Underpinning is an often-daunting renovation, where your basement floor is lowered to increase ceiling space. This involves altering the foundation of your house, and as a result should not be taken lightly. In 2012, this home in Toronto collapsed during an underpinning job.
In this edition of HomeStars U, Daniel Johnston, from True North Underpinning shares advice homeowners should know before hiring a contractor to do underpinning and explains an alternative method called benching.
Reasons for underpinning:
- Creates more livable space
- Income properties
- Correcting structural deficiencies like a sagging foundation
Do not underpin your house without a structural engineer
- City inspectors usually require a structural engineer to inspect the job after it’s complete.
- They will meet with you, go over the design, talk about the depth, problems and concerns that may arise.
- The foundation walls are marked out in sequence according to the engineered drawings. Usually 1,2,3,1,2,3, in sections that are three to four feet wide.
- All the earth from the sections marked 1 is excavated underneath the footing of the house, down to the required depth, which must be undisturbed soil. The undersides of the existing footings are cleaned of debris.
- The excavated sections are then formed, filled with concrete and vibrated to ensure even distribution and a level surface. The forms can either be filled over the height of the existing footing (over-pour method) or 2″ under the existing footing and the remaining space is filled with non-shrink grout.
- These steps are then repeated for the second and third sections.
- Once the three stages are completed, the footings of the foundation walls are extended down to the required depth. Now the remaining earth can be excavated to the same depth as the new footings.
- Finally the waterproofing, gravel and weeping tile can be installed, plumbing updated and a new concrete floor poured.
- This is mostly done in homes that share a common wall with another, where homeowners don’t have permission from the neighbor to excavate underneath, or, if the foundation has been deemed unfit to work on. Unlike underpinning, benching leaves the load bearing soil under the footings undisturbed and in place, encasing it in a concrete ” bench ” to keep the soil undisturbed.
- The bench is reinforced with rebar and anchored to the foundation walls and acts like a retaining wall for the load bearing soil under the footings.
- Once the benches are completed around the interior foundation walls, the soil can be excavated down to the same depth as the bench.
- These steps are followed by waterproofing, weeping tile, plumbing, gravel and floor installation.
- Varies on site conditions and engineering concerns
- For underpinning a 600 sq ft basement, adding about 2 ft: $30-35,000
- Benching is about two-thirds of the cost of underpinning
- Extra costs (approx. $10,000) for additional steel structures, plumbing and waterproofing
Hiring the right person:
- Experience is key. There are many different types of foundations and footings out there and you need a contractor who has worked with multiple types.
- Check as many references and read as many homeowner reviews on the contractor as possible. If you can, go to a site currently being worked on.
- Insurance is mandatory for underpinning; the company should be able to provide you a certificate from their insurance broker, stating your name and property as the policyholder.
When you get to the point of thinking about the fun part- the interior of your new basement – check out our Basement Board on Pinterest for lots of inspiration.
Best of luck with your project, and make sure you leave a review when it is completed, it benefits the whole community!