What Is a Floating Slab?
The slab isn’t anchored to the floor. This type of slab is typically used for shallow foundations such as garages, sheds, and light home extensions.
Why Use a Floating Slab?
The strip footing is installed below the frost level, typically 4′-0″ below grade. The cost of a strip footing and frost wall is a lot higher because there are more excavation, concrete, and labour required.
Floating Slab Construction
A building construction typically needs a solid foundation where it could bear the weight of the entire structure, including the roof, floor, and other loads it could contain.
But, small structures such as sheds, garages, backyard extensions don’t need elaborate and extra strong foundations as they’re generally light in structure.
The floating slab is an ideal solution for such basic foundation structures. This type of slab foundation is mostly used in northern climates as the slab doesn’t require concrete footing with deep extensions below the frost line.
The floating slab is sometimes called a raft foundation, and they’re generally constructed on soil with high proportions of sand, clay, or water.
A floating slab spreads the structure weight over the entire base instead of at strategic supporting columns. As they are shallow in-depth, they could ideally be used in specific construction locations.
How to Build a Floating Slab
A floating slab is the basic foundation structure used for buildings that don’t have basements (garages, sheds, barns, and even some homes in the high water table or coastal areas).
Concrete floating slabs may and, in areas that experience freezing temperatures, must be insulated.
Because there are no footings on this type of foundation, the insulation is either built directly in the concrete by sandwiching it between layers of concrete or by laying it directly on top of the concrete.
These additions are far cheaper than building a full foundation with footings and frost barrier as an excavation, and extra concrete costs far outweigh the cost of insulation.
Also, read: Difference Between Carpet Area and Built-up Area
Below Steps To Build Floating Slab
• Determine the area to your floating slab and then mark the four corners with 3-feet metal stakes.
• Determine the height of the top surface of the concrete slab. Once you’ve made this determination, use string to mark the height by wrapping it around the metal stakes. Use a level to ensure the string is straight, and the height is consistent.
• Measure out two feet in the perimeter (laterally) and mark off this area for drainage.
• Measure down 2 feet and 11 inches in the perimeter strings. Here is the starting point of the foundation for the slab. Excavate this entire interior area of the foundation to this depth. Also, excavate the 2-feet drainage section.
• Fill the excavated area 3″ deep with crushed stone to form a drainage area to the foundation.
• Cover the 3 inches of rockfill with 2 feet of sand.
• Compact the sand using a tamp (sand compactor) until it is 8 inches below the perimeter lines. Check the area in many locations to make sure the entire area is level.
• Place 2″-by-10″ (inch) lumber around the perimeter of the proposed slab to create the slab molds. Connect the corners with joints and secure them with framing nails. Make sure the forms are level.
• Insert support stakes [2-feet metal stakes with nail holes] each foot along the outside of the slab form. Secure these with framing nails as well.
• Remove the initial guideposts and perimeter string. At this point, it’s time to install any drainage lines or electrical conduit that must run through the foundation.
• Dig a 16-by-18-inch trench just within the slab form to provide extra support on the exterior of the slab to support walls.
• Install the 2″ (inch) foam insulation over the sand. Cut pieces to fill in the sides and base of this trench as well.
• Install a vapor barrier over all of the foam. Overlap pieces by 2″ to 4″ and secure with tape.
• Place the three-eighths-inch rebar at a criss-cross pattern within the vapor barrier. Be sure to use the 2″ rebar supports to lift the rebar off the ground so liquid concrete may flow around it.
• Add the five-eighths-inch rebar into the trench area. Make a cage formation with the bars to add extra support.
• Mix and pour your cement. If you’re working with a cement delivery service, they will determine the amount of concrete necessary for the pour. Otherwise, you’ll have to follow the calculations on your cement bag to determine how much to pour.
Advantages of Floating Slabs
The floating slab has the property of spreading the vertical loads or pressures coming over it to be distributed over a larger area.
The floating slab construction can be employed in site areas with lower load-carrying capacity and where investing large money on soil treatment is futile. This system lets us utilize over loose soil or from the soil using varying compressibility.
Floating slabs behave as a barrier for the entrance of the moisture coming out of the floor. It acts as a barrier between the superstructure and the floor.
This avoids seepage of water and frosting. For this purpose, the slab could be insulated for moisture or freeze. This was made their widespread usage in cold region construction.
Floating slab foundations are a good solution when it comes to the extension of houses. There’s a situation once we require a building extension without affecting the already existing foundation. This will not interfere with the building structures already built.
The floating slabs do not require the use of footer trenches. They may be poured with the help of trenching or digging, which can be economical.
In areas where there are possibilities of shifting in the earth layer, mainly because of high moisture content, the floating slabs are best suited.
The floating slab doesn’t cause any disturbance to the earth layer lying beneath. Neither will interfere with the quality.
Also, read: Testing for Silt Content in Sand
Disadvantages of Floating Slabs
Some limitations associated with floating slab would be its comparatively primitive technology, entrapped air within sealed floating foundation systems and lower layout resonance.
Floating slab foundations have the distinct disadvantage of lack of underground access for utility lines.