall blocks and big concrete retaining wall blocks of the search results |

Rectangle retaining wall blocks are the straight sided units. Round retaining wall blocks are radius face units cut to turn a curve, and most product lines also sell corners, columns and two sided blocks that are textured on both faces for serpentine walls and seat walls.

Face styles run to beveled, straight, tumbled, rounded fieldstone, chiseled and stacked stone looks. Pallet weights for 8 inch units run well into the thousands of pounds, so plan delivery by the pallet rather than the trunk.

Prices and product names change constantly and vary by region, so this page does not list them. Check the supplier for current pricing and, more importantly, for the manufacturer's rated height for the specific unit you are buying.

Nominal against actual CMU sizes, in inches and millimeters, are on cinder block dimensions.

Backfill and drainage: what gravel goes behind a retaining wall, and how much

Clean angular gravel, extending at least 12 inches behind retaining wall cinder blocks of either kind for the full height of the wall, over a perforated pipe at the base, wrapped in geotextile or landscape fabric.

That 12 inch figure is not a rule of thumb. CMHA's Guide to Segmental Retaining Walls states that the gravel fill should extend a minimum of 12 inches behind the SRW units regardless of the type. Pea gravel is the wrong choice. It rolls, will not compact, and clogs. Rounded river gravel fails the same test. A gravel backfill that cannot drain is just heavier soil.

Schematic section, not to scale. The 12 inch minimum is CMHA SRW-TEC-005.

How much gravel to backfill a retaining wall: multiply wall length by wall height by 1 foot for the 12 inch zone, divide by 27 to convert cubic feet to cubic yards, and add 10 percent for settlement.

A 20 foot by 3 foot wall gives 60 cubic feet, which is 2.2 cubic yards, or 2.4 with the allowance. That is how much backfill for retaining wall gravel you order, and the retaining wall block and gravel calculator works it out alongside the block, cap and base pad quantities for an SRW. The native soil beyond the gravel zone is the rest of the fill retaining wall volume and comes back out of the excavation.

The drainage sequence is the same for both systems. The perforated pipe sits directly behind the first course, sloped to daylight or to a storm connection. The gravel zone starts immediately behind the block and is filled as the wall rises, not afterwards. Fabric separates gravel from soil so the gravel does not silt up.

Water control is the whole point. Poor drainage is what makes a wall lean, bulge and fail. Weak block almost never is.

When to call a professional

CMHA names four conditions that put a segmental retaining wall in the hands of a qualified engineer: the wall carries a surcharge load, the wall carries live loads, the wall is founded on poor soil, or the design conditions require special consideration. Add the 4 foot permit threshold and that is the list.

Do it yourself fits garden borders, dividers under 3 feet, non retaining edging, and small terraces where the soil is granular and drainage is simple. A dry stack SRW wall at that height is a weekend project for someone comfortable with a level and a compactor.

Clay sits in CMHA's engineer list twice over. It is a poor soil condition, and CMHA's own gravity wall figure of 3 to 4 feet is explicitly reduced where poor soil or surcharge loads are present. Clay also holds water instead of shedding it, so in clay the drainage detail matters more, not less.

The failures, in rough order of how often they cause a wall to come down: no drainage, a footing above the frost line, no back batter, joints stacked vertically instead of staggered, and backfilling before the wall has cured. A cap and a sealer are the last step, not a fix for any of those.

Before you dig, call 811 for utility marking.

Frequently asked questions

Can you use cinder blocks for a retaining wall? Yes, under 4 feet, when they are laid in mortar on a footing with rebar and grout, and never by stacking them dry. A retaining wall needs weight, batter and interlock, or it needs reinforcement. A bare CMU has neither.

What is the difference between a cinder block and a retaining wall block? What resists the soil. A cinder block is a hollow unit for mortared walls and needs rebar, grout and a footing to retain anything. A retaining wall block is a solid, heavy SRW unit with a lip or pin that sets each course back so the wall holds by its own weight.

What blocks should you use for retaining walls? One of three retail systems: a plain gravity block, a pin system block, or an interlocking lip block with a rear flange. Above the gravity ceiling, an SRW unit rated for geogrid.

How high can a retaining wall be before it needs a permit? Over 4 feet total height under IBC 105.2. Height is measured from the leveling pad to the top unit, excluding the cap. A shorter wall carrying a surcharge may also need one. Check your municipality before you start.

What sizes do retaining wall blocks come in? Face heights of roughly 3 to 8 inches with lengths of 8 to 18 inches for retail and contractor SRW units, and 2 foot bin blocks for bins and commercial walls. The two most searched are the small garden unit and the 8 by 18 inch structural unit.

Can you dry stack a retaining wall? Yes, with SRW units made for it, inside two limits: the manufacturer's rating for retail flange block, commonly 24 to 36 inches, and the 3 to 4 foot gravity ceiling for full depth units. You cannot dry stack a standard cinder block retaining wall at any height.

What gravel goes behind a retaining wall, and how much? Clean angular crushed gravel, not pea gravel, extending a minimum of 12 inches behind the units for the full height, over a perforated pipe. For quantity: length times height times 1 foot in cubic feet, divided by 27 for cubic yards, plus 10 percent.

When does a wall need geogrid instead of just stacking? When it passes the gravity ceiling for its unit, usually 3 to 4 feet and lower in poor soil or under a surcharge. Geogrid is laid between courses and runs back into the fill, and anything above the unit's rated gravity height is an engineered design.

Sources

• CMHA SRW-FAQ-001, Segmental Retaining Wall FAQ. Permit threshold under IBC 105.2, gravity wall heights, how wall height is measured, terrace spacing.
• CMHA SRW-TEC-004 and SRW-MAN-003, cmha.org resource library. Geogrid reinforced SRW design and installation.
• CMHA SRW-TEC-005, Guide to Segmental Retaining Walls. The 12 inch minimum gravel fill behind the units, and the four conditions requiring a qualified engineer.
• Lowe's, Build a Block Retaining Wall. Interlocking flange block method and the 24 to 36 inch retail block height range.
• Home Depot, How to Build a Retaining Wall. Perforated pipe behind the full wall length, gravel covering the pipe, and clay soil holding water.
• How we source and correct figures and every constant on this site.