Retaining Wall Blocks: Types, Sizes and How to Choose

Which block is a retaining wall block?

These are dry-stacked concrete units made to hold soil. Mortar is not what keeps the courses together. The lock is built into the block, and that matters because a mortared wall and a segmental wall fail in different ways and get ordered as different materials. ASTM C1372 is the specification for the segmental units. Meeting it means an average compressive strength of at least 3,000 psi across three units, with no single unit under 2,500 psi, and overall dimensions within 1/8 inch. A split face is the exception, since the texture is supposed to be irregular. Where the wall stays wet and then freezes, durability comes from a proven field record or from the ASTM C1262 freeze-thaw test, not from the strength number alone. The details are in the CMHA sampling and testing note and on the ASTM C1372 page.

The lock itself depends on the brand. AB Classic uses a raised lip that sets the next course as you drop it on. Versa-Lok Standard uses two pins per unit. Rockwood Premier uses a bar molded into the bottom. All three stack without a mortar joint. A hollow 8 by 8 by 16 concrete masonry unit is a wall block too, but it is bedded in mortar and counted on the concrete block calculator. Do not swap the two when you are standing in the aisle.

What sizes do the catalogs print?

Order from the face, because that is what covers the wall you see. Depth, the front-to-back measure, is what gives the wall its weight. It does not change how many blocks fit along the run. Plants vary a little, and several of these lines are published as approximate, so treat the table as the catalog size rather than a promise that every block in the bundle is identical.

AB Classic weighs about 75 pounds. Its cap, AB Capstone, is 4 inches high, 12 inches deep, and 18 inches long, so the cap length matches the block and you are not cutting every piece to fit. The corner is 8 by 8 by 16 inches. Those three are on the Allan Block collection page. The smaller faces, Jumbo Jr, Lite Stone, and Junior Lite, are in the Allan Block residential guide. Versa-Lok Standard is about 82 pounds in the Versa-Lok Standard design and installation guidelines. Premier 6 is 44 pounds and Premier 8 is 54 pounds in the Rockwood Premier install manual.

How many blocks cover one square foot?

Divide 1 by the face area and you have the neat rate. AB Classic and Premier 8 need 1 block per square foot of face. Premier 6, at 0.75 square foot, needs 1.33. Versa-Lok Standard covers 2/3 of a square foot, so the rate is 1.5. Jumbo Jr and Lite Stone need 2. Junior Lite needs 4. That rate is only exact when the wall length divides by the block length and the height divides by the block height.

Real walls often do not divide evenly. The retaining wall calculator rounds each direction up before it multiplies, so a leftover half course becomes a full row you still have to buy. On the 30 by 3 foot wall below, 90 square feet at 1 block per square foot is 90 blocks. The rounded count is 100. The difference is the half course, and it is the reason a square-foot guess leaves you short on the last row.

When does the wall stay a gravity wall?

A short wall can hold soil with its own weight and with the backward lean of the courses. That is a gravity wall. Once the soil push is more than the blocks can resist, geogrid is laid into the fill so the face and a mass of soil act as one heavier structure. That is a reinforced wall. Which one you are building decides the trench, the drain, and whether you need an engineer before you buy block.

Allan Block describes the AB collection as a fit for common residential walls up to 6 feet, with a proper design once you go taller. Separately, a drain pipe is required on any reinforced wall, on gravity walls over 4 feet, and wherever the site drains poorly. A 4 foot wall can still be a gravity wall and still need that drain. The height for the drain and the height for a full design are not the same check. Both are in the Allan Block installation steps and on the collection page.

Versa-Lok Standard can stand without geogrid to 4 feet when the ground behind the wall is level, the soil is good, and nothing extra sits on top. Heavier soil, a slope, or a driveway above the wall means a shorter gravity wall or reinforcement. Walls over 4 feet usually need an engineer's plan. Where the grade in front is level, bury about one-tenth of the exposed height so soil in front of the toe helps hold the base. Those limits are in the Versa-Lok Standard design and installation guidelines.

Belgard publishes maximum gravity heights only for a very kind site: level ground above the wall, level ground at the toe, no driveway or other surcharge, clean sand or gravel within one wall-height behind the face, a friction angle of at least 34 degrees, and soil no heavier than 120 pounds per cubic foot. The height includes the buried part of the wall. Miss any of those conditions and the safe gravity height drops, or the wall needs geogrid. The heights come from the NCMA Design Manual for Segmental Retaining Walls, and the table is general information rather than a design you can build from. It is in Belgard's gravity-height note.

The code line is separate from the product chart. Section R404.4 of the International Residential Code requires an engineered design when a retaining wall is not supported at the top and retains more than 48 inches of unbalanced fill. It also requires engineering when the wall is over 24 inches and must resist lateral loads in addition to soil, so the wall stays stable against overturning. Cities adopt different editions and sometimes amend the section, so ask the building department which code you are under before you treat 4 feet as the only cutoff. A block maker's gravity chart can be shorter or taller than that trigger because it assumes a particular soil. Georgia DCA's summary of IRC changes from 2012 to 2018 is one public copy of that R404.4 wording: IRC 2012 to 2018 significant changes.

What lean does the block build in?

Each course should step back into the hill. That lean, called setback or batter, shifts the weight of the wall into the retained soil instead of letting the face tip outward. AB Classic is 6 degrees and AB Stones is 12 degrees. The steeper lean gives you more leverage, which is why a 12 degree system can go higher before it needs geogrid. Versa-Lok Standard sets back 3/4 inch on every course, about 7 degrees. Rockwood Premier 8 sets back 1 inch, which is 7 degrees, and Premier 6 sets back 1/2 inch, which is 3.5 degrees. You do not add this lean when you count blocks. You do check it with a level as the wall goes up, because a missed lip or pin lets the error grow with every course.

What goes in the trench and behind the face?

Most failed walls are wet walls. The stone under the first course makes a level, firm pad. The stone directly behind the face gives water a path out so it does not push on the blocks. The soil behind that stone is just backfill. Using one gravel for all three jobs is how the drain plugs and the wall leans.

Allan Block's wall rock is clean gravel from 0.25 inch to 1.5 inch, angular or smooth, with no sand, silt, or clay. Place at least 6 inches of it in the trench and compact it before the first course. For a wall under 4 feet, dig that trench 18 inches wide and 4 inches deep, then go deeper by whatever block you bury. On a taller wall the trench is 24 inches wide, and you excavate 6 inches plus 1 inch of buried block for every foot of wall height. The 6 inch figure is the rock you place. The 4 inch figure is the soil you remove on a short wall before burial is added. A short wall can still need the full 6 inch pad of rock. Both dimensions are in the residential guide and the installation steps.

Behind the face, fill the cores and a 12 inch thickness with the same wall rock, then compact the soil behind it in lifts no thicker than 8 inches. Thicker lifts look done and are still loose in the middle. Versa-Lok uses a granular leveling pad at least 6 inches thick and 24 inches wide, then 3/4 inch clear angular gravel at least 12 inches thick behind the units, starting at the grade in front of the wall. Keep that drainage gravel out from behind the buried courses, or you are loosening the part of the wall that should stay embedded. Soil behind the gravel goes down in lifts no thicker than 6 inches. That is the Versa-Lok Standard installation.

The NCMA Design Manual for Segmental Retaining Walls, third edition, calls for a compacted leveling pad at least 6 inches thick, reaching at least 6 inches past the front and the back of the bottom unit, and for 12 inches of gravel fill behind the units on every wall. Pavestone's Retani specification uses the same two minimums for that product: a compacted aggregate pad at least 6 inches thick, extended at least 6 inches in front of and behind the bottom block, and a drainage zone at least 12 inches wide. The Retani pad, graded to ASTM D448, is 100 percent passing 1 inch, 35 to 70 percent passing the No. 4, 10 to 35 percent passing the No. 40, and 3 to 10 percent passing the No. 200. Some fines are allowed so the pad packs tight. Allan Block's wall rock is the opposite: it is supposed to stay open. Order the rock the wall system names, then use the crushed stone calculator for the volume and the backfill calculator for the soil behind it. Open-graded drain stone is also on the gravel sizes chart and in the French drain gravel guide. The Retani gradation is in the Pavestone Retani spec.

Count a 30 foot by 3 foot wall

Mistakes to avoid

Buying from square feet alone. This 30 by 3 foot face is 90 square feet and still needs 100 blocks before caps or burial, because 36 inches is not a whole number of 8 inch courses. The catalog rate assumes a wall that divides evenly. Yours probably does not, and the last row is where you find out.

Treating every brand as a 4 foot gravity wall. Under ideal soil, Belgard's heights run from 3.0 feet to 10.0 feet and already include embedment. Versa-Lok Standard stops the unreinforced case at 4 feet when the backfill is level and unloaded. Allan Block talks about 6 feet for common residential walls, then a real design after that. Leave out the slope, the surcharge, or the poor soil the chart assumed, and a wall that looks short on paper can still need geogrid.

Using one gravel for the pad, the drain zone, and the soil behind it. Allan Block's wall rock is open, with no sand, silt, or clay. Pavestone's leveling pad is allowed 3 to 10 percent passing the No. 200 so it will compact. A dense driveway base in the drainage zone holds water against the face, which is the load the drain was supposed to remove.

Hiding the buried course inside the exposed height. The height you type is the face you are counting. Burial is a separate line, which is why the 100 does not include the extra 20. Allan Block adds the buried block on top of the rock depth. Versa-Lok, on level ground in front, embeds about one-tenth of the exposed height. Curves, a slope at the toe, and a surcharge above the wall are outside this straight-wall count, and they are the cases that move you from a catalog chart to an engineer.