Basement Lowering - Underpinning & Benching
A guide to basement lowering.
Article written by Austin Merricks
Ward Group Construction & Structural Specialists.
If your looking at the potential of underpinning/basement lowering your home then this article is a great place to understand what exactly entails in this type of foundation work. Below will explain the do’s, the dont’s, what exactly underpinning is, diagrams, photos, pricing for the typical industry costs, and how lowering your basement can improve a home's value between 10-15%.
What is underpinning?
“Underpinning” is a term that’s generally used to describe reinforcing an existing foundation. This may be to shore-up a structure that was poorly built originally or has degraded over time, we tend to see this in older homes within Toronto. Underpinning can also be performed to allow for extra weight being placed on the existing structure. It is recommended that when adding an addition onto a home, if the existing foundation is in pour condition and cannot carry the additional load then underpinning would be an option to make sure the structural aspects of your home are safe. Underpinning is also performed when a homeowner wants to have their basement foundation dug out to increase the ceiling height of their basement. This is also known as basement lowering.
Basement lowering is becoming very popular with older homes within Toronto as the housing market price has gone up, many would prefer to maximize the livable space of their home and turn the basement into a function-able renovated space rather than moving. When most homes were built, the basements were used as storage and not needed as a livable space.
Basement lowering is usually done in 2 ways;
“Underpinning” the existing foundation footing to allow for the new basement height, this method maximizes the space of your basement and is completed directly under the existing foundation. Underpinning is done in sections in a staged sequence, the typical underpinning project has stage 1 to 3 . Sections usually span 3 to 4 ft wide, with a depth of 2 to 3 ft. This varies based on soil conditions - such as compaction and the type of aggregate material your house was built on, and the existing foundation of your home. A staged approach (usually 3 stages) is used since the existing foundation wall is still carrying load and it is normally only acceptable to undermine a short section of foundation wall at a time. Note that It is vitally important to leave undisturbed sections of soil between each stage to ensure the continued support of the foundation wall during this construction process. By following the staged sequence process you allow the basement to be lowered in a controlled and safe manner. Thus causing little to no shifting or settling of existing house structure.
“Benching” is the term used to describe a concrete bench which is constructed over top of a stable excavation slope when a basement floor is being lowered without extending the foundation wall footings to a lower level.Thus leaving the existing footings as is and undisturbed. In benching, a stable excavation slope is maintained below the foundation wall footings and a concrete protection “bench” is constructed over top of the stable excavation slope. A concrete bench is the combination of concrete and rebar around the internal perimeter of your existing structure. This process is usually used as an alternative to underpinning, reducing the construction costs, or it is done to avoid undermining an existing neighbouring structure that could be undermined during the underpinning.
Note that if there is less than 48” between your house and the neighbouring house it is important to consider your options for basement lowering to ensure that the neighbouring footings don’t get undermined. Undermining neighbouring footings can cause serious issues and costly damages.
During any construction process it is important to notify your insurance company of the work you are going to be doing and get an extended construction/renovation policy.
The cons with “Benching” is the considerable large size of the benched footing itself. For example to lower your basement floor 6” below the height of the existing footing you will typically need to have a bench that extends 12” out from the foundation wall face. As the depth of the basement lowering increases so does the size of the concrete bench. A typical basement lowering of 18” using the benching method requires a typical bench size of 27” extended out from the foundation wall face.
Many who use the benching method will utilize this within their architectural design - such as having the mechanical aspects of their home placed on top on the bench. When considering the benching method you have to evaluate the added costs of adjusting your HVAC, Water Tank and any mechanic that may need to be re-routed/worked.
With a creative architect you can utilize the bench as a focal point of the basement. In the past we have utilized this space for custom shelving, storage units, TV/media displays, wine cellars, etc.
With any method of lowering your basement always consult with a licensed structural engineer who has previous references and experience in underpinning or benching of a structure.
Both methods of basement lowering require excavation and removal of dirt/aggregate. Forming the underpin/benching sections with wood. Then pouring concrete into the first staged section of the basement lowering project. For underpinning it is recommended you utilize a short pour system. Where a 2” space is left between the existing footing and the new poured concrete footing. The poured concrete should be left for a period of 48-72 hours before the 2” cap is hand packed with non-shrink grout. This is to ensure there are no voids between the existing footing and the new concrete. This method is recommended by the city of toronto building department.
See below for diagrams of both underpinning and benching methods to lower a basement.
Choosing the right contractor.
If there is one project you need professionals to do it is basement lowering. Ask any qualified and experienced engineer! Any mistakes surrounding basement lowering can put your home in risk of losing its structural integrity and can cause very expensive issues to arise. We ask any homeowner to educate themselves and have a good understanding with what entails within a basement lowering project.
What kind of insurance does the company have? Most construction companies will have a standard issue insurance, but for basement lowering and structural projects only choose a company that is insured not only for general construction but has a separate insurance policy specific to Underpinning, Basement Lowering, Excavation & Shoring. Make sure the contractor you hire also has a WSIB clearance certificate and safety board on site.
References: Is the contractor/company experienced in basement lowering? Over the past few years the city has cracked down on companies performing structural projects due to multiple projects taken on by inexperienced contractors which resulted in catastrophic structural failure. - Google: 77 Corley , 245 Brookdale Ave, 63 Maria Street. Many other projects have had issues but they have not made it into the news.
Ask Questions: Can the contractor answer all your questions, any concerns you have should be addressed.
Personally we have an open book standard, we will provide for the homeowner all the necessary insurance, documents, references, engineers we work with, licensing, in-depth project timeframe, scope of work, and costs of your project. Upon an initial site visit we conduct an assessment of current foundation conditions, of which we document. These assessments are done throughout various stages of the project and recorded in a master file of which the homeowner has access to for personal records.
Choose a contractor that has an experienced team and the knowledge required to carry out the project in a safe and professional manner. Choose a company that specializes in structural work, and isn't going to sub-contract the project out to another company.
You will find their are many large well branded construction companies , majority of their work they subcontract out. For anything structural we would recommend choosing a company that does the work in-house. Note that if you are using a general contractor for your project, ask to have a meeting with the company they would recommend to take on the basement lowering project. If they say they can do the work themselves, then refer to what is mentioned above, separate insurance and references.
Typical Costs of Basement Lowering:
The typical costs associated with basement lowering vary depending on the method, the depth of the underpin to reach desired height, access to allow for aggregate removal, and access to allow for new concrete.
Typical costs of Underpinning:
Linear ft of basement X cost per linear ft. Example: Total linear ft of home is 135 Linear ft. Cost per linear ft is $350.00 to $500.00
135 x $350.00 = $47,250.00
Typical costs of Benching:
Linear ft of basement X cost per linear ft. Example: Total linear ft of home is 135 Linear ft. Cost per linear ft is $275.00 to $400.00
135 x $275.00 = $37,125.00
Many other factors play a role in costs, be sure to have your contractor factor in removal of aggregate, new drains, waterproofing, weeping tile & sump pump tank, foundation repairs, framing, beams & posts, engineer reports, concrete pump truck for restrictions on access.
Also note a project can be a combination of both benching and underpinning. Many projects we have completed utilized benching on parting walls and underpinning of detached walls.
We have written this article to help educate homeowners about basement lowering projects, to truly understand underpinning and benching and help put trust in our company and our Openbook policy.
At Ward Group - We specialize in the structural aspects of homes! If you have any questions or would like a free no obligation quote please contact us!
Attached below are TACBOC standard detail example drawings for basement lowering.
Also see photos of our work and our team in gallery section!