Although a retaining wall for your backyard can seem like a simple solution, you can get in trouble in a hurry without proper due diligence. There are several reasons to contact an engineering professional before proceeding with your retaining wall construction. When in doubt, contact a professional engineer for their opinion. Before you begin, you should check your local building code to see if and when they require a building permit. Failure to do so could result in a stop work order, fines, and possible liens on your house by your city. You should also check with your homeowner’s association on their requirements for approval.
The International Building Code (IBC) states: “A building permit shall not be required for … retaining walls that are not over 4 feet in height measured from the bottom of the footing to the top of the wall, unless supporting a surcharge …” Many municipalities have adopted this code or added modifications to it. It is important to check your specific local building code. Let’s take a deeper look at the specifics of the code.
What is the Height of Your Wall?
It is a common misconception to only consider what you can see when thinking about a retaining wall, but the retaining wall starts at the bottom of the first block or bottom of the footing. For example, a retaining wall less than 3.5-ft in exposed height will typically have at least 6 inches (in) of embedment (buried wall) for a total wall height of 4-ft.
A building permit is typically required if the total height of the wall (exposed and buried) exceeds 4-ft for cantilever or segmental gravity (block) retaining walls. Some municipalities require a building permit for walls greater than 2-ft in height even if they do not have a surcharge. Some municipalities will allow a building permit without an engineer’s stamp for wall heights between 4-ft and 6-ft in height. Again, it is important to check your local codes.
In addition, many block manufacturers have height restrictions of their own. In general, the maximum height restrictions for a certain block system will vary between 2-ft and 6-ft. You should never exceed the height recommended by a block manufacturer. If you are building a wall taller than 2-ft, consult with the block supplier or an engineer to verify the maximum height for that block system.
Is There a Surcharge?
A surcharge is any object that could impose a lateral force on the wall. Any surcharge on the wall will require evaluation by an engineer. This includes vehicles, swimming pool, equipment (such as an air handler or pool pump), footings from a structure (such as your house or porch), or simply a fence on top of the wall. A fence imposes wind load on the wall, even if it is not attached to the wall itself. Trees will also create a surcharge behind the wall as they grow and increase in weight, the roots will push the face of the wall out, and the height of the tree will impose wind loads on the wall. Small shrubs and plants are acceptable behind the wall.
As a general rule, the edge of any surcharge object must be at least 2 times the total wall height away from the back of the retaining wall. This means if you have a total wall height of 4-ft (buried height plus exposed height), the edge of any surcharge should be at least 8-ft away from the back of your wall.
Sloping fill behind your wall is another type of surcharge that will impart a lateral load on your wall. Slopes are usually labeled by the horizontal (H) length for every 1-ft change in vertical (V) elevation. So a 10H:1V slope would increase 1-ft in height for every 10-ft in horizontal distance. You can differentiate a “slope” from a “flat” grade when the ground is steeper than 10H:1V. Any slope on top of the wall will require review by an engineer. A slope greater than 2H:1V should be avoided, even if the slope is a considerable distance away from the wall. A slope in front of the wall also needs special consideration. Below are some examples of when an engineer is necessary in regards to slopes around the wall. Note an engineer may still be necessary for other reasons.
Are You Planning on a Terraced/Tiered Wall?
A terraced wall is when two or more shorter walls are used in lieu of a single taller wall. The upper tiered wall exerts a surcharge on the lower wall and therefore they are treated as one wall unless the upper wall is offset (D) a minimum of 2 times the height of the lower wall (H1). The height of the upper wall (H2) must be less than the height of the lower wall (H1).
As an example, if there are two 3-ft tall tiered walls that are spaced 4-ft apart, these are treated as one 6-ft tall wall and will require a permit and an engineer. If the two 3-ft tall tiered walls are spaced 6-ft or more apart, they can be treated as independent walls. You should not have more than one terrace on a non-engineered wall. Note many landscapers will create several tiers to circumvent local ordinances for permits and engineering costs. This is a scam used by landscapers to try to undercut their competition, and you should heed caution as an un-engineered wall can fail with the first rain storm.
Is There Water Present?
If your wall will be built next to a pond, creek or other water sources, even if it is seasonal, contact an engineer. If there are irrigation lines, water mains, or other pressurized water sources, these could be a big issue if they begin to leak or burst. Water is the number one cause of retaining wall failures, and many additional measures are required to ensure you have a safe wall. When preparing your site plan, you should consider relocating or piping any downspouts away from your wall for added safety. Water should drain away from the wall, and never toward the wall. Water should never pond behind the wall. If water cannot be redirected away from the wall, contact an engineer. If there is groundwater on your wall site, or if the ground is constantly moist, contact an engineer. For more on drainage, check out this post.
Are You in a Seismic Region?
All retaining walls within a seismic area should be reviewed by an engineer. To help identify if you are in a seismic area, reference the map below from the United States Geological Service (USGS). Regions ranging from yellow to dark red require an analysis by an engineer.
Is Your Wall Near a Walkway?
Another thing to consider is whether your retaining wall is close to a walkway. Generally, building codes require a handrail if the vertical drop (or exposed height of your retaining wall) is greater than 2.5-ft and the walkway is within 3-ft of the vertical drop. In all cases, a fence or handrail is required if the retaining wall is greater than 6-ft unless there is some other means to prevent access to the top of the retaining wall. If you are in doubt about whether your wall should have a fence or guardrail, consult with an engineer. This could be a huge liability for you if someone were to fall off the edge of your wall.
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