In Montréal, most old houses are built on a stone foundation and include a crawl space.

When it comes to this type of residence, certain owners are inclined to expand their homes by digging up the space directly beneath the structure by proceeding with what is known in the industry as a "basement excavation."

An excavation consists of removing dirt and concrete found below the house. Usually, this is done with a jackhammer. A basement excavation is a complex task that, in most cases, requires the services of a contractor to ensure the safety of the whole operation.

Regulations, inspections, pros and cons, and excavation techniques. Keep reading for more information on basement excavations.

Benefits of Excavating a Basement 

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There are a lot of advantages to adding a basement should you choose to excavate the space beneath your home to increase the square footage of your house. First of all, this type of home improvement will allow you to increase the surface area of your home in due part to adding an additional floor. Furthermore, you will improve the internal structure and the foundations of your home, which will in turn increase its value.

The basement can fulfill several functions in a house (living room, storage, bathroom). Moreover, an important feature is that it provides easy access to plumbing systems, which is very practical and allows you to easily make changes to the system if necessary.

Another advantage is that a basement’s ambient temperature rarely ever varies during the summertime and wintertime. However, excavating can be quite pricy, which can present an obstacle to some. "Excavating a basement with underpinning foundations requires roughly four to five weeks and costs between $60,000 and $100,000," estimated Gaétan Filteau, a technical representative at Bisson Expert. 

To know more about excavation costs, check out our article dedicated to excavation costs.

Basement Excavation and Regulations

Prior to planning your excavation project, check with your municipality to ascertain whether your basement can be transformed. Depending on the space you wish to create, the regulations may vary. For example, your municipality may not allow you to build bedrooms but may allow you to build a basement. This is often the case when over half the basement's height is below the average exterior ground level.

To create a living space, a number of factors must be met, including natural lighting. In fact, the National Building Code of Canada requires for natural light to account for 5% of the bedroom space and 10% of the total dining or living room space.

Follow This Checklist Before Excavating

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Even though regulations may authorize you to jumpstart your basement excavation project, there are certain procedures to follow before undertaking the renovation. Should a problem occur, the homeowner is liable for any damage that may arise to neighbouring homes.

Before proceeding with any excavation procedure, you need to have insurance that will cover any damage that may occur, as digging up a basement is a relatively difficult process. A single mistake can literally lead to your home collapsing. It takes nerves of steel to excavate a basement, so choose a qualified and experienced contractor to handle the job.

Clay Soil

The Régie du bâtiment du Québec cautions homeowners regarding this aspect since, in Quebec, houses are commonly built on clay soil. This type of soil is not as dense and will compress rather easily, hence the need to take the necessary precautions to limit the risks of cave-ins during basement excavations.

The Régie du bâtiment du Québec urges homeowners to consult experts in order to properly evaluate the amount of pressure that the soil can withstand. It is therefore recommended to consult a geothermal specialist (or a soil laboratory) who can advise you on how to properly plan your excavation work.

Waterproofing 

Excavating a basement means lowering the basement floor by one full level. This may lead to water infiltration, as it leads to increased water pressure on the foundation. Bisson Expert strongly recommends using an interior drainage membrane to channel any water seeping in. This will ensure that both the exterior walls and the party walls are properly sealed.

Rotting Joists

Also, be mindful of the joists, since the excavated soil must not come into contact with them, as this might lead to eventual rotting. Furthermore, ventilate crawl spaces during the summertime to reduce the overall humidity level.

Steps to Excavate a Basement 

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There are several steps that must be respected before excavating your basement. First, you need to obtain a permit from the city in which you reside. As a matter of fact, only your municipality can authorize you to dig up your basement. Then, you will need to consult a structural engineer to determine whether the project is feasible. The engineer will draw up plans and provide a quote for your basement excavation project.

Basement Excavation Methods

Note that there are two methods used to excavate a basement. The first method is to add concrete under your existing foundation to extend the foundation downward. This should generally be done on foundation walls that are in very good condition.

The second method involves changing the entire walls of your existing foundation, with the exception of the party walls, with new concrete walls with new footing. This process has the added benefit of allowing for an exterior French drain as well as a waterproofing membrane, while also benefiting from larger window openings to let in a maximum amount of natural light.

Excavation Process

Once you have settled on a particular method, the next step is to excavate the interior of the structure to the desired depth to lay the new concrete footings. These footings will then support the new steel columns, which will in turn reinforce the existing central supports of your house.

If the latter is in poor condition, it will then have to be replaced. A 4-inch slab of concrete has to be put in place. Then, you can apply a smooth finish to leave the concrete exposed in its natural state, or you could choose to cover the slab with a ceramic or hardwood floor.

For more information on the benefits of a crawl space, check out our article What Is a Crawl Space and How to Know if Your House Needs One.