Underpinning near you

With full underpinning, new concrete is poured beneath the existing footings so the walls sit on a deeper base, and the new section of wall below the old footing becomes part of the foundation. The basement keeps its full floor area right up to the walls. It takes more engineering and stricter sequencing, since each section briefly leaves a length of wall without soil support until the new concrete is placed and has hardened.

A bench footing leaves the original footings where they are. The floor is dug out inside, stopping short of the walls, and a sloped or stepped band of soil along the perimeter is encased in a concrete ledge, the bench, that keeps the soil under the footings from moving. It generally disturbs less and stays clear of a neighbour's footings, but the ledge takes up floor space around the room and has to be planned into the layout.

Many older homes have basements built for a coal bin and a furnace, not for people: low ceilings, ducts and beams at head height, and a thin floor over bare soil. Lowering the floor makes room for comfortable living space, proper insulation and a new floor assembly, and it's often needed to reach the minimum ceiling height that building codes set for habitable rooms and secondary suites. Planning the finished space, from layout to egress windows, is covered on our basement renovation page.

The crew follows the engineer's drawings and works in short sections in a set pattern, so most of each wall always rests on solid ground.

Lowering the floor brings the living space closer to the groundwater, and the old drainage goes out with the slab. That's why underpinning and basement waterproofing are almost always one project: new weeping tile is laid at the new footing level and connected to a sump pit or to whatever discharge route the municipality accepts, and the walls get a waterproofing treatment from inside or outside. Our weeping tile and foundation waterproofing pages explain the options.

The plumbing drops too. Floor drains and the main building drain are rebuilt at the new depth, and with the floor already open it's the right time to add a backwater valve. Fixtures that end up below the level of the municipal sewer need a pump to lift their wastewater.

Lowering a basement by underpinning is structural work and needs a building permit. Toronto, for instance, requires a permit to underpin an existing house, calls for a professional engineer's design and on-site review when new footings go below an adjacent building's footing within the soil's angle of repose, and in every case requires an engineer's testing and reports at the first and last stages of construction. In Quebec, municipalities and Montréal's boroughs issue the permits and set their own document requirements, so ask early whether plans sealed by a member of the Ordre des ingénieurs du Québec are needed. Creating a secondary suite can bring zoning and code requirements of its own.

Where the job includes digging outside, such as a basement entrance or exterior waterproofing, buried lines must be marked before the first shovel goes in: in Quebec, the Régie du bâtiment du Québec asks contractors to file a free Info-Excavation request at least 4 business days ahead, and in Ontario an Ontario One Call locate is free and legally required at least 5 business days before digging. Indoors, the contractor also needs to know where the water and sewer service lines run, since they often come through the basement wall or floor.

On a detached house with space around it, the neighbours mostly notice the noise and the trucks. On a semi or a row house, the wall you're digging under also holds up the house next door, and deepening its footing affects both homes. Digging below an adjacent building's footing is one of the cases where Toronto requires an engineer's design, and engineers often treat a party wall differently from the other walls, with a bench or a particular sequence.

Rules on notifying an adjoining owner or getting their consent vary by province and city, so ask your permit office and, if the wall is shared, a lawyer or notary what applies to you. Whatever the rules say, talk to your neighbours before the design is final.

Many families move out for at least part of the job. The furnace, water entry and electrical panel are often in the basement and may be moved or shut off for stretches, jackhammering and concrete work are loud, and dust travels up through the house however well the stairway is sealed off.

In many cities underpinning goes ahead year-round, but winter adds steps: fresh concrete has to be kept from freezing while it hardens, and outside digging in frozen ground is slower. Ask how cold weather will be handled, and check whether your home insurer needs to be told about major renovations or a house left empty.