A timber retaining wall is a cost effective option for flattening out your property. As we learned in this post, all wood in a timber retaining wall must be pressure treated to UC4A or UC4B standards (UC4B preferred). The timbers shall be 6″x6″ or 8″x8″. To further help with wood protection, we will ensure the wood does not contact natural ground and the wall has proper drainage.
Tools
- Hand Tamper*
- Shovel*
- Gloves*
- Wheel Barrow
- 4-ft Level*
- Torpedo Level*
- Sledge hammer* or Deadblow*
- Drill (if using screws)
- Circular Saw or Chainsaw
Materials
- Pressure Treated Timbers
- Galvanized or Stainless Nails/Screws
- Clean Crushed Rock
- Filter Fabric*
- Rebar*
- Paintable copper based wood preservative*
- Construction Adhesive*
- Perforated Drain Pipe*
- Pipe outlet or Wall Drain Pro*
Foundation
Dig a trench that is at least 12-inches wide. For walls between 0-2 ft, dig an 8-inch minimum depth. Compact the bottom of the trench using a hand tamper. Check out more information on preparing the foundation here.
After compacting the foundation, place the base material. The base material must consist of at least 4-inches of clean crushed rock such as No. 57 stone. The clean crushed rock will remove water from under the wall helping to prevent water damage to the timbers. For more information on retaining wall backfill materials, check out this post.
Compact the base material using the hand tamper.
Setting First Row of Timbers
Place the first row of timbers directly on the crushed rock base. Start at a corner or structure that you are abutting against. Leave no gap between the timbers. Then, ensure the timbers are completely level along the length of the timber by using a 4-ft level*. Use a torpedo level* to level the timber from front to back. Use a deadblow* or small sledge hammer* to adjust the timbers.
Drill two holes at 4-ft apart on each timber on the bottom row. The holes should be the same diameter as the rebar. The length of the rebar* should be at least 2-ft. Drive the rebar through the holes with a sledge hammer.
Subsequent Rows of Timbers
Stagger the joints of the next row of timbers from the ones below. This helps the stability of the retaining wall. Aim to place at least a quarter of the length of the timber past a joint (for 8-ft timbers, this is at least 2-ft). Also, set each row of timbers with a 1/2-inch setback from the one below. This dramatically helps with the stability of the wall.
Note: Many retaining walls are built without offsetting each row. These walls also lean out and fail over time. Vertical wood walls also look like they are leaning out, even if they are built straight. If there is no convincing you of building your retaining wall with a 1/2-inch offset, then batter the timbers toward the backfill side by 5-degrees. This will help the retaining wall look vertical after completing the wall. To accomplish this, tape 6 quarters on one end of your torpedo level. Place the side with quarters near the fill side of the timber when leveling from front to back.
Attach each row of timbers to the one below using 3/8″x12″ galvanized nails*, 12″ stainless steel screws*, or 12″ ACQ approved coated screw*. Because the copper in the pressure treatment is corrosive, use hot-dipped galvanized or stainless steel nails/screws. Do not use electroplated (or zinc plated), uncoated, painted, or other type of coating unless it is rated for ACQ pressure treatment. These alternative coatings are fine for an above ground deck, but they do not provide the necessary corrosion protection needed for pressure treated wood that is in contact with the ground. Space the nails or screws every 2-ft along the timber.
Installing Drainage
Water cannot easily flow through wood like it can through porous segmental blocks. This means you need special accommodations to ensure water does not build up behind the wall. Place a minimum of 12-inches of drainage stone (a clean crushed rock such as No. 57 stone) behind the timbers. This, combined with the clean crushed stone used for the base, will help the water drain underneath the wall.
Run a 3-in or 4-in perforated pipe behind the wall and outlet every 30 to 40 ft. Plan an outlet for the pipe the lowest points on the wall. The lowest point may either be at the end of the wall or somewhere in the middle. An outlet in the middle will require a hole in the timbers.
Compact the drainage stone with a hand tamper. Continue placing the drainage stone to within 4-inches of the of the wall. When you get to the top of the wall, place filter fabric above the drainage stone. Extend the fabric up the back of the timbers and attach with construction adhesive. Extend the filter fabric at least 6-inches past the drainage stone.
Cutting Wood
Timbers are too thick to cut with a standard circular saw in one cut. To use a circular saw, you will need to cut one edge, rotate the timber, cut again, and continue this until you have complete sawn through the timber. Alternatively, use a chainsaw to cut the timber. Wear a mask to avoid breathing in any dust from cutting pressure treated wood.
Pressure treatment only extends an inch or so into the wood. So, when you cut through the wood or drill a hole, the cut edge will be untreated. Treat any cut edge with a paintable copper based wood preservative*. Use a foam paint brush* to apply the preservative.
Final Thoughts
A timber retaining wall can be a great feature for your backyard project. The steps for building a wall up to 2-ft tall are simple. Use UC4B pressure treated wood, install clean crushed aggregate on all surfaces that touch the wood, pin the bottom row of timbers, and coat all cut edges. In the next post, we will go over timber walls greater 2-ft and how to install deadman anchors.
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