How to Build a Timber Retaining Wall
Retaining walls can be constructed from stacked field stone, block, concrete, or timber. Timber retaining walls have been used installed in residential landscapes for generations. They are used to hold back soil on steep grades, provide level access to homes or walkways, or are used for landscaping and gardening functions. These structures can also increase usable yard space by terracing sloped properties.
Along with sloped landscapes where water runoff causes hillside erosion, ideal locations for a retaining wall include spots downhill from soil fault lines and where the downhill side of a foundation is losing supporting soil or its uphill side is under pressure from sliding soil.
If your property needs a retaining wall, or if the one you have is falling apart you may be able to build a timber retaining wall yourself.
You may want to consider hiring an engineer to design and certify your retaining wall if it is going to be higher than 4 feet. You should consult with your local building inspector and zoning department to see what permits are required. Simple garden retaining walls may not require any inspections or plans, but larger retaining walls or retaining walls located near easements or property lines will often require a site plan be submitted with specifications on the timber wall construction and costs.
If you look around your neighborhood you may see several properties with existing timber retaining walls. Of these, there may be a few that were improperly installed and show signs of bulging or leaning. Most often this is due to poor drainage or improperly securing the timber retaining wall to the slope.
Small retaining walls have to withstand enormous loads. A 4-foot-high, 15-foot-long wall could be holding back as much as 20 tons of saturated soil. An 8-foot high wall would need to hold back as much as 160 tons. This is why walls over 4 feet high need to be engineered and may not be suitable for a do it yourself project. If you need a taller wall, consider step-terracing the slope and use two walls that are half as tall.
For a timber retaining wall you can expect to pay between $10 and $15 per square foot, calculated by the area of the wall face; a 4’ x 15’ wall would be 60 square feet or between $600 and $900. Preparing the site may raise costs substantially if it contains clay soil, heavy rock, or a natural spring. Expect to pay an additional 10 percent if you hire a landscape architect or engineer. You may be able to have the design cost included in the price if you get several bids.
Overview: Retaining Wall Construction
As stated before, retaining wall failures are primarily due to poor drainage. A good retaining wall begins with landscape fabric, gravel backfill, and 4-inch perforated drainpipe.
Timber walls should be built on a trench filled with about 6 to 8 inches of compacted gravel so that the top of the first timber is even with the grade of the soil.
Before adding gravel, lay down enough landscape fabric to contain the new gravel. Form the fabric into a large C shape, with the open mouth of the C facing downhill. The fabric should wrap around and create a border between the gravel and topsoil to keep sediment from clogging the gravel and drainpipe.
Replace the removed soil with 3/4 inch gravel or "bank-run" gravel (washed stones 1/4 inch to 6 inches in diameter). Shovel at least a 4-inch layer of gravel onto the landscape fabric. Grade this layer so it slopes 1 inch for every 4 feet, allowing water to drain away. Then lay in 4-inch perforated PVC drainpipe at the base of the wall and cover it with gravel.
Shovel in backfill as you build the wall, one tier at a time. Don't add all the backfill at the end—it won't compact thoroughly. Tamp down the gravel as you go with a heavy hand tamper. Behind the top tier of the wall add 6 inches of topsoil and lightly compact it.
All retaining walls should lean into the hill 1 inch for every 12 inches of height. Timber walls 4 feet or higher should be tied to the hillside with "dead men" anchors (6-foot-long, T-shaped tiebacks buried in the hillside) attached to the wall every 8 feet, extending 6 feet back to a 2-foot-wide T-bar.
Use 8-foot-long, 6x6-inch pressure-treated wood designated "For Ground Contact," and have all materials delivered. Pin the first tier of timbers to the ground with #4 rebar (1/2-inch diameter).
Use an end cut treatment for all cuts and drill holes. Water-based copper naphthenate is a wood preservative as well as an insect repellent.
Tools and Materials
Pressure treated timbers; quantity depends on size of wall
4-inch perforated drainage pipe
#4 Rebar in 4-foot lengths
Timber Wall tie screws, ¼-inch by 10 inches long.
Digging Bar, for removing stones
Contractors and homeowners used to use galvanized spikes to secure the timbers together in a retaining wall. The problem with this method is that the timbers can shift position and also become change the level of the wall. The nails also had to be pre-drilled. By using timber wall screws you won’t need to pre-drill any holes and you won’t shift the wall around pounding in the nails.
1. Set up wooden stakes and Mason’s strings to represent the height and perimeter of the retaining wall.
2. Stretch another level line near ground to represent the top of the first course of landscaping timbers.
3. Dig a 12-inch-deep trench around perimeter of retaining wall.
4. Use a hand tamper to compact the bottom of the trench.
5. Add about 6 inches of ¾-inch stone to trench, then compact the stone with hand tamper.
6. Drill a series of 9/16-inch-diameter holes in the first-course landscaping timbers.
7. Set a timber in the trench and check it for level. Use a sledge hammer to pound the high end of the timber until it is level. Don’t try to adjust the height of the gravel in the trench.
8. Stake the timber in place by using a small sledgehammer to drive 4-foot-long rebar spikes through the holes and into the ground.
9. Use a circular saw to cut timbers to length; use a 3-inch-wide foam brush to apply wood preservative to all freshly cut ends.
10. Measure across the width, from one side timber to the other, to ensure the two are parallel; make adjustments, if necessary.
11. Set the second row of timbers on top of the first course, making sure to stagger end joints by at least 4 feet.
12. Use an impact driver to fasten the second course to the first with timber wall screws.
13. Pour ¾-inch stone behind the wall and then drape the soil behind the wall with landscaping filter fabric.
14. Lay a length of 4-inch-diameter perforated pipe on top of the stone and then cover the pipe with more stone.
15. Pull the filter fabric over the stone-covered pipe.
16. After installing the third course of timbers, make a T-shape dead man (tie-back) from two short lengths of landscaping timber.
17. Dig out a T-shape trench into the hillside behind the wall and lay in the dead man. It is important not to remove any more soil than is necessary for the dead man to sit in the trench. Remember to apply wood cut preservative on all ends of the dead man.
18. Fasten the dead man to the retaining wall with two timber wall screws. Then stake the dead man to the soil with two rebar stakes. Install one dead man every 6 to 8 feet around the entire wall.
19. Continue building up the retaining wall, using screws to secure each timber.
For safety and appearance, cut the free ends of the timbers that are not buried in the soil at a 45 degree angle or and angle that matches the slope of the ground.