How to build a cinder block retaining wall
by Jim Bessey
There was a time when cinder block retaining walls were all the rage. That was during post-war American building boom fifty years ago. Times have changed, and now these utilitarian walls more typically adorn commercial sites.
Somewhere in between, the use of once-popular cinder block lost ground to the more contemporary "concrete block." With the that change came a lot more options that once again made block retaining walls an attractive choice for suburban homeowners.
This isn't your grandfather's cinder block wall.
We've all seen those aging, crumbly cinder block constructions. That is exactly the image that convinced so many of us to choose more modern variations. Today, many landscapers prefer dry-laid walls composed of pre-cast concrete blocks designed specifically for the job. They're very pretty, easy to work with, AND extremely expensive. For those of us on a budget, choosing a cinder (concrete) block retaining wall could be just the right choice.
The block wall advantage:
Modern concrete block is inexpensive and very durable. It's widely available for home delivery, and offers unsurpassed strength in many applications. Today's cast block is available in a variety of dyed colors and an assortment of facing finishes, although both choices add to the cost. Block retaining walls are a good choice for long straight runs, especially for heights above 16" or so. They are perfect for perpendicular walls that meet at a 90 degree corner. You can, however, lay ordinary rectangular blocks in gentle concave or convex arcs.
But first, the disclaimer:
Laying block isn't an ideal DIY project. You can't easily do the job piecemeal, or without the correct tools. Mixing and applying mortar properly can be exhausting and frustrating. You'll need to start with a decent-quality footer, too. This isn't a half-weekend task that offers easy shortcuts. Let's have a look at what's involved.
Tools required:
* 4' Level, 2' level, string-line, string-blocks, and wooden stakes (optional: builder's level/transit)
* Shovel,garden hose, mixing hoe and tray (or power mixer)
* Mason's block-laying trowel, striking tool and block-chisel (optional: power saw for cutting masonry)
* Measuring tape
* At least one strong back (helpers always appreciated!)
* Optional: backhoe (can be rented for small jobs)
Now that we've weeded out the faint of heart, we can proceed to the job of building a concrete block retaining wall. For simplicity, we'll consider a fictional sample wall project in your back yard.
Preparation:
A pitched yard with a low corner is an ideal candidate for a cinder block wall. For our example, we have an area about 20 feet square that could be about two feet lower than the rest of the back yard. By retaining the higher ground, you can create a patio or garden in the remaining low section. Step one is to lay out the general dimensions of this project. Before you begin, check with your local building department to see if you need a permit. In addition, you should call for a utilities stake-out to avoid digging surprises. This service is generally free, and should only require one toll-free phone call.
If you have access to a simple builder's level, use it (and a helper) to facilitate layout and to find the lowest point for your wall layout. You can do the same thing with stakes, a string line and string-mounted level (cheap!).
To properly mark a right angle, use the old 3-4-5 triangle trick: any triangle whose sides are identical multiples of 3 and 4, with a hypotenuse formed of the same multiple of 5 will be a right triangle. For instance, use a 9-foot board and a 12-foot board (2 x 4's are fine). Set one end of each board at your corner point in the shape of an "L" at roughly 90 degrees. Move the long ends of the boards until they measure exactly 15 feet from point to point on the diagonal. (3 x 3, 3 x 4, and 3 x 5 are the multiples used there.) Whichever method you use to establish a right angle, mark it with stakes beyond the area to be excavated. You'll use string, later, to find the lines again.
You can dig it.
This is the hard part. You'll want to move some earth around for leveling. You're also going to need a footer, or your new retaining wall won't last long. For this, you'll have to dig a trench slightly wider than the wall you envision, about 8 inches deep and about one foot wide. The top of your trench will be level with the lowest point of your proposed wall.
For large projects, using a backhoe for this step is all but mandatory. If you're going to be moving a fair amount of dirt anyway, you may want to go ahead and hire or rent one. Use the trench's walls as forms if the surrounding dirt is solid; otherwise form-up your footer trench using 2 x 6 (or 2 x 8) lumber. Back-fill or stake the wooden forms for rigidity. Make certain that your forms are level. Add about 2 inches of compacted gravel in the bottom for drainage.
The tough part: pouring footers.
Our sample project includes two walls, each about 20 feet long. Therefore, we have about 40 feet of footers that are 2/3's of a foot in cross-section (width times depth). So, we'll need to pour almost 30 cubic feet of concrete - just over one cubic yard, far below the minimum pour required for ready-mix delivery. Unless you're prepared to pour a concrete patio at the same time you pour your footers, you're going to be mixing about 30 bags of concrete by hand. Yikes!
There are two ways you can make this part of the job tolerable. First, if you have plenty of able-bodied help, simply mix the load bag-by-bag and work quickly (mix wetter than normal). In the absence of strong helpers, you can mix the concrete right in your trenches. Although this method is unacceptable for home-building, it's fine for the much lighter load of a small retaining wall. Here's how:
1. Purchase about 30 bags of cheap concrete mix and 80 feet of 1/2" metal rebar. Have these delivered to your job site. (The big stores offer trucks you can rent, too.)
2. Using a garden hose and nozzle set to "shower," lightly wet your gravel base.
3. Open about 1/3 of your concrete bags and spread the dry mix uniformly into your trenches, wetting the mix lightly as you go along.
4. Use a mixing hoe (larger than a garden hoe, with two holes in the center), mix the dry concrete with the water you've applied until it's like thick pudding.
5. Repeat #3 and #4, until your footers are about 2/3's full of wet concrete.
6. Lay two rows of rebar down the center of your trenches, about 4" apart. Tamp each metal bar down into the wet concrete until it disappears.
7. Repeat #3 and #4 for the final third of your concrete. Your hoe will scrape the rebar repeatedly; this is fine, if aggravating.
8. Use a 3-foot scrap of 2 x 4 lumber to level-off the top of your concrete footer. It doesn't have to be pretty, just solid and level.
9. Break time! Your new footers need to "set up" overnight, and your building department may want to inspect your work.
Finally - it's time to start building something!
Before you begin, you'll need to purchase your materials. Your block retailer can help you figure materials. We're building about 40 feet of walls, about two feet high, with one inside corner and two exposed ends. Here's a rough list for our fictional project:
1. Standard 2-core block, 8 x 8 x 16 inches - 80
2. Corner blocks, full size (for exposed ends and corners) - 5
3. Half-blocks for ends, 8 x 8 x 8 inches - 4
4. Cap blocks, for the top, 2 x 8 x 16 inches - 30
5. Mortar mix, 80# bags - 10 (this is a wild guess, subject to verification by your retailer)
6. Optional: appx. 40 linear feet of block-reinforcing wire mesh (not needed for this sample project)
Again, have these materials delivered, or rent a truck to carry this heavy load.
Laying block is skilled manual labor. Remember our original layout, the one at right angles? Before we set any blocks, we need to re-create our lines. Ideally, our lines will fall at the precise center of our new (dry) footers. If your lines are offset, simply measure and mark true center. We'll be using standard 8" block because it's cheap and always available. For colors and fancy faces, it's the only choice for homeowners. Mark new lines 4" from center, on the high side of your future retaining wall. Move your stakes and strings to match these lines, which will form the outside plane of your block wall.
Block layouts work best in multiples of four feet. This is why we're building two 20' walls. However, any multiple of 8" is workable. You'll begin by placing the crucial corner and both ends. One end will be a full block and the other a half block - check for even multiples of 16" in each direction. This is the make-or-break part of the set-up. Each of these three blocks must be set level with the others. The same (outside) face of each block are aligned with your layout strings. Each end-block will measure exactly 20 feet from the outside point of the corner block.
Don't worry, this isn't rocket science.
Mix one bag of mortar to get things started. You can mix by hand or rent a power mixer. Perfect mortar is smooth, sticky, and the consistency of thick pudding. Follow the on-bag instructions for proportions; if your mortar won't hold a shape, it's too thin. Place a few pounds of fresh mortar on a scrap of plywood ("mortar board"!). Using a good quality mason's trowel, lay a bed of mortar beneath each of your starting blocks (the corner and ends). Tap each block lightly with the trowel handle until they are level in all directions and with each other. Use a builder's level or string-mounted level to check this.
It's more like Lego.
We'll want to build each end and the inside corner to a height of 24" for this project - 3 blocks high. Do this by adding one block at a time to form a step pattern, using mortar joints as close to 1/2" thick as possible. The corner will end up using 6 blocks to form what looks like two 3-step stairs meeting at right angles. One end will use 4 full blocks and one half block, while the other uses two full blocks and two half blocks. If you've ever played with Lego building blocks, this will come easily to you. Use your level and measuring tape to check for level and plumb (straight up) and mortar thickness as you set each new block.
And it gets easier.
This method of building up the corner and ends sets you up for speedier work for all the middle blocks (the standard 2-core ones). Your block retailer should have supplied you with wooden string blocks that hook onto the corners of concrete block (possibly for a small charge). These wood blocks hold your string line the way you do when you're flossing: you hook the string level with the top of the first course of blocks, end to corner to end. Once you set the line, you can align and set each block exactly level (the long way). You still have to check level side-to-side. You also have to measure out 4' markings, so your block won't "stretch."
Laying the first row is good practice for working with mortar. As you proceed, you'll get a feel for when your mortar is just right and for how much to use. Getting the "mud" to stick to the upright ends of each block for the in-between joints takes some getting used-to; there's a wrist motion to it that's hard to describe. Experiment a little. If you have trouble getting the hang of it, check YouTube for a how-to video.
Round two:
Once your first row is laid, move on to the second row of blocks. Reset your string to be level with the top of the second row, about 16" above the footer. These mortar joints will all show, so technique and joint thickness matter. Excess mortar will squish out as you settle each block. "Strike it" by slicing off the excess with your trowel; rescue larger blobs for re-use. Whenever you need a short break, use your striking tool (similar to a spoon) to smooth and slightly recess the joints. Have a small bucket of water handy to keep this tool wet for best results.
One special note: if you're concerned about water pressure from the high side of your new wall, you can add "weep" holes in the mortar joints. Do this by inserting 8" cut pieces of copper or pvc tubing (1/2" outside diameter) between rows one and two. These go in the intersection of the mortar joints, about 4 feet on center. It's a small detail, not always needed, but important if there's going to be groundwater flowing heavily from uphill.
Add the third row the same way, being careful not to greatly disturb the first two rows.
Cap it all off:
Strike and smooth all of your mortar joints. There's only one row left to do. At this point, you will have become fairly adept at slinging mortar - now that the job's nearly done! All that's left is to add the final row of solid blocks as a cap. We've chosen 2 x 8 x 16 inch for this part; you can make a different choice if you like for a decorative flair. It's critical to seal-off all those hollow spaces inside your new wall, so don't skimp on your "between" mortar joints. You can still use string lines, but be sure to use a good 4-foot level for this final step to keep your cap row nice and flat.
Break time - take a picture!
Laying that final block provides a great feeling of accomplishment. Pour yourself a cold drink and snap a photo of your handiwork. (Clean up any mortar-blob mess you've left behind first.) Don't lean against your freshly-laid block wall until it's dried a bit first. You can begin to back-fill the high side of your wall, but do so tenderly and don't go higher than halfway the first day.
Back-filling your new wall is hard work. Now's the time to draft your kids, the neighbors, passers-by, a brawny Boy Scout troop - anyone who can handle a shovel or rake. The whole idea of building a retaining wall is to create more usable lawn space from a sloped area. That means you have to move quite a lot of earth. Wear sunblock and drink plenty of liquids. For a gorgeous finishing touch...
Plant some flowers along your new retaining wall.
After all that hard physical labor, you're going to want to show off your efforts to your family, friends, and neighbors. Splurge on some peat moss, a few flats of annuals, and some mulch. Nothing makes a project shine brighter than the little finishing touches. Add some low-voltage lighting. Install a small section of decorative railing. Buy a fancy bench to place in front of your wall. The point is to FINISH the job in style. When you're done, and the tools have been returned to the rental place or your garden shed, gather your helpers around for a "job well done" photograph. Smile!