Stem Wall vs. Retaining Wall in ADU

Why build retaining walls or stem walls? Which one to choose? When you construct an accessory dwelling unit (ADU) on uneven ground, it may need additional structural support to maintain the integrity of the ADU as well as the safety and longevity of the unit. There are different site conditions that may require retaining wall construction, each of which leads to potential problems if you leave the ADU walls unreinforced.

Retaining walls serve the purpose of building a stable and secure foundation, preventing wall failure and potential issues with soil erosion, and stabilizing slopes. So, let’s delve into ADU construction considerations related to retaining walls and their differences from stem walls!

Retaining Wall Design

A retaining wall is a structure aimed at restraining soil or water to a declivity, if it doesn’t remain there naturally. If your ADU project requires landscaping, you may need to hold soil between varying elevations to optimize the use of your uneven land and boost the stability of the new unit.

The elements of retaining wall designs encompass a base slab that helps distribute the wall’s weight. The base may include a toe slab and a hill slab, contributing to the strength and sturdiness of the wall. The next part of the retaining wall is the drainage system, consisting of drainage stones and weep holes. It prevents water accumulation behind the wall.

The visible part of the wall that stands on the footage is the wall face. It can be a concrete slab, a wall made of natural stone, or other materials. It holds the backfill—the soil itself, which exerts pressure on the vertical stem wall. The part that resists lateral pressure (the pressure of the earth on the wall side) is the vertical stem. This element transfers the vertical weight to the wall’s base, counterbalancing the whole construction.

Retaining Wall Purpose

Why do people opt for building retaining walls? Are exterior walls not enough? Unfortunately, in some areas, earth pressure is so high that structures designed with less bearing capacity can fail to support their own weight.

Retaining walls have a range of functions, such as:

• Stabilizing inclines

Water and soil retention are not the only things mandated by local building codes, which makes it impossible to obtain permission if you fail to comply. There’s a more pressing issue with that. If you don’t retain soil mass, it can lead to the risk of landslides, presenting a significant threat to ADU owners, homeowners, and neighbors.

• Preventing soil erosion

Retaining walls hold back soil, resisting soil erosion (the natural movement of soil from one location to another under the influence of wind or water). Soil erosion causes soil infertility, which is bad if you’re dreaming of a nice garden. It also damages the environment, weakens the foundation of buildings, increases the risk of collapse, and alters the natural landscape of your land in a negative way.

• Generating functional areas

If you plan to build an ADU, you definitely need adequate space for a foundation building. In fact, the retaining walls confine soil within a set boundary, freeing more usable land for accessory dwelling units to fit. More so, the robust foundation it allows for is essential for the prevention of structural damage, problems with windows and doors as they sag, mold and mildew, or even flooding.

• Enhancing the property’s aesthetics

Retaining walls typically have the purpose of improving your safety and construction quality. A retaining wall has a side benefit. If you choose a suitable design (e.g., natural stone veneer), a retaining wall can create cozy outdoor spaces with plants, water features, decor, lights, and garden furniture to complement your house’s aesthetics.

Whether your primary goal is to prepare suitably, even land for an ADU, safeguard your property against landslides, take care of your future soil condition, or make a lovely sight, you can apply to us for professional retaining wall services!

Types of Retaining Walls of ADUs

Retaining walls differ in design as they serve diverse purposes. There are the following types of retaining walls:

• Gravity retaining walls

Gravity walls rely on their own weight to oppose lateral earth pressure. They usually don’t need extra reinforcement to effectively resist lateral forces, thanks to their unique design.

• Piling walls

Pile walls use special wall components, such as piles (steel bars), that go deep into the ground. This type of retaining wall is suitable for areas where the state of the soil doesn’t allow for conventional building. Then allow the load to shift down to more stable soil layers.

• Cantilever retaining walls

This vertical wall, called a cantilever retaining wall, supports lateral loads thanks to a cantilever (a horizontal beam). One of its ends goes deep in the soil and gets fixed there, while the other end extends out to prop up the backfill. Cantilevered retaining walls provide higher levels of stability and sturdiness for ADU construction.

• Anchored earth walls

Anchored walls utilize anchors that may use steel cables or other materials. Builders install them deep underground, behind the retaining wall, to counteract the lateral pressure. Where other types of retaining walls are not enough, anchored walls provide extra reinforcement.

• Counterfort retaining walls

Thin vertical slabs carry the name “counterforts.” They stretch from the footage of the wall to its top, inside the retaining wall. They are most suitable for taller walls, helping to distribute the pressure more efficiently and promoting their resilience.

We’ve established that the type of retaining wall depends on its design and method of fixation. Some retaining wall types use nothing but gravity force, while others come with more elaborate fixators such as piles, cantilevers, or anchors, all of which cater to the peculiar conditions of an ADU project.

Other Retaining Wall Varieties

Retaining wall options also vary depending on the construction method. For example, CMU walls utilize poured concrete blocks. You pour the concrete monolithic slab and rebar in place and then seal them to safeguard the construction against weather elements. These walls are mostly vertical or near-vertical.

Another way to create a retaining wall is by interlocking keystone blocks with each other, creating a more sloping angle. These blocks enable you to make reinforced soil layers arranged into beautiful landscapes.

Stem Wall and Retaining Wall: Differences

If you wonder what the biggest difference is between a stem wall and a retaining wall, let’s have a closer look at what a stem wall essentially is. Surely, they both aim to hold vertical loads and ensure the stability of the building situated on the land. However, they have several key differences.

A stem wall is a wall connected to a foundation. Stem walls are perimeter walls located below ground level, stretching vertically from the footing of a concrete slab that constitutes the foundation. So, basically, they’re vertical walls created as part of ADU or main house construction. Stem walls reinforce the foundation.

Unlike stem wall foundations, retaining walls are above-the-ground structures that are standalone. They resist soil pressure and stabilize the slopes to create buildable space for your ADU or other structures within specific space constraints.

To sum up, diverse sites demand different protection measures, and it’s up to you to decide what serves you the best. If you live in a coastal area that is prone to flooding and need to raise your ADU, you might prefer stem walls. If your land often experiences heavy rainfall and has unstable soil, a retaining wall may suit you better.

ADU Stem Wall Construction

What are the advantages of stem walls? There are plenty of instances where homeowners undeniably prefer stem walls to retaining walls. These scenarios include smaller plots of land where you lack space for a retaining wall or when there’s no need for costly grading and soil retention. Instead of preventing the soil from moving into your land, stem walls support the weight of the ADU structure, providing significant help to load-bearing walls within a structure. It can be critical in cases where your property needs additional elevation and stability.

Stem Wall Purposes for ADUs

The primary purposes for opting for stem walls in your ADU project involve the following:

1. Increasing robustness of the foundation;
2. Lifting the ADU above ground level on inclined land sections and allowing to avoid extensive leveling when it’s too costly or impossible;
3. Strengthening the structural integrity of the ADU;
4. Creating a barrier against the penetration of moisture and pests;
5. Creating crawl spaces for convenient access to utilities (plumbing, electrical lines, etc.);
6. Opportunity for elevated ADUs to offer scenic views;
7. Opportunities for landscaping as well as architectural enhancements like unique trim or façade details.

Thus, stem walls for an ADU project bolster foundation strength, raise buildings on uneven landscapes, improve ADU’s durability, and provide defense from dampness and vermin.

Stem Wall and Retaining Wall Building Process

No matter if you use these constructions for commercial buildings or for residential ADUs, stem walls and retaining walls are very distinct when it comes to their building. When creating a stem wall, you typically excavate the ground for the foundation first. Then, you set up forms around the ADU perimeter to create the shape of a stem wall and pour concrete in there.

As for the retaining wall building, you also excavate the trench for it and add proper drainage to prevent water accumulation. Next, you erect a wall made of any material you choose for it.

Stem Wall and Retaining Wall Materials

Most often, a stem wall uses such materials as reinforced concrete to strengthen construction and add stability to ADU’s foundation. On the other hand, a retaining wall provides more versatility in material use. You can make them out of concrete blocks, natural stone, bricks, steel sheets, or timber. It depends on the desired functionality and the kind of look you’re aiming for since, unlike stem walls, they’re visible structures that can complement your ADU house aesthetics.

Stem Wall and Retaining Wall Construction Costs

Generally, stem walls are more cost-effective than retaining wall construction. However, it’s important to note that distinct site conditions, labor costs in your district, and the materials you select can significantly affect the price. If a steel sheet retaining wall may turn out to be quite inexpensive, a natural stone created on a plot with very uneven terrain can result in big expenses.

Frequently Asked Questions — FAQ

What are the four types of retaining walls?

The most common types of retaining walls encompass gravity retaining walls, cantilever retaining walls, piling walls, and anchored retaining walls. However, there are also other types of retaining structures used in the construction industry, such as counterfort retaining walls, crib retaining walls, gabion retaining walls, CMU walls, etc. There are different types of retaining walls and construction to cater to diverse ADU projects.

What is the purpose of a stem wall?

The purpose of a stem wall is to reinforce the ADU foundation. Concrete stem walls are useful to raise the small house from the ground level in cases of uneven terrain surfaces and simultaneously add stability to the construction. They connect the foundation with the exterior walls of the ADU and dispense the building’s weight onto the reinforced concrete foundation.

When to use a stem wall?

Stem wall foundations are very common for hillside areas or areas prone to flooding and seismic activity. Other situations that may require stem walls are sites with unusual soil conditions (e.g., moisture-sensitive soils) or other land-specific factors like unstable or expansive soils, erosion, and areas with increased hydrostatic pressure in the soil.

What is retaining wall failure?

Retaining wall failure is a situation when a retaining wall collapses or otherwise stops performing its functions to withstand lateral earth pressure. This might happen for such reasons as excessive pressure, poor design, poor-quality installation, improper drainage, unforeseen geological circumstances, and others. That’s why we recommend regularly checking retaining walls to ensure their structural integrity and effective performance.

Is it better to choose a stem wall or a crawl space for my ADU?

This choice heavily depends on what purposes you want to achieve with the construction. If you pursue to enhance the stability of your ADU, then it’s better to elevate your ADU above ground level with the help of stem walls. Crawl spaces, on the other hand, are more suitable for creating accessible areas beneath the floor. You can use it for storage, convenience during construction work, or better ventilation.