Sunday, December 10, 2023
Are you a real estate investor looking to expand your portfolio by adding an Additional Dwelling Unit, like a Garden Suite, to your property?
In this blog post, I will explain the four most common types of foundations, their pros and cons, potential scenarios where one is better than the other, and price ranges on a scale of low, moderate, and high.
Key takeaways you'll learn in this article:
Read on to lay the foundation (pun intended) for your Garden Suite project.
A poured concrete foundation is created by pouring concrete into molds, known as formwork, which are set atop undisturbed (compacted) soil.
The poured foundation is done in two stages.
First, forms are placed for the concrete footings, which distribute the weight of the structure above to the soil underneath. Once formed, concrete is poured and allowed to cure (harden).
See the image above for an example of the concrete footings.
Second, once the concrete footings have cured, the foundation walls are then formed and poured on top.
Steel rebar is added to reinforce the foundation walls to enhance its strength.
Foundation walls are not limited to just poured concrete. They can also be built using concrete blocks, or insulated concrete forms (ICF).
This type of foundation provides a strong and durable base for the structure and is commonly used in residential and commercial construction.
What are some PROS for this foundation type?
- Proven method and commonly used in seasonal climates.
- Known for it's strength and durability, the poured foundation can be adapted handle heavier loads by increasing the size and thickness of the footings and foundation walls, by adding more steel rebar, or by using a higher MPa concrete.
- Formwork can adapt to a wide assortment of architectural and structural designs.
- Great for seasonal climates because the depth of the foundation will be below the region's frost line.
- The space within the foundation walls can be used for a basement, crawl space, or backfilled with soil or granular material. (Note: This is the only foundation type you can build a basement with.)
What are some CONS for this foundation type?
- Poured foundations often take longer, as they involve more excavation, formwork installation, and curing processes.
- Because they can be more time-consuming and labor-intensive to construct, which can increase the overall cost of the project.
- Without proper construction techniques and rebar reinforcement, poured foundations are susceptible to cracking, and water leakage.
What are the potential scenarios where choosing this foundation type would be best?
- Formed and poured footings and foundation walls are well-suited for more complex shapes, and when you want a crawl space or basement.
Where is this on the price sale?
- Poured foundations are high on the price scale compared to the other foundation types.
- Pricing will vary based on the material used, depth, the amount of soil needing to be removed, site accessibility, and the size and complexity of the structure.
- A 4-inch concrete slab with a granular base, and rigid insulation (or vapour barrier) underlayment is required when adding a basement or crawl space. Driving the cost up further.
An engineered slab on grade is also formed and poured.
However, this is done in a single step.
After topsoil and tree roots are removed from the site, a granular base is layered and compacted on top of the undisturbed subsoil.
This slab is then formed, lined with rigid insulation, and reinforced with steel rebar and welded wire mesh, before the concrete is formed.
In the image above, you will see this step just before concrete is poured.
The slab on grade provides a stable base for structures while maintaining consistent contact with the ground.
What are some PROS for this foundation type?
- Engineered slab-on-grade foundations are more economical than the poured foundation.
- They can be installed relatively quickly (1-2 weeks).
- Placing rigid insulation underneath offers good thermal mass properties for regulating surrounding soil temperatures to prevent frost heaving in subzero temperatures.
What are some CONS for this foundation type?
- They may be more prone to damage from soil movement and settling, particularly in areas with expansive or unstable soils.
- They may be less suitable for structures with complex architectural shapes or structural requirements.
- If insulation and moisture barriers are done incorrectly, they may be susceptible to moisture infiltration.
What are the potential scenarios where choosing this foundation type would be best?
- Suitable for flat or gently sloping sites.
Where is it on the price sale?
- Pricing for the engineered slab on grade is moderate.
- Price varies based on factors such as site conditions, like grading and subsoil type, as well as the complexity of the shape.
Helical piles are steel shafts with helical plates that are twisted into the ground below the building.
The helical plates (imagine the threads on a screw) provide resistance against the surrounding soil, enabling the helical pile to support the building's weight and any additional loads (e.g., window, snow, people, furniture).
Helical piles are like sticking your beach umbrella in the sand so the wind doesn't blow it over, just at a larger scale.
This type of foundation is often used in areas with weak soil conditions or to retrofit existing structures.
This option is relatively quick and easy to install, making it ideal for projects with tight timelines.
What are some PROS for this foundation type?
- Very versatile as they can be installed in a wide variety of soil conditions
- They can be installed quickly with minimal disturbance compared to traditional foundation types
- Helical piles can provide high load-bearing capacity, making them suitable for supporting heavy structures.
What are some CONS for this foundation type?
- Limited to a small number of manufacturers.
- The costs can add up very quickly based on the quantity, depth, and design.
- They may not be suitable for very deep foundations or for sites with hard rock or other obstructions.
- The steel components of helical piles may be subject to corrosion in certain environments, requiring additional protective measures.
What are the potential scenarios where choosing this foundation type would be best?
- Helical piles are ideal in areas with expansive soils or poor ground conditions. For example, lots with a large slope, or high water table.
Where is this on the price sale?
- Price ranges from moderate to high.
- The number of helical piles, size, and depth required will impact pricing.
- Other considerations that will add cost:
Concrete piers consist of concrete columns, called piers, that are set into the ground to provide a stable base for the building.
The piers are typically spaced evenly throughout the building's footprint and are connected by beams to distribute the weight of the building evenly and prevent settling or shifting.
This foundation type is suitable for sites with stable soil conditions. However, it may not be the best choice for larger structures.
What are some PROS for this foundation type?
- They can be adapted to various site conditions, like changes in grade.
- Relatively simple to install.
- Require less soil to be hauled away.
What are some CONS for this foundation type?
- Probably not a viable option for larger structures with greater load-bearing support requirements, such as Garden Suites or Laneway Houses with a second storey.
What are the potential scenarios where choosing this foundation type would be best?
- Suitable for areas with stable soil conditions.
Where is this on the price sale?
- Price ranges from low to moderate.
- The number of piers, size (diameter), depth, and amount of rebar reinforcement needed for concrete piers will vary based on soil bearing pressure.
- Other cost considerations are identical to helical piles.
There is no one-size-fits-all solution when it comes to choosing the right foundation for your Garden Suite.
Each option has its own list of pros, cons, and potential scenarios where one might be a better choice over the others.
As always, it’s important to consider factors such as site conditions, budget, and timeline when making this decision.
But whether you decide on a poured foundation, an engineered slab-on-grade, helical piles, or concrete piers, working with experienced professionals, like an Architectural Designer, Structural Engineer, and General Contractor, is vital.
Start your Garden Suite project by building a solid foundation.
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