How to Build a Treated-Wood Retaining Wall
No heavy timbers—build this sturdy retaining wall with lightweight, off-the-shelf treated lumber
IntroductionEliminate steep, difficult to mow slopes, stop erosion and create attractive planting beds with an easy to assemble wood retaining wall. It uses lightweight materials to create a strong durable wall. It uses standard treated dimensional lumber. You can easily adapt it to your site. And you can preassemble much of it in your garage!
- 3-lb. hammer
- Adjustable wrench
- Air powered framing nailer
- Circular saw
- Drill bit set
- Drill/driver - cordless
- Extension cord
- Hearing protection
- Mason line
- Safety glasses
- Socket/ratchet set
- Tape measure
- Utility knife
- 1 gal. wood preservative
- 16d galvanized nails
- 1x2 stakes
- 2x6 brown treated lumber
- 2x6 green treated lumber
- 2x8 brown treated lumber
- 5-1/2 in. x 3/8-in. hex head bolts with 2 washers and a nut (6 sets)
- Galvanized joist hanger nails
- Landscape fabric
- TS18 Twist Strap
Wall design and materials
Wall step up
This system allows you to step up along the primary slope as well as up the slope.
You don’t have to be a master carpenter to build this retaining wall. If you’ve ever built your own deck, relax—you already have the tools and skills you need. This wall is easier and more forgiving than any deck design. We’ll demonstrate the assembly, layout, backfilling and anchoring techniques that will make you comfortable tackling this project yourself.
This wall is unusual in that it doesn’t rely on dead weight or deep pilings to keep from caving in or shifting. Instead, it gets its soundness from the inherent strength of a rigid triangle assembly and the dirt that’s piled behind it. The very earth that’s the enemy of most retaining walls actually works with this design to give it strength. You don’t have to dig the wall in. You bury it in place!
The stanchions (post assemblies; see Photo 6 and Fig. A) have two short boards underneath the post to prevent settling. The headers, footers, braces and shelves anchor the stanchions deep into the side of the hill to keep the wall from tipping out or sliding away from the slope. A special steel strap (Photo 2) ties the 45-degree brace to the top of the posts to keep everything straight up and down.
Your walls can be as long as you want, but limit the height to 4 ft. Add more terraces as needed to retain higher slopes, stepping them back about 4 ft. If upper tiers will rest on earth that’s been disturbed from building a lower wall, you might have to wait as long as six months for the soil to settle naturally, especially in dry weather. Hasten the process by soaking the backfill (the fresh dirt behind the lower wall) with a sprinkler a few hours a day for a few days, and then allow a couple of extra days for the soil to dry out before starting higher tiers.
Every slope is different, so plan to establish a level base, then step it up or down as needed to handle hills that slope sideways. We stepped our wall in 5-1/2 in. increments so the 2×6 wall planks would continue to neatly stack at these transitions (Photo 10). To handle the transition between tiers, install bulkhead planks (Photo 15) between the upper and lower posts. First you have to make the lower and upper stanchions line up exactly. Use a carpenter’s square or a 4-ft. square to extend a straightedge to line up the first upper stanchion with the first lower stanchion.
We built the exposed surfaces of this wall with brown pressure treated lumber and below-grade members with less expensive green treated lumber. We chose the brown simply for its looks. If your lumberyard doesn’t carry brown, use green for all the parts and stain any exposed wood after the wall is up. Both the green and the brown wood used in this wall are rated for “ground contact” (not for below grade or foundation use). However, this wall will easily last a few decades—especially if you brush or dip the cut board ends with wood preservative (Photo 4). If you want your wall to be around for a century, use foundation-grade treated wood. But you may have to do a little detective work to find a supplier. Get out the Yellow Pages and start calling lumberyards. Most lumberyards will special-order it and charge more for the upgrade.
Special Tool #1: Skid Steer Loader
When you get into big retaining walls, you have to move a lot of dirt. It’s tempting to rent your own skid steer and do the excavating yourself. Don’t waste your time and money! We rented one and spent seven hours doing what a professional could have done in an hour and a half at a much lower cost. Hire the pro.
Special Tool #2: Builder’s Level
A good-looking retaining wall depends on level footings, and the best way to establish level over long distances is with a builder’s level. If you’re building a wall with only a couple of 4-ft. sections, you can get away with using a 4-ft. level, but for a longer wall, rent a builder’s level.
How to set it up:
- Select a spot where you’ll have a clear view of the entire project. It should be well away from the wall so it won’t be disturbed during construction. Drive the tripod tips into the ground with your foot until the table is fairly level.
- Mount the scope on the table and snug up the nut that holds the scope down.
- Rotate the scope until it’s directly over two of the four adjusting nuts and twist the nuts until the bubble is centered in the vial. Then rotate the scope 90 degrees so it’s over the other two adjusting nuts and level it again. Repeat the process until the bubble stays absolutely level as you rotate the scope.
How to use it:
A builder’s level has to be absolutely level to give accurate readings. Check the vial frequently during the day to make sure it stays level. Sometimes the legs get bumped or stepped on. You’ll have a lot of work to redo if you’re unknowingly using a scope that’s not true.
Looking through the scope, you’ll see crosshairs that will tell you the exact height of whatever is being measured (see Photo 7) relative to a “benchmark,” or starting point. In this case, the benchmark is the first footing. Take a reading from the first footing with a tape measure—let’s say it’s 58 in. If the next stake is 56-1/2 in., it’s 1-1/2 in. too high.
It’s easy to get confused. Just remember: If a stake reading is a larger number than the benchmark, the stake is too low; if it’s a smaller number, it’s too high.
Project step-by-step (16)
Excavate and assemble the posts
Assemble the posts
Build a 90-degree assembly jig from 2x4s (shown in blue) to keep the stanchions square while you nail. Screw the jig to a sheet of plywood. Short 2x6 blocks hold the header 1-1/2 in. off the table. To assemble the post, center the 2x4 in the middle of the first 2x6 and nail it with 3-in. galvanized nails, keeping the top ends flush. Nail the header onto the bottom of the vertical 2x6 (flush with the 2x4) with two 3-in. nails at two opposite corners.
Add reinforcement straps
Nail one end of the strap to the end of the 2x6 brace, filling all the holes with 1-1/2 in. galvanized joist hanger nails. Rest the bottom of the brace on the header and adjust the brace until the top of the hanger is about 3 in. from the top of the post. Nail the hanger onto the post with joist hanger nails and tack the brace to the header with two 3-in. galvanized nails.
Finish the stanchions
Tack the second 2x6 post member to the 2x4 with three 3-in. nails and bolt the assembly together with 5-1/2 in. x 3/8-in. bolts, washers and nuts (see Fig. A for Fastening positions).
Brush preservative into raw ends
Coat (dip or paint) the ends, especially freshly cut ones, with wood preservative to prevent rot. Center and nail two footers to the bottoms of the post and header. Nail two shelves on the top of the header, one on each side of the brace (see Photo 6 and Fig. A). Use two 3-in. nails for each. If you're hardy and have lots of time on your hands, you could do all the excavating and backfilling by hand, but that's a huge job, especially for this design. Because the stanchions extend into the hill 4 ft. behind the wall, it's necessary to dig out 4 ft. behind the entire wall level with the footings. It took three separate sessions with the loader to dig out and backfill our double-tier wall.
First we dug the lower level back 5 ft. behind the face of the wall, then installed the lower wall. Then, we filled in behind the lower wall and prepared the hill for the upper wall. After the upper wall was finished, we backfilled and graded the entire area surrounding the walls to get it ready for sod. A single-tier wall would require only two sessions with the loader.
Figure out how many stanchions you need (See “Ordering the lumber and hardware” below), and assemble them in your garage (Photos 1-3).
Ordering the Lumber and Hardware
- Calculate the number of stanchions:
- Figure out lumber quantities by first determining how many stanchions you need. Divide the total wall length of each tier by 4 and add 1 to get the total number you need to build. For each 40-ft. section, we needed 10 stanchions plus one for the end, making 11 for each wall. Two walls required double that number, or 22 stanchions.
- For each stanchion, order:
- Three green treated 10-ft. long 2x6s for the below-grade components for every two stanchions.
- One brown treated 8-ft. 2x6 for every post.
- One brown treated 4-ft. 2x4 for every post.
- Enough linear footage of brown treated 2x8 to cap the walls ordered, in any multiple of 4 ft. Estimating the number of 2x6 planks for sheathing the wall is easy. You'll need two linear feet of 2x6 for each square foot of wall surface area. Order the 2x6s in multiples of 4 ft. so the splices will land behind the 4-ft. spaced posts. You can use 8-ft., 12-ft. or even 16-ft. lengths.
- For each stanchion, order the following hardware:
- Six sets of 5-1/2 in. x 3/8-in. hex head bolts with two washers and a nut.
- One Simpson TS18 Twist Strap (or the equivalent; straps are probably special order) and about 20 galvanized joist hanger nails.
- A quarter pound of hot-dipped galvanized 16d nails for fastening the 2x6 planks on the back of the posts.
- 20 sq. ft. of landscape fabric.
- Three cubic feet of gravel (any type) for backfilling.
- One cubic foot of sand for each post footing.
- A 2-ft. long 1x2 stake for every stanchion.
- One gallon of wood preservative for all cut ends.
Position and level the stanchions
Level the first post footing
Dig the first end post footing by removing the top 4 in. of soil in a 2-ft. square area. Pound a stake near the center of the hole, keeping the top of the stake even with the top of the hole. Pack the hole with sand, then smooth it with a flat board.
Plumb the stanchion
Set the first post on the sand footing and adjust the stanchion until it's plumb and roughly square to the direction of the wall face.
Position the other stanchions
String a mason line from the first stanchion to the other end of the wall to provide a straight layout line. Adjust the string to minimize extra digging. Position the other stanchions by measuring from the first stanchion (see Photo 8) and pounding a stake in the center of each location a few inches back from the line. Level the tops of the stakes with a builder's level. Dig 4-in. footings around each stake, pack each footing with sand, and smooth the tops even with the top of the stake.
Brace the stanchions
Plumb and brace the first stanchion. (You'll be anchoring the others to it.) Set additional stanchions along the string line. Set the second stanchion 3 ft. 9-3/4 in. from the first stanchion (center to center, not the space in between) and all the others at 4-ft. increments. That way the 8-ft. wall planks will always meet in the middle of a stanchion. Set the bottoms first, then plumb them. Hold the stanchion tops in place by tacking on a 2x4. Mark the 2x4 with the proper spacing and tack it to the other stanchions.
Square the stanchion
Square each stanchion to the mason line with a carpenter's square. Hold a 1-1/2 in. thick block against the header to fur it out for squaring. Shovel dirt over the end of the header to help lock the wall in place. Follow Photos 5-9 for a detailed how-to.
Install the wall planks
Cut and nail the wall planks
Cut the first row of planks to fit between the headers, then nail the boards into each stanchion with two 16d galvanized nails. Add planks, alternating the joints behind posts for a stronger wall.
Work planks around braces
Slip in the planks that fit behind the brace before installing the row directly below it. (It's impossible to put them in later.) Then push that row down into place and nail.
Cut a slot for the strap
Notch the row directly over the braces by holding them in their proper positions and tapping them against the sharp edge of the strapping. Using the indentations as a guide, make a couple of passes through the backside of the board with a circular saw to create a slot for the strap. Follow Photos 10-12 for a detailed how-to.
Backfill and finish up
Add landscape fabric
Staple landscape fabric onto the backside of the wall before backfilling.
Use slip forms to backfill
Build two 2x4 and 1/2-in. plywood slip forms to allow you to backfill with a thin layer of gravel behind the wall. Shovel about 1 ft. of earth against the slip form and fill it to the same level with gravel, then pull the slip form up and repeat the process until the wall is filled to within 1 ft. of the top.
Add bulkheads as needed
Slip the first bulkhead plank into the stanchion recesses, then level and nail through the outside corners of the 2x6s. Drop in and nail the rest of the bulkhead planks.
Install 2x8 cps
Screw the 2x8 top caps to the top of the stanchions with two 3-in. deck screws at each post. For bulkhead spans longer than 4 ft., nail a vertical 2x6 stiffener to the backside of the wall planks. It'll keep the wall planks in line and add strength to the top cap in case someone uses it for a step.
Establishing good drainage with a layer of gravel is essential for a long-lasting wall. Because the gravel is porous, it drains water and separates water-laden soil from the wall. If you don't use the gravel and you live in a frosty climate, frozen soil can grab hold of the wall, lift it out of position, and permanently warp it.
Install a pair of homemade “slip forms” (Photo 14), then shovel in a thin vertical layer of gravel between the wall and the backfill dirt. Build the slip form by cutting a 2-ft. x 3-1/2 ft. piece of plywood and nailing a 2-ft. length of 2x4 at each end to provide the space for the gravel. Cut in a couple of handholds near the top for easier lifting. Shovel about 1 ft. of dirt against the form, then shovel 1 ft. of gravel into the form. Slide the form up until the gravel is within 1 ft. of the top of the wall. You'll end up with a hill of hand-shoveled dirt that traps the gravel against the wall. When you're through, you're ready to have the skid steer finish the backfill.
After you backfill, the dirt will settle significantly. We filled the walls all the way to the top, and after a couple of weeks, the dirt had settled almost an entire foot. Then we leveled out the earth by hand and used wheelbarrows to fill the rest with high-quality topsoil for the plantings.
Cap the wall with a 2x8 for a finishing touch if you wish. The cap is strictly cosmetic and doesn't add to the structural integrity of the wall. If you opt for it, span 12-ft. 2x8s over the posts and secure them to each post with 3-1/2 in. deck screws.