Retaining walls are a great way to level out a sloping yard or maintain two different levels of soil. Whatever your purpose is for building a retaining wall, you might be wondering how deep the retaining wall posts should be. Well, we've done the research and have an answer for you.
Retaining wall posts should be at least as deep as the amount of soil they will be holding. A good rule of thumb is to halve the height of the wall and then add 4 inches. This is how deep you should place your posts.
Now you have a general idea of how deep to place your posts, but keep reading as we elaborate on this further. We'll discuss some other ways people calculate the depth of their retaining wall posts. Additionally, we'll answer some other questions you might have about retaining walls, including what is the strongest type.
Depth Of Retaining Wall Posts
If you're getting ready to dig your holes for your retaining wall posts, you likely have a good idea of how tall your retaining wall is going to be. One method is to dig your holes at least half the height of your wall.
Another method used by someone who frequently constructs retaining walls is to dig at least 3ft down. 3ft is a good depth to hold the soil sturdily behind most retaining walls.
Then there's the method we mentioned above that is used by a landscaping company located in Australia. Though they use mm instead of inches, the concept is still the same. Half the height of your wall and add four inches. For example, if your wall is going to be 30 inches tall, you should place your posts at least 19 inches deep. The additional 4 inches account for any topsoil you might add.
How far apart should retaining wall posts be?
Now you know how deep they should go, but what about how far apart the posts should be?
A good distance between retaining wall posts is 3ft. You can increase or decrease this amount by a couple of feet if you'd prefer that for aesthetic reasons. However, you should keep the distance below 7ft to avoid the horizontal planks from buckling. If you begin spacing out your posts 3ft apart but find you have some distance left to cover, it may be worth extending the wall to cover the last 3ft. Or, you can divide the length of the wall by the number of posts you want to space them evenly.
Retaining Wall Trench
To build a retaining wall, you will need to excavate a trench for the base of the retaining wall to be built in. The wall footing and base of the wall will need to be buried in the ground, just like the posts. Just like the posts, there are important recommendations for the depth you should place these as well. To account for both the footing and base of the wall, trenches should be dug deep enough to allow three inches of gravel footing and then half of the first row of blocks or wood plank.
Retaining Wall Footing Depth
The wall's footing is what goes underneath the wall to keep it level and secure. Some choose to compact gravel for their footing, while others use concrete pours. Gravel is a good option if your wall is less than 15ft long. The footing depth depends on the height of your wall. In most cases, a good rule to follow is that the depth of the footing should be an eighth of the height of the wall.
However, the footing of the wall should also be below the frost line. In northern states, the frost line may be further down than an eighth of the height of the wall, so be sure to go by its depth instead of the walls.
Retaining Wall Base Depth
The base of the wall will also need to be below ground level. If you use blocks to build your retaining wall, make sure the first row is buried halfway up the blocks. Typically this is between 4 to 6 inches deep.
What is the strongest type of retaining wall?
There are a few types of retaining walls. Retaining walls made of concrete retainer blocks are typically the strongest and easiest to install for homeowners. Concrete block retainer walls will work for most retainer wall needs. In general, steel pile retaining walls are the strongest and are often installed temporarily at construction sites to help support foundations. Other retainer wall materials include wood, bricks, and pavers. We've included some image examples below.
Concrete Block Retaining Wall
Here is an example of a block retaining wall. These are the most durable type of retainer walls for homeowners. They are also one of the easiest to install, especially for DIYers. However, they can only be used for retaining walls no taller than 4ft.
Paver/Stone Retaining Wall
Paver or stone retaining walls can be more decorative and are also pretty sturdy. However, they often require more architectural expertise to build, making it worthwhile to contact a professional.
Brick Retaining Wall
Another sturdy option, bricks, will work well for constructing a retaining wall. Installing a brick retaining wall is not as easy as using interlocking concrete retainer wall blocks, though, and will be more time-consuming. However, some homeowners prefer the look.
Wood Retaining Wall
Wood retaining walls are the least sturdy of the bunch. They cannot withstand a heavy load, so they should not be used for any heavy-duty retaining. However, they are a great option if you're looking to create raised flower beds.
Steel Pile Retaining Wall
Strong and durable, but you can see why you might not want to used steel in your backyard. It's the least attractive of the materials and definitely more suitable in a construction zone or commercial setting.
Do I need drainage behind a wood retaining wall?
Wood is more porous than other retaining wall materials such as brick or cement, but it's still a good idea to add drainage of some kind to your wood retaining wall. Even though it's less likely to buckle from too much water, wood is more susceptible to rot and decay, which can cause the retaining wall to fail.
To ensure longevity and decrease the chance of the retaining wall failing, all walls should have a drainage method. Fortunately, there are a few ways to add drainage to your wall.
Retaining Wall Drainage Methods
Adding drainage stone, a drainage pipe, filter fabric, or weep holes are some ways you can add drainage to your retaining wall.
Drainage stone should be added behind your wall. It should extend at least a foot back and up to within six inches of the top of the wall. It is a type of gravel that you can find at most hardware stores.
Another option is to install a perforated pipe along the bottom of your wall. The pipe will need an outlet, though, so keep this in mind.
Filter fabric can be placed directly behind the retaining wall to help with drainage. It can also be placed on top of your layer of drainage stone under your topsoil.
Click here to see Filter Fabric on Amazon.
Weep holes are just holes drilled into the wall to allow water out. If the water can not be released, pressure will build and can cause the wall to fail. Weep holes are more important in masonry-built retaining walls rather than wood, however.
If you're concerned about erosion affecting your retaining wall, check out our other blog post on the topic here:How To Stop Erosion Around A Retaining Wall.
How long do you wait to backfill a retaining wall?
Concrete takes time to cure, so you should not backfill a retaining wall right away. Depending on the type of concrete used and the humidity of the area it is curing in, it can take up to three weeks to a month to reach full strength. However, most retaining walls under 4 feet can be backfilled after 7 days.
We've given you several methods for deciding how deep to bury your retaining wall posts, so hopefully, you now feel more confident about your project. If in doubt, though, bury them at least 3 feet as this should be sufficient for any retaining wall you want to build yourself. Happy Building!
For more reading on the topic of retaining walls, check out some of our other articles below:
How To Build A Retaining Wall On A Slope
How Long Do Retaining Walls Last? [Inc. Wood Ones]
How deep should retaining wall posts go? ›
As a general rule the depth of the holes are equal to the height of the wall plus 100mm for cover on steel.How deep should a 4x4 post be for a retaining wall? ›
Dig post hole so diameter of the hole is 3 times the width of the post (i.e., the hole for a 4” wood post should be about 12 inches wide). The depth of the hole should be 1/3-1/2 the post height above ground (i.e., a 6-foot tall fence would require a hole depth of at least 2 feet).How deep should a retaining wall pile be? ›
The depth of the holes should be equivalent to 70% of the height of the wall. Examples: wall height 1M – hole depth 700mm, wall height 1.5M – hole depth 1.05M. If you have an engineer's design, the hole depth will be specified.How deep should a trench be for a retaining wall? ›
The trench should be deep enough to bury at least half the height of your first course of blocks as they sit on a 2- to 3-inch base of gravel. Depending on the size of your blocks, this depth will be about 4 to 6 inches. The trench should be twice as wide as a single block.Does a 2 foot wood retaining wall need drainage? ›
Most retaining walls require drainage and are built with a perforated pipe set behind the wall in a gravel base.How deep should sleeper posts be? ›
Holes should be dug at 2010mm centres for 2.0m sleepers and 2410mm for 2.4m sleepers (Note: a similar allowance of 10mm can be used for other lengths). Hole diameter must be at least 450mm for a one metre high retaining wall. For a one metre wall, the post holes must be dug 1100mm deep.Does a 4 foot retaining wall need drainage? ›
A drainage pipe might be needed if: The retaining wall is at least four feet high or taller. Clay or other poor draining soils are behind the wall.How deep should you bury a 4x4x12 post? ›
The general rule of thumb when setting a post is that the depth of the post's hole needs to be one-third to one-half of the actual above-ground height of the post.How deep should a 8 foot post be in the ground? ›
How Deep Should a Fence Post Be? For an average fence post, about 6 to 8 feet tall, prepare to dig a post hole about 2 feet deep. To install a fence post, you'll need a shovel or post digger, a 6-foot level, soil, and gravel or crushed stone. For gateposts, you'll need concrete too.How thick should a 6 foot retaining wall be? ›
Base thickness = 1/8 of the height of the wall but not less than 12 inches. Stem thickness = 6 inches + ¼ inch for each foot of wall height.
How much gravel do I need behind retaining wall? ›
In order to provide proper drainage, at least 12 inches of granular backfill (gravel or a similar aggregate) should be installed directly behind the wall. Compacted native soil can be used to backfill the rest of the space behind the wall.How deep should a footing be for a 4 foot retaining wall? ›
The general rule of thumb is to bury about one-eighth of the height of the wall. For example, if your wall will be three feet (36 inches) tall, the first course of blocks should start five inches below soil level. The gravel base should start three inches below this.What is the best backfill for retaining wall? ›
The best material for the backfilling of a retaining wall is gravel, and it should be well graded. The main reason for using gravel is because it does not retain water (small void ratio); hence lateral loads experienced will be minimal. You should also have weep holes for draining excess water that may be retained.Do you need landscape fabric behind retaining wall? ›
Filter fabric is an essential step in the process of building a retaining wall because it lets water through but prevents dirt, bark dust, or other landscaping from migrating into your gravel base.What is the best gravel for drainage behind a retaining wall? ›
The best gravel for using underneath a brick patio or concrete block retaining wall is often referred to as “processed gravel” “crushed base” or “bank run gravel.” This type of gravel has a mixture of fines that aid in compaction. It may look like sand with rocks in it.Does a retaining wall need a column? ›
When considering Retaining wall with greater heights, introduction of columns can give a better economy. This paper conducts a thorough analysis and design of Cantilever Retaining Wall (CRW) and Column Cantilever Retaining Wall (CCRW) for a span of 20 m and height from 2.5 m to 5 m, along with its footing.Do I need a French drain behind a wood retaining wall? ›
If you're building a retaining wall, add a French drain behind the first course of stones or blocks. Otherwise, water moving down the hill will build up behind the wall and undermine it. The pipe should rest on the same compacted gravel base or concrete footing that supports the wall.Do all retaining walls need weep holes? ›
Weep holes are small, evenly-spaced holes along the bottom section of your wall. They protect the structural integrity by allowing underground water to seep through, preventing pressure build-up. Every retaining wall should have them.How deep should a garden bed post be? ›
Dig the first hole.
The hole will need to be 350mm deep to leave enough length on the post holders for the 400mm high walls of your garden bed. Keep your hole narrow, so you have less to dig and less to back-fill. Post hole diggers are designed for digging small holes.
A rule of thumb is to dig your trench an eighth of the wall height plus three inches. For example, if the retaining wall is 48” high, your trench would be 6” + 3” = 9” deep to account for 3” crushed rock base.
How far apart should sleeper retaining wall posts be? ›
1m high wall = 2m posts. Post spacing is crucial as sleeper length can vary up to 10mm in length, for the no fence install add 30mm to the overall distance and add 40mm if you intend to put fence brackets on top of posts. If you use 2m sleepers spacing between posts will be 2040mm with a fence and 2030mm without one.What happens if you don t put drainage behind a retaining wall? ›
If there is no drainage, water will compromise the safety and structure of your retaining wall. It's vital that the wall is repaired or replaced if measures to ensure drainage weren't taken. This can lead to an extensive amount of damage on the property, the expense of repairs and installation of a new wall.Does a 12 inch retaining wall need drainage? ›
Do you need drainage behind a retaining wall? Yes, which means you need backfill too. This backfill is the soil that's located in the first 12 inches of space right behind the retaining wall in order to have proper drainage, and it can be either gravel or crushed stone.
Water pooling or soil becoming saturated due to poor drainage can more than double the pressure that the retaining wall must support. While there are significant safety factors calculated into retaining wall design regulations – without proper drainage, eventually the wall will fall.How big of a hole do you need for a 4x4 post? ›
The diameter of your post hole should be three times the diameter of your post. So, if you're planning on using a four-inch round or 4x4-inch square post, your post hole will need to be 12 inches in diameter. For a six-foot-high fence post, we would need a hole that's 36 inches deep and 12 inches in diameter.How long does quikrete take to set in a post hole? ›
The concrete sets up in 20 to 40 minutes, so you can quickly move on to the next stage of the project (a great convenience when setting fence posts) or backfill the hole to finish the job. Under normal curing conditions, you can apply heavy weight to the post (a basketball backboard, for example) after just 4 hours.Can you put dry concrete in a post hole? ›
Another benefit of dry cement is that it will allow water to drain around the fence posts. Installing a fence post in dry cement is pretty simple. You want to pour about 5 inches of the dry cement into the hole around the fence post and tamp it down. Then repeat this process until the hole has been filled.How far down do you dig a post hole? ›
Posts need to be set below the frost line to prevent heaving. For regular privacy fences, holes need to be at least 36 inches deep or half the fence height, whichever is greater, plus six inches for base gravel.
The general rule is when setting a fence post into the ground, the height required above the ground should be divided by 3 and this figure is the hole depth required.How many feet apart should posts be? ›
The standard spacing for fence posts is 8 to 25 feet apart, depending on the type of fence. A standard wood fence would require posts 8 feet apart, while a high-tensile wire could span 25 feet.
How deep should footings be for a 6 foot wall? ›
All footings should be a minimum of 150mm (6") in depth, with the bottom 350 - 400mm (14-16") below ground level on most soils. For clay soil however, thicker and deeper footings should be used.Do I need rebar in a retaining wall? ›
Retaining walls must be stronger than freestanding walls. Insert rebar in the footing when you pour it; this should be done at every three blocks or at intervals specified by your local codes.How deep should a garden wall footing be? ›
Start digging at the lowest part and go down until firm ground is found. For a light garden wall, a trench 30 cm (12 inch) deep should be sufficient if the soil is firm and well drained. But on unstable or weak ground, make it 46 cm (18 inch) deep.Do you need footings for a garden wall? ›
You should always build a garden wall on a solid foundation of a trench filled with concrete. This is called the 'footing', and if it's substantial and accurate, your wall will last longer and be easier to build.What is a typical footing size? ›
Under every house is a foundation, and under most foundations are footings. Most of the time we take footings for granted, and usually we can: For typical soils, a common 16- or 20-inch-wide footing can more than handle the relatively light weight of an ordinary house.What is the minimum footing thickness depth? ›
The minimum depth of footings below the surface of undisturbed soil, compacted fill material or controlled low strength material (CLSM) shall be 12 inches (305 mm). Where applicable, the requirements of CBC Section 1809.5 shall also be satisfied. The minimum width of footings shall be 12 inches (305 mm).How wide should retaining wall posts be? ›
The post holes into which the retaining wall posts will be placed shall be 450mm diameter with minimum 100mm concrete cover below the post. Footing depth is typically equal to the height of the retaining wall. For this example, therefore, the post hole is 1200mm.How big should a footer be for a retaining wall? ›
A typical footer is twice the width of a wall and equal in depth to the wall's width. The footer should rest on a 6" gravel base set below the frost line.