Updated: May 10, 2019
As the base your home will sit on, the foundation is a key part of the structure and key to the success of your build. In choosing the right type of foundation, there are a number of important factors to consider. Here we discuss and take a look at common foundation options.
From the Ground Down
The first and most important factor is your lot. The most obvious characteristic will be whether the grade is flat or sloped. Steeper grading will require more complex solutions, you'll find more on this below in the foundation options. Before any decisions can be made, soil testing and a geotechnical report will be needed. Soil can vary greatly in composition and its ability to handle the great weight of your house. Movement in the soil - compression or expansion - can compromise and cause serious damage to a home. Even with other homes in close proximity, testing the bearing capacity of where you plan to build is necessary. What may appear to be solid soil on the surface can be very different 5' or 10' beneath, where weight will ultimately transfer to.
Determining foundation depth is another primary consideration. Your geographic location will be one determinant in the required depth. Foundations will normally need to go below the frost line to avoid ‘frost heaving’. Typical frost lines we see in Canada range between 2’-6’ below the surface. Building code climate zones will take this into account and provide design requirements. Connecting back to your soil testing, in some cases it may also be necessary to go deeper than this to find good bearing soil.
In addition to soil conditions noted above, the water table height may also determine suitable foundations for your lot and must be identified.
Common foundation options:
1. Crawlspace or Full Basement
Two of the most commonly used foundations in Canada are crawlspaces and full basements. Both are perimeter foundations that spread weight and transfer down to footings that sit on the soil. A crawlspace will typically go as deep as the frost line (or slightly below) and as the name implies, allow you just enough room to crawl into. This provides the solid bearing needed and saves on excavation and concrete costs compared to a full basement. Crawlspaces can be insulated and conditioned, or not (if not the homes floor system must be designed with building envelope/insulation). In a full basement the depth will go not only to the frost line, but extended further (8' to 10') to allow for suitable living space. While more expensive than a crawlspace, it is a great way to provide extra square footage at a marginal cost by not having to increase the footprint of the house. By far, poured concrete is the most common way to build a crawlspace or basement, however there are instances where concrete blocks or preserved wood may be considered. Full basements are a great choice with steep grades, for many the potential of creating a walk out basement is highly desirable. A high water table may limit the use of these options.
Next we take a look at pile foundations, which are a series of load bearing points that transfer the weight into the ground. Unlike the systems above, the weight is not spread, but rather each point handles a concentrated load. This can be advantageous when dealing with weak layers of soil and a deeper foundation is required to find end bearing. Friction piles are also used in some applications. While concrete or wood piles have been popular in the past, helical or screw piles have emerged as a precise and economical option with very little disturbance and fast installation. When considering piles you will also want to consider how to finish around them. Your structure will rest on the piles, raised at least 6” from the ground and you will likely want to cover them to match the home, and/or close the gap between the home and ground to keep animals from underneath. Piles are also a flexible way to deal with complex grades. Piers can be added creating open space or raise the house up further and accommodate parking, storage, etc. underneath.
3. Slab On Grade
Finally, slab on grade can be an effective and economical foundation. As the name implies, a concrete slab is poured on top of the ground as the base for the building. You may be asking what about that frost line protection? Skirt insulation around the slab can be installed to protect the soil under the home and prevent freezing and thawing. Additionally, adding rebar and thickness to the slab can help protect from movement and cracking. The size of the building can also play a role in what frost protection is required/not with slab on grade. With slab on grade extra consideration must be given to items like plumbing and how to run them. Slab on grade foundations are most compatible with flat land, and an effective option with high water tables.
Each of these options will affect the design of the rest of your structure, and should be determined before moving past conceptual. It’s also important to consider what utilities and services you have, and how they will fit with foundation design. The point of entry of deep services should be considered as they will typically be coming from below the frost line (and where exposed will require insulation/heat tracing). If no basement (or enough crawlspace room) to house mechanical and utilities, you will need to make room on the main floor's plan.
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