This is an overview of the process involved in lowering a basement floor. This is not an exhaustive report. Rather this is a basic overview to give depth and perspective to homeowners so that they can understand and appreciate what is necessary to lower a basement floor.
There are many reasons to lower a basement floor. The reasons for doing so are as diverse as the conditions for each project. Some wish to turn a low headroom area into functional living space while others may be interested in converting a normal basement into one that has extra high ceilings for a movie room with stadium seating. Regardless of circumstance, the process of lowering an existing basement floor is one that requires the contractor to possess skill, expertise, experience and consideration of the below elements.
One of the most important aspects to consider is the type of foundation that exists in the area where the floor level is to be lowered. Each foundation type will have inherent structural aspects that will dictate how your project will unfold and the tactics you will employ to safely lower the floor while ensuring the safety of the structure. Let's look at some of the most common foundation types and how they differ.
Traditional Block with Footer - This foundation type in almost all cases will have a footer that is a monolith. This is important to note because it lends to overall stability of the entire structure when small section of the home is temporarily undermined to facilitate lowering of the floor. A monolithic footer also provides some degree of uniformity in the sub-floor area. This degree of uniformity usually allows the contractor to use the same support methods throughout the project rather than shifting and adjusting to different conditions under each section of foundation. Although there are no absolutes, in most cases you will not find surprising conditions once the floor slab is removed. A less complex job is always an advantage for all involved.
Stone/Sandstone/Fieldstone - Due to the fact that stone foundations literally come in all shapes and sizes, I am going to discuss them in a very general sense. Certainly each will be different, and require specific investigation, but there are some common hallmarks that can be discussed for the purpose of this article. The first thing to discuss is the absence of a footer, which is very common for this foundation type. The next thing to note is the fact that rather than standardized masonry units, you are dealing with large stones of inconsistent size, weight and shape. Lastly, in almost all cases you are dealing with a very heavy and thick foundation. Thus, with all of this considered, you have a foundation that is much more difficult to temporarily support when lowering the floor.
You will almost certainly have non-uniform conditions under the walls. Therefore the methods and equipment used on one section may not work for another section. This will slow down the pace of work as you adjust to each new condition. Moreover, as sections are undermined, small pieces of the stone foundation may break loose and warrant spot repair before work can proceed. Another challenge is the thickness and weight of most stone foundations, this dictates that your temporary reinforcement be substantial and able to carry heavy loads while still being light enough to transport to the job-site. In short, a stone foundation project will most likely take longer to complete.
Poured Concrete - We can classify poured concrete foundations into two categories; those with footers and those without footers. Those with footers are very similar to traditional cement block foundations with footers and treated very similarly. With that said, those without footers are treated quite differently.
If the poured concrete foundation is a monolithic pour, but does not have a footer, it is still a very strong foundation and able to handle reasonable undermining with temporary support during the excavation phase of lowering a basement. The only additional consideration would be the overall weight of the wall. Because the wall is poured concrete and not hollow block, it will weigh more and require more robust supports.
Some poured concrete walls are pre-cast panels that are connected on-site as the house is built. In some cases a base of crushed stone is used rather than a traditional concrete footer. This type of foundation is one of the most difficult to lower the floor level. The fact that the walls are comprised of concrete panels makes it harder to support in long sections. Thus smaller, more localized areas have to be done. This can drastically slow down the pace of work but is necessary to protect the integrity of the foundation.
The desired depth of the new floor is the biggest driver of cost and work duration in a basement lowering project. In some cases the property owner underestimates the amount of work and excavation necessary to create the finished floor height. For example, if you wanted to lower a floor from its current position down 12-inches for a total drop of 1-foot, you would have to initially excavate 20-inches or more. The reason for the additional 8-inches is that you must lay a minimum 4-inches of gravel base and 4-inches for your concrete floor to give you the finished height of 12-inches lower. In most cases a 20-inch excavation will be deeper than the footings of a home and thus warrant reinforced cement knee-walls around the perimeter to support the home and prevent collapse or undermining of the foundation. This process of excavating and then supporting areas is what allows the floor to be dropped safely.
Not all foundations are healthy or strong enough to incur the temporary stresses associated with a basement lowering project. Homes with cracked or bowed walls as well as other foundation problems may not be a good candidate for this type of work. In some case repairs to the foundation must occur before the floor lowering process can begin.
Each home will require some degree of support during the lowering phase. The most common area that needs addressed is the internal support post(s) of a structure. In some cases these supports are sitting atop the floor that will be lowered. Therefore compensation for their removal during the lowering phase must be considered.
Depending on the desired depth, the new floor height may be deeper than the primary foundation walls and warrant temporary support. If the substrate next to or underneath a section is being removed and eventually replaced with reinforced concrete footers and knee walls, these areas must receive reinforcement to prevent movement during this process.
As with any retrofit, specific circumstances may exist that warrant specific measures based on the work environment you have. When lowering a basement you may need any manner of braces and supports to ensure the safety and stability of the home and foundation you are working with. This is one of the major reasons it is imperative to deal with a seasoned contractor who has performed this type of work in the past and have the necessary skill set and manpower to handle a project of this type.
In most cases the major plumbing of a home leaves the house at a depth lower than the basement floor. If you are dropping the floor elevation the homes plumbing becomes an area of discussion. Although in some cases a client will elect to have the plumbing remain at its current elevation, and thus be exposed above floor level, the most common approach is to lower it along with the floor. If you are considering a project where the plumbing will be lowered, it is imperative that you consult a licensed plumber to ensure the feasibility and practicality of such a project.
Access for Debris Removal
Access to the basement area is one of the most important aspects, vis-a-vis debris removal, to consider. Whether you have a small excavator and conveyor belt system or you will be using manpower to bucket out the material, access will be very important when considering cost and work duration. In some projects an egress will be cut out of the foundation to allow quick and easy access. Still others will not have the ability for this luxury. Some homes, with integral garages, will have built in access. Each environment will be different and access must be discussed.
It is always good practice to consult the local code enforcement office before a project such as this to procure the necessary permits. Additionally, as excavation will be occurring, an audit of underground utilities should be conducted.
By definition, you are creating an area of the foundation that is deeper in the ground than was originally constructed. This negates the home's waterproofing defenses that have been calibrated for a floor height that no longer exists. Because most basement lowering projects occur primarily on the inside of the home, there is little opportunity to expose the foundation and lower footer drains and apply wall membranes. Therefore, internal waterproofing practices are usually employed to protect this new, deeper area. Internal French drains with sump pumps are the most common practice used as they can be built to the specific conditions you have at your project. Additionally, when installed correctly, they are an extremely reliable method of waterproofing a basement.
Once the above work has been completed you are now ready to pour the basement floor. Unlike many traditional basement floors, which are termed "floating" as they sit atop a footer and have no real load bearing duties, this floor will be charged with the load bearing responsibility of locking in the base of the new reinforced concrete knee walls. The floor should be a minimum of 4-inches thick and be poured in such a way that it touches all of the newly poured knee walls. The floor should have a 4-inch gravel base and use post-cured control joints rather than expansion joints. This will allow for crack control and lateral stability whereas an expansion joint could compress if the base if the knee wall was moving inward. With that said, the size and stability of the knee wall should be calculated by a professional engineer so that this movement never exists and the floor as a load bearing member is relegated to a fail-safe.
An additional note is that all support posts and columns should be set on independent footers which sit below the floor slab. The posts should be put in place and then the floor slab poured around them.
Doors and Means of Egress
During this type of work it is common to include a permanent door or means of egress in the foundation. Assuming sound construction principals are followed, this can certainly be accomplished during the time the project is underway. In many cases the temporary egress used to remove material from the basement is used to create a permanent door or stairwell for future use by the property owner. Additionally, if the area is to be converted into a bedroom or other living space, local codes may mandate a secondary means of egress. Naturally this would be included in the scope of work you would be performing.
Selecting a Contractor
As with any complex duty or task, you want to use competent and well-seasoned people. Considering the fact that ones home is in many cases their biggest asset, the reason for using an experienced contractor to lower a floor is compounded. As discussed above, lowering a basement floor is a highly complex process. Moreover, it is only performed by experts. Make sure to seek a truly seasoned contractor and ask for references of similar work.
This is one of the most important - and overlooked - areas for selecting a contractor. To perform basement lowering a contractor must have specific foundation repair insurance, which is extremely expensive to carry. The cost of this insurance is such that only those who regularly perform this work would justify the cost. Ask for a copy of the contractors insurance and call the carrier to ensure this type of work is covered. Remember that general liability insurance, regardless of the limits, does not cover this type of work.
Almost as important as Foundation Repair Insurance is Workman's Compensation Coverage. Work Comp is extremely costly for a company to procure and maintain, but it is an absolute necessity. If a worker were to get injured, and the company did not have work comp, the homeowner could be forced to pay medical bills or be liable for physical harm.