You’ve probably heard the term footing being tossed around in the construction world. Many people know that footings are crucial for a building’s structural stability.
The way footings fit into the foundation construction picture can be confusing. Do all foundations have footings? Do all footings have foundations? These are questions we get asked a lot.
If you want some clarity on the role footings play in foundation construction, stick around.
What is a footing & what is the purpose of a footing in construction-
The very base of a foundation is called a footing aka footer. Traditionally, it is wider than the foundation wall and will be placed approximately 12 inches below the frost line. Footings are typically made of concrete reinforced with rebar.
The purpose of a footing is to create an attachment point between the foundation and the ground. Essentially, it prevents the house from settling or sinking into the ground. This helps to support the foundation, prevent settlement and provides an enormous amount of stability for the structure as a whole.
Structural engineers are responsible for designing the right size and style footing to support a structure.
What is the difference between a footing & a foundation-
Let’s start with this statement. “All structures have foundations while most (but not all) foundations have footings”. So, a footing is a component of a foundation. Not all foundations need a footing .
Foundation definition- In engineering, a foundation is the load bearing element of a structure responsible for connecting it to the ground and transferring loads from rooftop to soils.
There are three main types of residential foundations: basement foundation, crawl space foundation & slab on grade.
- Basement foundations are the deepest and consist of foundation walls sitting on top of foundation footings. Full basements traditionally match the floor space of the floor level above and are usually between 8-9 feet tall.
- Crawl space foundations utilize small foundation walls (under 4 ½ feet tall) which sit upon footings. Typically they are unfinished with a dirt floor and are often narrow with just enough room to crawl.
*Example of a crawl space foundation. The engineer literally had to “crawl” in order to inspect the foundation and make a structural assessment.
Fun fact- You will see a lot of these crawl space foundations in the 1950’s St. Louis Park (MN) homes. Essentially, upon building, a homeowner chose not to put in a full basement (back in those days, basements were damp & dingy) but due to the soils & climate, footings were required. That is the only reason a crawl space was essential; in order to create stability with footings. In modern days, trusses have made load distribution much easier and therefore you do not see as many crawl space foundations.
Note- Not all crawl spaces are accessible. Our engineers run into crawl space footing access issues periodically. This makes it challenging for homeowners wanting to put on an addition. If you can’t see the existing footings you really can’t engineer a building. You need to know what type of support is there before adding more loads.
- Slab-on-grades foundations are a solid concrete slab that sits upon the soil. Even though the foundation is one poured slab, there are still footings that exist as thickened concrete at the edges or below load bearing walls more towards the middle of the structure (there are other styles of footings depending on soil types too).
Slab-on-grade foundations are less common in areas with frost heave issues since they are more prone to cracking.
To learn more about foundations click here.
Let’s move on to the footings.
Footing definition-A footing is a part of the foundation construction for a building, creating an attachment point between the foundation and the soil.
When learning the difference between footings & foundations, many people use the analogy of a human leg and foot. You can think of the leg as the foundation and foot as the footing. The foot connects to the ground and supports the structure (foundation) above. Footings actually connect to the ground, help to spread out the loads over the soil and do help support the foundation and other loads above.
So, just to be clear, the main difference between a footing and a foundation is that the foundation transfers building loads down to a footing while a footing then transfers loads to the soils. Both footings and foundations are typically made from concrete.
What type of footing is most common in residential construction-
The most common type of footing in residential construction is poured concrete or concrete block footings (reinforced with rebar) underneath foundation walls. This is the standard type of footing that you will see with most homes.
Now, this doesn’t mean that this footing will work for every scenario. That is why every single foundation must be designed by a structural engineer. Structural engineers consider factors such as soils, water management, climate and loads when designing footings and foundations.
These factors greatly influence the style and size of a footing. For instance, poor quality or wet soils in combination with a heavy load (i.e. a large commercial building) may require heavy duty footings that could be 8 feet deep by 5 feet wide reinforced with rebar.
Keep reading to learn more about other common types of footings.
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Common types of footings-
Before we can hop right into footings, we need to talk a bit about the main types of foundations. This is because footing type will depend upon foundation type.
There are 2 main types of foundations, shallow & deep. Depending on which type the foundation is, there are many unique types of footings.
Shallow foundation: Small & Light Buildings
Shallow foundations are used when the existing soils at a shallow depth are able to sustain and support the structural load demands of a building. Typically, the depth of a shallow foundation is less than its width. This style of foundation will be used for small & light buildings.
Footing types: 4 types typically used in shallow foundations
- Individual or isolated footing:
This type of footing is easy to remember because the name depicts its function; they support individual “isolated” columns. They are typically used when soil bearing capacity is very high, the loads pushing on the soils are low and the columns of the building are not closely spaced.
There are 3 main types of isolated footings.
- Simple (also called a pad, flat or plain footing)- They can be square, circular or rectangular in shape.
- Sloped (also called trapezoidal footing)- This type of footing maintains a 45 degree angle on all sides of the footing and also uses less concrete than other types of footings.
- Stepped- These footings were used back in the day but not so much in modern times.
- Combined footing:
A combined footing is one that supports 2 or more columns. This type of footing is used when columns are closely spaced. The combined footing is necessary to prevent overlapping of isolated footings. It may also be essential to use a combined footing when a column is close to a property line or a sewer pipe.
There are 3 main types of combined footings:
- Rectangular combined footing- When one of the projections of the footing is restricted.
- Trapezoidal combined footing- When the load on one column is much greater than on the other.
- Strap beam combined footing- When one column is located on a property line resulting in unbalanced loads on different parts of the footing.
- Strip footing (aka continuous footing or wall footing): This type of footing is responsible for supporting long masonry walls. Typically, the width of the footing is 2x the width of the wall (or more) sitting on it. The size will vary depending on soil bearing capacity. Sometimes there may not be a wall sitting directly on top of the strip footing, but a very heavy load, such as a kitchen countertop, on the floor above. The strip footing will support the additional loads.
- Raft footing (also known as mat footing):
Raft footings are a type of combined footing used when columns in a structure have heavy loads and poor soil bearing capacity. Conditions like this would require a footing to have a larger area to disperse loads properly. However, this can present issues with overlapping footings. Raft footings solve this issue by supporting all the existing columns with a single large footing. You might see this type of footing in silos, industrial structures or certain structures with basements.
Let’s move on to footing types that can be found in conjunction with deep foundations.
Deep foundations>Heavy loads & low soil bearing capacity
Deep foundations transfer loads further into the earth than a shallow foundation would; we’re talking anywhere from 10-300 feet deep. There are many reasons a geotechnical or structural engineer may recommend a deep foundation for a building. Some of these reasons could be poor soils, heavy structural design loads or other site constraints.
Footing types: 2 types
Long columns made of steel or RCC to support the structure above. These are used in weak soils, high ground water table, expansive soils, exposure to uplift forces or when there are soil erosion issues.
Prefab hollow cylinder is sunk into the ground to a planned depth, filled with concrete to form the foundation. This type of footing is typically used in bridge pier construction or for foundations underneath water.
Now that we’ve provided you with a general outline of some of the most common footings types, let’s talk about sizing footings.
What size footings do I need-
The International Building Code does provide general guidelines for footing sizing via depth & width. Remember, every footing must be designed by a structural engineer since there is no way a single code could factor in specific details about each project such as soils, conditions, climate & structure loads. These factors will impact the size and type of footing required.
In general, one story buildings with footings on undisturbed soil with LBVs between 1,500 and 4,000 should have a minimum width of 12 inches.
A two-story building needs, at a minimum, a 15 inch wide footing with a 1,500 LBV soil.
12-inch (at a minimum) wide footings can be adequate on two-story buildings with a LBV soil of 2,000 or greater.
Wondering what LBV means? Well, it’s the load bearing value or load bearing capacity. Soils need to be able to tolerate the pressure exerted upon them by a footing. Different soils have different load bearing capacities affiliated with them. The IBC chart below depicts the load bearing capacity (aka pressure) in pounds per square foot of various soils. Structural engineers will use these charts when designing footings.
Footing depth will depend upon the specific conditions of the property & the structure loads. Some footings can be shallow while others must be quite deep as we discussed above.
But, a general rule of thumb for footing depths in residential construction is a minimum of 12 inches below previously undisturbed soil. They also need to reach at least 12 inches below the frost line.
Certain wet soils, like expansive clay, may have special requirements to prevent settlement or heaving. See the IRC’s definition of expansive clay soil in the photo below.
A small amount of settlement will not make or break a house. However, if settlement is uneven (differential), damage can occur. Even ¼” of differential settlement can cause cracking in a home. If you notice large amounts of settlement in your home, it’s a good idea to have a structural engineer perform a structural foundation inspection to ensure your home’s foundation and footings are structurally sound.
Note- These general guidelines may or may not apply to structures such as sheds and decks. Ask your building department for information on this.
To read more about building codes on this, click here.
*CBS structural engineers inspected this foundation wall to ensure its integrity remained intact.
What are footings made of-
Most footings are made of concrete with rebar reinforcement. Concrete is strong, durable and lasts a very long time.
You may have heard of wood foundations. These foundations still have a concrete footing. Wood foundations should never be used in northern climates.
Where are strip footings used-
People often ask about strip footings and where they should be used.
First off, what is a strip footing? It is a long continuous block of concrete that helps to spread the weight of loads from the structure above to the soils.
It looks just like regular concrete footings that are poured in a home, but it will extend under specific areas of the home to support various loads. The strip footing could be located under the concrete slab or be even with it.
Strip footings can span very long distances. One example is with a large open basement floor plan. If there was a heavy kitchen island with granite counter tops on the floor above this span, a strip footing may be required.
Do footings need to be level-
The top of a footing has to be level. It’s best if the bottom of the footing is level as well. However, sloped grades may require the builder to “step” the footing by even as much as a 10% slope.
The picture below is taken straight from the IBC.
How do footings support the home-
Footings, along with the foundation, are the reason that your home does not slowly crack and fall down. This is because they transfer the enormous loads above down to the soil. Footings are even more important in areas with wet clay-like soils.
If we did not have footings, your home’s foundation walls may just sit upon the dirt. Ultimately, you may experience settlement, ceiling & floor sagging or possible roof leaks because of this.. Your home may not hold up as well.
Also, mortar does degrade over time, so if you ever wanted to do an addition, the dirt sitting next to your foundation walls may be the only thing holding those walls up. This could be a very dangerous situation.
Can concrete footings be poured in sections-
Yes, concrete footings can be poured in sections. This is not very common in residential construction. Pouring one continuous slab is quicker, easier and ensures that the slab cures all at the same time.
However, every project is different and there are situations where pouring in sections is beneficial. The most important thing is that the soils underneath the proposed slab are a) correct soils (conbit or other) and b) are properly compacted. These factors are the key to avoiding concrete cracking and heaving due to frost heave.
How long do footings take to set-
Footings are typically made by pouring concrete into a form. The time it takes for concrete to move through the chemical processes necessary to reach full strength is called curing. Curing starts immediately after a concrete footing is poured. After about 1 week, the concrete will reach about half of its final strength. Full strength concrete will occur after approximately 1-2 months. Typically the building process can begin when the footings & foundation are about 50 percent cured.
What is a continuous footing in construction-
A continuous footing is one that supports the whole length of a masonry wall. Sometimes it may be referred to as a strip footing. Typically, it is made of reinforced concrete and is designed to spread out and distribute loads to a wider area as it transfers the weight to the soils. It is a common footing choice for residential construction.
We hope that by this point you have a much better understanding of what a footing’s role in construction and engineering is. If you take anything from this blog, understand that footings are the final load bearing element that transfers the weight of a structure to the soils. Footing design will vary based upon factors such as soils, structural loads, climate and other conditions. They provide an incredible amount of support to the building at hand and help to prevent settlement, cracking and upheaval. If you have any further questions, please reach out to us at 763-544-3355.