If you’re frustrated that your crawl space area is impossible to access and wished there was a full basement there, then a conversation with an underpinning contractor should be in your future. While an underpinning project is complex and time-consuming, the return on investment can be significant for the added square footage and foundation stability it’ll provide.
About basement underpinning
The process of lowering a basement is often done for a variety of different reasons:
- Converting an unfinished or crawl space into a usable area with more headroom;
- Additional support for cracking foundation walls;
- Stabilizing the original foundation;
- Increasing load capacity for a new storey.
It’s important to follow safe building practices and code requirements related to excavation, electrical and plumbing re-routing, and concrete construction. A foundation and underpinning contractor will be aware of these safety practices and building codes to ensure your underpinning projects meets the right standards.
Here’s an overview of how a basement underpinning gets done:
Evaluation and Design
It’s important to determine the right excavation level, footing arrangement and details for moving and extending load-bearing walls. Most municipalities require stamped engineering drawings be submitted prior to construction to obtain the proper permits.
Preparing the Site
A professional will need to re-route furnace connections, ductwork and equipment to another location. Temporary supports or vertical jacks will need to be installed to hold the building during underpinning. Steel or wood beams can be used to disburse the weight of the home and removed once the job is done.
The underpinning process can be completed in several different ways, but each requires excavation and removal of excess soil under and around the basement. This can be accomplished by hand, with shovels, or heavy equipment such as backhoes and soil loaders. The latter are more costly, but save significant time and labor. The Traditional Mass method is used when the foundation needs strengthening. A mass concrete slab is poured in the area where the soil was removed and provides a more efficient distribution of weight.
- Concrete beams are also used in conjunction with the mass base with the Beam and Base method. This gives even more support to the structure and balances the load evenly.
- Mini-piled underpinning utilizes 15 cm to 30 cm steel-encased piles that are driven into pre-drilled holes. They rest on more stable ground 5 to 15 meters below the current footers.
- A fourth method requires the existing concrete floor be removed. Excavation is done in several sections and filled with concrete. Non-shrink grout fills the space between the old and new concrete.
Now that the basement has been lowered and a beautiful new, useful space has been created, you’ll need to ensure the area stays dry. The foundation walls are now in place and the old plumbing can be removed to make way for new. A reliable waterproofing system should include:
- An external membrane;
- Internal weeping tiles;
- A waterproofing membrane;
- A sump pump.
Don’t underestimate the power of hydrostatic pressure and its ability to force water inside your basement. Your new space is no exception and will be susceptible to water leaks and flooding if you don’t prepare for it early. Installing an external membrane system will keep a lot of ground water from entering the home. Next, you should backfill any exposed foundation walls. Inside, adding a drain tile system before the floor is poured is a fairly simple process.
- Dig a trench around the perimeter of the room, lay gravel in the bottom, insert a plastic, perforated pipe and membrane and run it to the lowest corner in the basement. Here, you’ll want to dig a sump pump pit about 2 feet deep and 18 inches wide.
- Pour a layer of gravel in the bottom, lower a submersible or a pedestal sump pump into the pit, and connect it to the drain tile.
- As water fills the reservoir, a float is activated, turns on the pump and safely moves the water through a pipe to the outside.
- Cover the drain tile with more debris-filtering gravel around the entire interior of the room.
- Before closing up the sump pump, make sure it’s fully operational. It’ll be a lot easier to make adjustments before the floor is installed.
- To get ready to pour the new concrete floor, a layer of gravel 4 to 6 inches thick should be spread evenly throughout the basement.
- Pour the new concrete slab and allow to cure for several days.
Once the floor has dried, there is one more waterproofing process you must complete. Low-VOC content, low odor liquid waterproofing should be applied to walls, ceilings and hard-to-reach places. This liquid membrane provides excellent coverage, is impervious to hot and dry conditions and won’t crack or shrink.
You may want to consider adding a walk-out basement. As long as one wall is exposed, you can add full-size windows, a door with easy access to the outside and the room will always be brightly lit. Remove the temporary beams used to support the building and enjoy.
Completing the underpinning job will give you the extra space you’ve been dreaming of and increase the value of your home for years to come.
Experts from Royal Work Corp. are always at your service when it comes to basement underpinning or waterproofing of any size and complexity. Follow them on Twitter @RoyalWorkCorp or check out their blog for more tips.
Let us put you in touch with one of many underpinning contractors who can assess your home’s needs and do the work for you. The faster you get started, the faster you can enjoy your new basement!
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