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3 Bombproof Retaining Walls For Sonoma County, CA

Retaining walls look easy, right? It's just a little digging, a few blocks, and it's go-time, right?


We could go on and on about different retaining wall failures, but there's three key components to a good retaining wall:

FOOTING - What a retaining wall sits on.

MATERIAL - What your wall is made out of.

DRAINAGE - The system which lets soil water drain from behind the wall, preventing a blowout due to hydrostatic force.

So without further discussion, and in no particular order, here's my very favorite retaining wall designs.

1) Piling-And-Board wood wall

REGULATIONS: No more than 4 foot tall without an engineer designing and signing off on the wall.

A piling-and board retaining wall (let's just refer to it as the wood wall) is cost effective and alluring to penny-pinching prospective clients. The materials are lightweight and readily available. Skilled woodworkers can make quick work of this wall. Little dirt has to be excavated, and no baserock needs to be trucked in, making this a great option for limited access locations.

Most walls use a foundation made of noncompressable base material as a footer, like on your house or garage. Wood walls gain their strength from their giant concrete piers. We like to dig the piers 3x the size of the post with our power-auger, fill the piers with soupy, plasticized 4000psi (very strong) concrete to fill in all the gaps and allow for little adjustments, and leave them alone for one dry day before constructing the wall portion. There's no doubt our walls will hold up!

The downsides: Appearance, soil concerns, and lifespan. We can make the prettiest, toughest, tightest-tolerance wood wall in the world, but pressure-treated lumber (Yes, it HAS to be treated) just doesn't fit the vibe of some landscaped areas! Rejecting an idea just because it's not that pretty to you is valid to us. And rejecting the wooden wall idea based on the wall's short 30 year average lifespan is doubly valid. Contractors may shudder at the idea of drilling through solid rock, so in areas such as Cazadero's mountaintops, or high points in Marin County with loads of serpentine rock in the soil, a different wall style may be more practical.

2- Concrete Masonry Unit (CMU) Wall

REGULATIONS: No more than 4ft tall without an engineer designing and signing off on the project.

Cinderblocks, or CMU's are popular due to their predictability, strength, and in-stock nature - CMU's of high quality are available at almost anywhere building supplies are sold. They're like legos, but for adults with contractor's licenses. Many homes are built, in part, or as a whole, with CMU's, often serving as a "Stem Wall" for framers to construct base boards and framing upon.

The appearance of a typical CMU is dull and drab, and not particularly in style right now. (were they ever?) But don't fear - CMU's can easily be gussied up with decorative caps, stucco, paint, ledger panels, veneers, or natural stone overmolding. The standard grey ones are available anywhere, but CMU's can be ordered in a variety of colors, sizes, and appearances. An example of a "textured edge" block can be seen at the bottom of this page.

CMU's as a standalone item are rather brittle, which is why minimizing hydrostatic loads by installing a drain behind them is a good idea. A CMU wall built as a retaining wall must sit on a reinforced concrete base, twice the width as the width of the blocks, and ought to be set in "S" mortar and grouted. Grout, for wallbuilders, means a soupy concrete mix that is poured into the cavities of the blocks. Also inside these cavities is rebar, spaced at every-other-cavity and tied deep into the concrete foundation. Rebar helps the wall stay together, even if a crack or accident occurs. Of course, there's no way this wall is weeping. Installing a drain behind the wall is absolutely crucial.

The drawback to CMU walls is the labor and legwork they demand. There's not a steep learning curve, but concrete and mortar are unforgiving materials to work with. Plus, there's many steps and materials needed. Dig, then install bars, forms, then cement, then lay blocks, then grout blocks. As a project manager, this means calculating and arranging delivery of many orders of different materials onsite and staged out in a sensible manner. A bit of a headache! As a laborer, journeyman, or foreman, this means lots of skills to either learn or perfect throuought the course of a project. A fun challenge, but it's a time-hog having to switch gears so often. Overall, the CMU wall is a great, strong, and long lasting choice for a litany of wallbuilding concerns.

#3 - Board-Formed Concrete Retaining Wall

REGULATIONS: Up to six feet tall, without needing an engineer to design or sign off on the wall.

Concrete, with a skilled team and a good readi-mix supplier, can be used to build nearly anything. Walls are no exception, and while most concrete walls are built with plain-old smooth forms, we strongly recommend board-formed concrete. It's a little more prep work, but the payoff is a one-of-a-kind product which draws to mind the old addage of the snowflake - they all look similar from a distance, but no two are the same.

When a concrete wall is constructed, workers build an incredibly strong wooden frame called a "form" for the concrete to fill up and set up inside. Since concrete moulds like play-dough and dries as hard as a rock, this means the concrete will take the shape and texture of any form it touches. This has bugged constructors for years, and countless hours have been spent to make concrete walls perfectly smooth. But one day, somebody smart likely said, "What if we used really cool boards, so the concrete would form to them and resemble wood when we removed the forms?" And that's exactly the look you get with a board-formed wall. A less "edgy" look can be achieved by filling gaps between boards with silicone sealant. To create a more professional finish, oftentimes workers will spray the wooden forms with a special biodegradable oil to keep the concrete from sticking to the boards and being difficult to pull from the forms when the concrete has set. Using rough-cut lumber can result in a granier, knottier look, and using different sized lumber (one row of 2x4, one row of 2x10) will make it truely unique.

Like a regular, smooth concrete wall, this one needs lots of rebar reinforcement, and a sturdy foundation, up to 18 inches deep or more for tall walls. If local codes and the integrity of the forms allow it, occasionally a contractor will order concrete for a wall with a plasticizer. This allows the concrete to flow like water and fill more gaps with minimal air pockets, which are unsightly and can manifest themselves as pockmarks in the finish. Some contractors prefer using glass-reinforced concrete, at the expense of the finish. Glass reinforced concrete is tougher, but it can be more difficult to get a nice finish out of. Personally, I'm not into fancy mixes and special tricks. A good high portland, small rock, 5 slump mix thoroughly vibrated in the forms and poured in the wee hours before the sun dries it out will get you there. And of course, a well flowing drain behind the wall is key to preventing your board-formed concrete from becoming a board-formed blowout.

The downsides - This is NOT a DIY project. Yes, it CAN be done, but unless you're a concrete worker doing this as a side project or a retired contractor, this is one to phone in. There's endless tips and tricks associated with this concrete work. Entire books can, and have, been written on vertical concrete work. I'm NOT an expert, and there's loads of dedicated concrete workers who stand head-and-shoulders over my abilities. There's no shame in knowing your limitations - I won't touch most board formed concrete walls unless I know without a shadow of a doubt I can do a superb job.

Board formed concrete combines skilled woodworking with skilled masonry work. Many, many board feet of lumber go into forming even the smallest of walls, and at the end of the job, it's all got to go to the recycler's. That said, lumber prices will drastically impact the cost of your board formed concrete project. Portland cement, sand, and rock prices will also play havoc with your budget, but wood seems to be the more volatile component, price-wise. Really, you're building two walls - a wooden wall, then a concrete wall. Keep that in mind when you're planning a project. Depending on the site and economic conditions, a CMU wall may be cheaper, or more expensive. Consider getting a quote for each from your contractor.

The options don't stop here. These are simply my favorite options. Traditional wooden homes in forested areas with soft soils are typically the best fits for wooden walls, older southwestern-style houses in more arid locations like Sonoma or Healdsburg are perfect fits for concrete-masonry-unit walls, and board-formed concrete walls are good choices in modern, angular houses, like the Dr-Seuss esque homes on the coastline or one-off custom builds done in the past 10 years. But don't paint yourself (Or, in this case, wall yourself) into a corner based on this alone. If you're considering a wall, call 707-755-0612. We'll help you pick the best one for your needs, and build it to last a lifetime. We are licensed, insured, and have a kind and able team, and can accomplish nearly all wall-builds in house.

- Sam W

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