I’ve been conversing with a new landscape designer in Ohio, and sharing with him some of my thoughts and work processes. He asked me how to determine when a retaining wall is necessary and when it’s not, and I kind of ran with it. I ask myself that same question every time I design a project with grade changes on the site, and it’s a decision that’s impacted by not just structural requirements, but also my client’s budget and my own, personal design philosophy. Here’s what I wrote:
Easy question first: retaining walls. Now, here’s a caveat- while my design aesthetic is either very formal, classical garden-style or very loose, natural, blurring-the-edge-of-the-woods (it all depends on the site and architectural context), my heart is with the mid-century modernists. Form follows function, and the goal is restraint. Why make the client pay to build more than they need?
As that relates to retaining walls yes vs. no- in my opinion, a retaining wall is a last resort. If the only way to make the space function as required is to build a wall, build a wall. Otherwise, figure out a different solution. Obviously you need to consider your local factors- soil type, how much load is uphill, how much water is coming from uphill (and at what kind of velocity), and where the water is going from where you are. But typically, if I have a grade change of 12 inches or less- I prefer to cut and roll it. If you do that, you can even plant grass on the slope and still maintain it, if you grade properly. 24 inches or less? If I can, I’d rather roll it. Something to keep in mind is that the top of your wall doesn’t HAVE to be even with the top of your wall. If you need to retain just a little bit of soil, you can use a curbing product like granite cobbles or whatever Techo-Bloc’s curbstone is. You can also do a short (8-12 inch) drystack fieldstone wall, and allow the slope to pitch into the wall or curb. A wall can also be used for aesthetics (to visually expand a space, or define a courtyard) or multiple function (like a seat wall). So I’m not saying walls are bad. Just make sure you have a reason for wall, not just “there’s a grade change, so I must need a wall.” I’ve had jobs where I could’ve gotten away without a wall, but a little one did make it look a little tidier, and probably prevented mulch landslides until the groundcover got established. So… use them, but have a reason.
You’ll get a feel for where you need a wall and where you don’t the more projects you get under your belt. I really have to bite my tongue looking at some of these online landscape forums, because these guys are clearly all about strutting around and seeing who has the bigger wall. Think of it this way: 18-22 inches to top of wall, you have a seat wall. Those are functional and don’t really detract from the space. 24-36 inches, you’re still not impacting your aesthetics too terribly much. 36-60 inches, you’re going to start feeling boxed in. Anything over five feet and a) you need an engineer, and b) you just made your space feel smaller. And, you’ve dramatically upped the cost.
Example: I just presented a design for a client. They have a modest house on a pretty standard-sized lot. Off the back of the house, you go about fifteen feet and the ground shoots up at at least a 40% grade. However, it’s a 20 year old home, and the slope is heavily wooded. With all the brush, it’s a stable slope. They’re also not having water penetration issues in the rear of the home, so, my design didn’t include a wall. I kept the design very simple and functional. They have a lot of shade and they’re not super into gardening, so I did what made sense for them. They could renovate the entire landscape- including a new front walk and a small BBQ grill patio in the back, both made from natural stone- for around $20K. Or, for that $20K, we could do a 42″ wall made of 6×6 p/t timbers that wouldn’t buy them any more usable space. Needless to say, the client’s thrilled.
Just a little food for thought. Have a great weekend!