Underpinning in the GTA

What Is Underpinning?

The definition of underpinning is a solid foundation that is laid below ground level to support or strengthen a building. This can be an effective workaround for people who need some extra room, but don’t have the square footage or funds to afford a complete extension. With underpinning, you can enjoy the perks of a full height finished basement without building into your backyard. As you can imagine, the process is very labour intensive and requires significant experience to pull off.

Underpinning is the process of extending the underside of a foundation wall to a lower level. This is normally achieved by excavating below the existing foundation wall in short discrete sections and then by filling in these sections with concrete in such a way that the foundation wall is made to extend to a lower level.  For existing houses this is usually done from the inside of the basement in a series of stages determined by a structural engineer.

A staged approach (usually 3 stages) is used since the existing foundation wall is still carrying load. Normally it is only acceptable to underpin a short section of foundation wall in order to avoid any undermining or safety issues.  It is vitally important to leave undisturbed sections of soil between each stage to ensure the continued support of the foundation wall during construction.

Depending upon the type of foundation wall present the underpinning sections may be shorter or longer but typically the underpinning sections are installed in 4’ sections maximum for concrete or block and 3’ sections for stone or rubble. By following a staged sequence of underpinning the entire foundation wall can be successfully lowered in a safe and controlled manner without causing any significant shifting or settlement of the original house structure.