Giovanni Valle is a licensed architect and LEED-accredited professional and is certified by the National Council of Architectural Registration Boards (NCARB). He is the author and managing editor of various digital publications, including BuilderSpace, Your Own Architect, and Interiors Place.
Laying out the footing is one of the first steps in the construction of a house following excavation. The dimensions of a footing can vary depending on the structure’s type and size as well as soil conditions.
A footing is a part of the foundation construction for a building, creating an attachment point between the foundation and the soil. Footings consist of concrete material placed into a trench. The role of a footing is to support a building and help prevent settling.
The rest of this article will outline topics related to the above question, including a footings’ role, why the soil type matters while placing a footing, footing size, dimensions, and common footing problems.
The Role of a Footing
No one can deny the importance of a proper house foundation. Unlike poorly installed gutters, which may not have much impact, improperly built foundation footings could eventually bring down an entire building. Footings are particularly important in places with poor soil conditions. Proper placement of a footing is important to provide ample support for a foundation and, eventually, the structure.
Concrete footings may provide the necessary support in projects such as pergolas, decks, retaining walls, and other construction types. Footings must be thick enough, wide enough, strong enough, and must have enough reinforcement to support a building. Concrete has high compressive strength but low tensile strength. Therefore, under loading, concrete is susceptible to cracking and breaking.
Construction crews place vertical and horizontal steel reinforcements inside the concrete while building a footing. By doing this, they combine both the compressive and tensile strength and make the footing far superior and less susceptible to damage.
Considering the Soil Type
Footings help spread the weight of a building so that the soil can bear a building’s weight. The load spreads out at a 45-degree angle within the footing and spreads out in the soil at a steeper angle of 60-degrees. It is important to compact the soil directly under the footing because it takes the greatest load.
Contractors use specialized equipment while compacting the trench bottom. The ideal equipment will depend on the soil type. The equipment used to compact gravel or sand soils may differ from the equipment used to compact clay or silt. Failing to compact the soil might lead to settlement of up to ½ inch (1.3 cm) in the first 6 inches (15.2 cm) of the soil.
What happens if you dig too deep and then replace the soil to recover the grade? You will be putting back soil that has expanded by 50%. Therefore, under pressure, this soil will re-consolidate and lead to settling. If you replace soil into the trench, it is crucial to compact it thoroughly. You might also consider using large gravel, which is self-compacting.
Footing Size and Dimensions
How do you determine the ideal footing size and dimensions? You should consider the soil’s bearing capacity. The lower the soil’s bearing capacity, the wider the footing should be. The ideal footing thickness should range between 8 and 12 inches (20.3 and 30.5 cm). The footing depth will vary depending on the soil strength and the frost line.
Some footings may be shallow, but others must be deep. It all depends on the prevailing soil conditions. If you are building a heavy house on weak soils, you need a footing that is 2 feet in width or more. However, when building a light building on strong soil, narrow footings of 7 to 8 inches (17.8 to 20.3 cm) will be adequate.
What is the ideal footing dimension for undisturbed soils? Undisturbed soils refer to soils that have not been hoed, tilled, graded, or turned over by man or a machine. Undisturbed soils provide a stronger foundation than soils that have been disturbed significantly. Most experts recommend that footings should extend at least 12 inches (30.5 cm) below undisturbed soils. The footings should also extend for 12 inches (30.5 cm) below the frost line or should be frost-protected.
Different leveling requirements exist for the bottom and top of the footing. The top of the footings should always be level, and there are no exceptions to this requirement. The bottom of the footing should also be level if possible. However, there are some exceptions regarding the footing’s bottom, especially if you build on sloped ground. On a sloped grade, contractors can step the footing with a 10 percent slope.
Here is a video on how to lay a foundation:
Common Footing Problems
Experts may encounter several problems when placing footings. The most common problems include misplaced footings, placing footings below the water table, and spanning over soft spots. What should you do when building footings when water is pooled in a trench? It is advisable to place large cobbles in the trench and compact them down into the mud.
Even if water and muck still fill the spaces between the stones, contact between them will help provide a proper bearing. When you cast the footings, you should use a stiff concrete mix. While building in certain areas during the wet season, groundwater may move into the trench. You may consider pumping out the water if it will not flow right back.
Footings are sometimes misplaced, and this is often evident through walls that do not align to the center of the footing. Contractors have to place the foundation wall properly to provide ample support for a building. This means that contractors should place the foundation wall off the center of the footing.
The method of correcting a misplaced footing will vary depending on the soil type. In soils with a bearing capacity greater than 4,000psf, contractors may excavate next to the footing and place compacted gravel into space. This strategy will ensure there is ample support for the wall. In weaker soils, contractors may augment the footings with concrete and steel. The first step is excavation, but instead of using gravel, drill the footing and place steel rods.
Soft Spots in the Soil
It is common to encounter some soft spots, even in good soil. Contractors often identify these spots while driving stakes for the footings and a stake just disappears with one blow. There is a cause for concern if a stake sinks easily under hand pressure. To address these issues, contractors often have to excavate down past the soft spots and end up placing deeper footings.
The other option for dealing with soft spots while placing footings is to excavate or remove the soft soil and replace it with low-strength concrete or compacted gravel. However, the typical solution is widening the footing. For instance, if you have a 16-inch (40.5 cm) footing, you may consider widening it to 32 inches (81.3 cm) to increase the bearing area. It is important to note that increasing the footing width also calls for an increase in the thickness.
If a footing is too wide and not thick, it may experience a bending force that might crack the concrete. The footing projection on either side of the wall should not be greater than the footing depth. A common misconception in dealing with soft spots is adding steel along the footing’s long dimension. Even if many contractors use this method, it does not help solve soft spots.
Footings play a crucial role in supporting the foundation and the buildings. Contractors should place footings under every wall bearing a load. Therefore, basement walls and the exterior foundation require footings. Any interior wall requires a footing as long as it is load-bearing. You can identify load-bearing walls from your home plan blueprints.
- Concrete Network: Footings 101 – A Guide to Concrete Footing Construction
- Concrete Network: Bearing Capacity of Soil – Bearing Pressure Chart
- Concrete Network: Concrete Footing Size & Dimensions
- Concrete Network: Footings Below Water Table, Misplaced Footings & More
- The Spruce: Guide to Foundation Footings Building Code
- Construction Mentor: Concrete Footer
- The Washington Post: Ask the builder: The Right Way to create a house’s footer
- eHow: How to Build a Footer for a House
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