Home & Garden Columns

About the House: Using Wooden Retaining Walls in Your Home

By Matt Cantor
Thursday May 22, 2008 - 10:28:00 AM

Ours is an fluid landscape and all its houses, merely mobile homes. For those of us who live in the hills, it seems as though we live upon soils that have long been bored with their current circumstances and long to see the world. They just won’t stay put.  

Also, the steeper the soils, the more active they are. This is a source of great frustration for those of us who deal with the manifestations of this effect on an annual basis in the form of cracked houses and driveways that keep trying to change address. 

Additionally, there is the problem of soils that continually erode downward into our yards.  

There’s a manageable solution to spilling soils as well as the less dramatic conundrum of needing a bit of flat ground on which to set a table and chairs: A retaining wall. 

I see a lot of failed retaining walls and I also see a lot of very expensive concrete replacement. In fact, there’s a relationship between these two. Because many of the recommended repair are so expensive, we all live with a lot more ancient failed retaining walls that never get replaced. 

A wood retaining wall can fix this problem but it must be done properly or it can soon fail from rot or lack of strength. 

If you choose to build (or conscript for) a wooden retaining wall, here are a few of the things you can consider to make this job work out well. 


First: Plant those roots deep.  

One of the most common failings I find in retaining walls is the simple overturning of the entire wall system due to lack of depth. 

Have you ever noticed an entire hillside of trees that all angled from the base of the trunk upward to one side. This is due to earth movement. When surface soils slip or “creep” (this is an actual soils engineering term), the lower layers often move at a slower rate making the entire soil body rotate like the arm on a clock. This literally takes a tree and turns it toward the ground. Retaining walls do the same thing. Top layers often move more rapidly while lower layers stay relatively more stable. If you go deep enough (and the wall is strong enough) you can greatly slow this process. My rule of thumb is to make sure that my posts are as deep below ground as they will be above ground. A three foot high retaining wall will, then, have posts recessed below ground three feet. This is pretty rough logic but its a place to start. 

On flat ground, a retaining wall need not be so deeply footed but, if you want that wall to last, its not that much more work to bury those posts nice and deep. Also, you’re not paying for all the parts of the wall to be added below grade, just the post. 


Second: There’s strength in numbers.  

More post per length is one of the easiest ways to make a stronger retaining wall. One of the most common failings I see in the building of retaining walls is that the builders seem to fall into the trap of building all of them pretty much the same way with posts at about four foot spacing (or six), when the loads may vary from very light to extremely heavy.  

If you’re building a four foot high retaining wall that had a “surcharge” or slope heading uphill from the wall, you’re dealing with a lot of weight and may want to bring your posts as close as two feet apart (although this will vary with the size of posts and the type of board that will run between posts). The point is that a great wooden retaining wall (and keep in mind you’re saving a great deal of money over a typical concrete one) has adequately sized posts and plenty of them. 


Third: Put Your Shoulder Into It. 

When installing posts, its best to orient the post so that the thinner dimension faces into the wall. Some posts for retaining walls are square in cross-section, such a four by four (or better, a six by six) but it is a better use of wood and money to use an uneven dimension such as a four by six or a six by ten. You can then employ the strength of the wider part by letting it extend back from the wall. Again, you are going to face the smaller dimension against the wall. When you face the wall, you’ll be seeing the smaller dimension. This is analogous to the two by eights or tens (typical) below your floor. We press on the narrow face and try to stretch the long face, which is much harder than the other way. A great small retaining wall can be built using four-by-sixes or four-by-eights in this way. 


Fourth: Relieve The Tension.  

When building a wooden retaining wall, its best to cut the soil back behind the wall prior to construction so that you can install the wooden planks across the back side of the posts, between the posts and the soil. This will force the planks against the post making the nails or bolts virtually superfluous. Once you’ve attached the planks, its good to backfill the area between the soil and the wall with a large sharp-edged gravel that will work to distribute the force of earth against more of the wall and will also help to maintain drainage of moisture behind the wall. When soils get a chance to drain out behind a wall, the weight stays lower, there’s less movement and the wall last years longer. Of course, one of the things we love about wooden retaining walls is that they possess inherent drainage in that their inevitable gaps allow water to leak out. Nevertheless, drainage rock at the back of such a wall makes things work even better. It also lessens the fungal growth that eventually destroys all wood retaining walls that don’t fail from lack of strength. This brings us to my last item. 


Fifth and last: Species Matters. 

I’m a redwood guy. It might be because I just love the look of redwood but I there it is. Also, contrary to much eco-thinking, redwood can be quite eco-friendly, as long as you leave anyone older than the first Roosevelt presidency to itself. Redwood is a fast growing tree and can be forested without clear cutting to the joys of both Spotted Owl and human interloper. If you buy redwood, go for tighter more uniform grain and look out for pithy soft woods that will rot more quickly. If you inquire, you can find out where the wood was grown and how it was harvested. For the true tree-hugger, there’s also recycled redwood. 

If you choose to use pressure-treated wood and don’t mind the green roller-marked appearance, be sure to use suitable metal hardware since this PT wood contains a great deal of copper (much more than in the past since the removal of the arsenic) and is highly corrosive to ferrous metals such as steel and iron. 

If you’re considering one of the loose or pinned-block retaining wall system, keep in mind that these are not much stronger than graded soil itself and I’ve seen many that are buckling or tipping over as a result of being used where real resistance is needed. Concrete has roughly the same mass as consolidated soil, so the only real advantage that these systems provide is in erosion resistance. The don’t really hold a hillside together. 

Lastly, for those of you weekend warriors who want to take a shot at constructing a retaining wall, wood will be a more manageable project. It can be done without elaborate form-work, pumped or wheel-barrowed loads of heavy and impatient concrete and without bending rebar until you’re cross-eyed. The concrete retaining wall is definitely in the Master class, while a wooden one can actually be a beginner’s project.  

A brief paid visit from an engineer or talented general contractor might not cost as much as you think and can provide you with the specs you’ll need to do it right. If you hire out your wooden retaining wall, you can also talk about making it pretty. I’ve used large “malleable” washers for mine as both practical bonding and as motif (think of the Cannery in San Francisco where they used large star-emblazoned malleables all over to tie brick to framework). A retaining wall can actually be attractive if done well and can also ascend into fencing if you wish. 

It’s an unfortunately reality that many of us will be forced at some point to buy a retaining wall but, at least, with a wooden one, you won’t end up also suffering a financial landslide.