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House Foundation: Excavation & Pour Foundation

At last, it's time to kick off the construction of your dream home! The initial and vital phase in building your future abode involves laying the foundation. The home foundation serves as the essential load-bearing component of a building and is typically constructed beneath the ground's surface. At the very least, a house foundation must excel in three crucial functions: providing sturdy support for the structure above, preventing groundwater infiltration, and serving as an effective barrier against water and soil vapor. Foundations typically come in three main types: Concrete slab, Basement and Crawlspace.


Excavation

Generally, the foundation excavation should extend at least 600 mm (2 feet) below natural ground level upon completion. Ensure that the foundation's width is at least three times the width of the supported wall, and the concrete thickness must be a minimum of 300 mm (1 foot).

The depth of the trench can vary, but in the majority of instances, it measures 1 meter deep (equivalent to 3.5 feet) and has a width of 60 cm (which is approximately 2 feet).

Concrete Slab, or Slab-On-Grade

The concrete slab-on-grade is often seen as the top choice, but ICF operates similarly to exceptionally robust poured concrete, rendering it even more robust than the conventional poured concrete. When it comes to affordability, the most budget-friendly foundation for a house is a concrete slab foundation at ground level. The slab foundation is a solid concrete slab that is usually 4 to 6 inches thick in the center and rests on the ground. The edges are generally thicker, about 24 inches wide, to allow for extra strength around the perimeter.

Concrete slabs are a better choice in regions susceptible to wet weather.

Concrete slab is the most common and cheapest, and costs around $7 per square foot*.


Basement Foundation

Basement foundations are usually the most expensive of the home foundations, ranging from $9-$12 per square foot*. They can be more costly if the home is built on bedrock that requires major excavation or removal by blasting.


Crawlspace Foundation

Foundation with a crawl space is a foundation with a narrow gap between ground and floor (but not a full basement).

Crawlspaces are more suitable for colder climates, where the risk of utility lines freezing within a concrete slab is higher. They are also ideal for dry climates. Foundations with a crawl spaces excel in areas with problematic soil types, such as expansive clay, which can lead to significant issues for concrete slabs.

The crawlspace foundation will cost around $13 per square foot*.


Slab-Off-Grade

Slab-off-grade is another type of foundation provides for the construction of a concrete slab above the underlying grade, thereby not relying upon the grade for support.


Foundation Base with Gravel

To prevent the concrete from cracking and breaking you'll need a foundation base. For a concrete slab, it's advisable to use gravel with a size ranging from 3/4 to 1 inch. Keep in mind that the specific gravel size and shape may be governed by local building codes, which can vary depending on the project. In instances where the concrete slab doesn't bear a heavy load, it's possible that a subbase of sand may be used instead of gravel.


The Footing

The foundation's footing plays a vital role in evenly distributing your home's weight, ensuring it remains stable and prevents any sinking or shifting. It's an essential element of the foundation since it has the most significant interaction with the soil beneath. Typically, the footers extend a minimum of 8 inches beyond the foundation walls, with 4 inches on each side. However, it's essential to consider local building codes and specific project requirements, as these measurements can vary.


For example: Two-story buildings require a minimum of 15 inch wide footings for 1,500 LBV (most clay-content) soil. For 2,000 LBV (sand, clayey sand, etc.) soil or greater, two-story buildings may have 12 inch minimum wide footings.


The Foundation Wall

A typical foundation wall minimum thickness of 8 inches applies to walls eight feet or less with no more than 7 feet of soil pressing against it. Foundation walls taller than 8 feet with more than 7 feet of soil pressing against it requires a larger foundation wall 10 to 12 inches in width.


Reinforcing & Pour Concrete

When it comes to house foundations, poured concrete stands out as the most robust option available. Its exceptional strength and durability allow it to be molded into various shapes, making it an ideal choice for bearing the weight of a house. It's worth noting that pouring a concrete foundation can be a significant expense, often ranking as one of the most costly aspects of building a home, frequently surpassing other items on the budget. Additionally, the cost of pouring the foundation for a larger home expects to be higher. It's essential to consider factors like the foundation type and the local climate when making this crucial decision.

The concrete used in footings and foundations should possess a minimum compressive strength of 2,000 pounds per square inch (PSI). It must be prepared and delivered in compliance with the guidelines specified in ASTM C94 (Ready-Mix Concrete), or alternatively, it can be mixed on-site.


Concrete footings and slabs on grade typically require a concrete of 3,500 to 4,000 PSI.

Increasing the cement content in the mixture results in a higher PSI rating for the cured concrete. For instance, a standard 3000 PSI concrete mix includes 5 cement sacks, but to attain a 4000 PSI rating, 6 sacks of cement are required. The additional cement is the reason for the increased cost of higher PSI mixes.


Rebar size for house footings

The resistance of rebar to corrosion is influenced by its type, with the grade specifying its maximum tensile and yield strength, and the size or diameter dictating its suitability for various construction applications. Typically, you'll need at least 2 x #4 (ø 1/2 inches) bars for residential footings. These rebar rods should be positioned in a grid arrangement, with no more than an 18-inch gap between them. Adhering to these minimum specifications is crucial to guarantee the footing's ability to support the load and stress from the structure above.

Concrete Grade

Rebar Size

Good for

Cost per Cubic Yard**

2,000÷3,000 PSI

Patio; sidewalks

$150÷$170

3,500÷4,000 PSI

Footings; slabs; driveway pavement; garage slabs

$170÷$190

5,000÷7,000 PSI

Beams; suspended slabs; reinforced walls

$200÷$240

7,500÷15,000 PSI

High-load bearing (columns)

-

** This is an approximate price range. Refer to your local suppliers for quoting. Usually, concrete suppliers require MOQ of one truck load (~10 Cubic Yard). With delivery cost.


Compressive strength is normally tested at seven days and then again at 28 days.

The cost of concrete mix and the necessary supplies for mixing concrete is quite affordable. If you're willing to invest some time in mixing your own concrete, you can potentially save money by taking on the task yourself. For smaller concrete projects, a lot of people opt for hand-mixing cement in a wheelbarrow.

Concrete costs $4–$8 per square foot on average*.

TIP: Our experts strongly recommend against attempting to pour your own foundation. It can prove to be a costly, time-consuming endeavor and poses potential safety risks for those attempting it as a DIY project. For this particular task, it is advisable to seek the expertise of a professional.

Many local code authorities typically adopt the IBC and/or IRC as their base codes, but they have the flexibility to exclude, adapt, or enhance specific code requirements to align with local circumstances and legal obligations. Building Codes provide essential guidelines for foundation footings, although it's important to note that these aren't step-by-step instructions for constructing footings. Instead, they serve as crucial parameters that must be followed and customized to comply with your local code mandates.


The key factors that can help you prevent issues with your foundation work are as follows:
  • Site Grading: Ensure the ground slopes away from the foundation at a minimum of 10% for a distance of 10 feet or more.

  • Site Grading Exceptions: If necessary, the ground can have a 5% slope when combined with effective water diversion methods.

  • Soil Load-Bearing Value (LBV): Use either presumptive undisturbed soil or disturbed soil with a minimum load-bearing value per square foot.

  • Footing Depth: Dig to a minimum depth of 12 inches or below the frost line for stable footings.

  • Footing Width: Maintain a minimum width of 12 inches for adequate support.

  • Top Footing Level: Always keep the top of the footing level for proper foundation stability.

  • Bottom Footing Level: Allow for a slope of up to 10% at the bottom of the footing.

  • Spread Footing: Ensure that the spread footing is at least 6 inches thick to provide sufficient strength.

  • Concrete Strength: Use concrete with a minimum strength of 3,000 PSI, but in some cases, you may require up to 4,000 PSI for added durability.


I genuinely wish that this blog post proves to be a valuable resource, assisting you in avoiding any potential hiccups during your home construction journey. Best of luck as you embark on the exciting path to realizing your dream home!

 

* This is an approximate price range. Refer to your local suppliers for quoting.

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