Floating Slab | Float Slab Foundation | Floating Concrete Slabs

8 min readOct 8, 2022

Floating Slab | Float Slab Foundation | Floating Concrete Slabs

What Is a Floating Slab?

Floating Slabs are concrete slabs that are laid on the ground without any anchoring, such that it simply sits and floats on them. The floating slab, as the name suggests, looks like a plate, which is only placed on top of the water, with no connection between them.

Why Use a Floating Slab?

Floating Slab

Using a floating slab foundation is considered one of the most cost-effective methods of building a foundation of a house. Normally, a strip foundation is used for house foundation construction using a frosted wall above.

Strip footings are construction lower than frost level of area and which is typically kept between 3 ft to 4 ft below grade level. The construction cost of strip footing and front wall construction is generally higher compared to normal foundation construction which involves a larger area to be excavated for a foundation with more concrete work and labor work is required.

Construction of Floating Slab

Construction of Floating Slabs requires details. The proposed building requires a solid and strong foundation to fully support the entire superstructure’s weight on it. As mentioned they are more commonly used in garages, extensions to a house or shed do not necessarily require a large foundation. These buildings have less structural load compare to others.

Hence Floating Slabs are found to be most suitable and economical as compared to conventional slab construction. The main construction layers in a floating slab are shown in Figure-1. The construction includes the following steps:

1. Site Excavation for Floating Slab Construction

Site Excavation For Floating Slab

The first step to be done in floating foundation construction is to clean the land and excavate the area as per the plan. This is the preliminary preparation for construction.

The step also includes creating drainage in the sides to drain water through the gravel base as shown below. The constructed trench is sealed with geotextiles and gravel as shown below, which will drain water from the concrete slab foundation.

2. Laying Gravel Base for Drainage

Install a gravel grid before pouring gravel. Add an 8- to 12-inch gravel base of various rock sizes, compacting each layer for durability.

3. Reinforcement for Slabs

Reinforcement Of Floating Slab

A slab mold is placed for the casting process. For reinforcing slabs, reinforcing bars are provided as per the dimensions and code.

4. Casted Floating Slab

Concrete is poured into the slab formwork. This compresses either my labor or a mechanical vibrator, depending on the area and workmanship. Proper compaction has to be done to make the slab fully productive. Appropriate treatment has to be taken for about 2 to 3 days.

How to Build a Floating Slab?

Floating Slab Components

Step: 1 Prepare A Ground Floor Slab

Firstly, Drive the four stakes at each corner of the floating slab. You also use line, level, or builder’s level to check the slope of the ground at the construction site. If there is an excessive slope at the site and flattening it means moving tons of soil which will increase construction cost.

So, try to cut down the higher side and fill the soil on the lower side of the slab. It will reduce the cost of moving soil from the side as it settles down in the same place.

Step: 2 Mark and Lever On Side

Prepare a wooden board form of size 2x12 length or join it with a cleat to mark one face of concrete forms and fix it with corner stakes.

Again level 2x12 wooden board and fix it with the second corner of the slab. 2x12 boards are best suited for constructing 5 in tick slabs for garages and sheds.

Step: 3 Do Bracing of Stakes and Side Forms

Fix the Manson line on the top of the 2x12 wooden board. Align this 2x12 board to match the string and brach it or fix it with 2x4s spaced pairs which are placed at every two feet apart.

Forms are braced to ensure sides remain straight in position during the pouring of concrete. Because fresh concrete can push these board out side which result in curve shape of slab on the sides of flooring slab.

Step: 4 Fix Other Side with Stake and Level it

Prepare a second 2x12 wooden board with having a size equal to the width of the slab. Fix one side end with braced form and set it at 90 degrees using diagonal length calculation. Drive stake fixers at the end and level it and brace 2nd wooden board form.

Similarly, add the 3rd and 4th sides of floating slab formwork.

Step: 5 Level and Fill Work

Now, Fill 3 inches of granular soil keeping five inches margin from the top of the forms. Level and tamp this fill properly.

After filling we mush have to check that the 5-inch margin is left out in the whole slab area are. For that stretch, the string between the top of two forms at the various positions and check the distance.

Step: 6 Compact the Fill

Properly compact the soil with a plate compactor. Left 12x12 In deep and wide trench around edges of slab to thickness the edges of the slab.

Step: 7 Insert Steel Rebars to Increase Slab Strength

Now, Spread a 5 mm thick plastic sheet to cover the entire area of the slab. Fix two steel bars of 1/2 inches dia to stake keeping at least 4 inches distance from a wooden board from work.

Step: 8 Prepare Slab Partition

Spread the grid of steel bars on a plastic sheet each at least 4 feet spaced from each other. Divide the entire slab into various parts with the use of 2x4 wooden boards.

Keep this board align with string stretched between top edges of 2x12 wooden planks.

Step: 9 Calculate Volume of Concrete

Calculate the volume of concrete needed to pour the floating slab by multiplying its length, width, and depth together. This will give to the volume of concrete on cubic feet.

Now, divide the concrete volume by 27 to get a cubic yard of concrete. Place your order of Ready Mix Concrete at least one day before slab casting.

Step: 10 Pour Concrete and Properly Level it

Use a Wooden screed board to leveol the concrete and give it a smooth finish. Take numbers of passes over concrete surface to finish it well.

Step: 11 Make Round Concrete Edges

Rounding off concrete slab edges can be done with an edging tool. Remember, perform this work after surface water from concrete is disappeared from it.

Step: 12 Insert Achor Bolts

Insert 1/2 inch dia anchor bolts at every 6 feet distance apart. Keep about 2.5 inches of the bolt above the concrete surface.

Step: 13 Prepare Expansion Joints in Concrete

Make expansion joint in concrete to avoid random track by using groover tools.

Step: 14 Do Proper Curing and Let It Harden

Do proper curing of concrete with water at least for 24 houses and let it gain sufficient strength. You can also use a curing compound to make the curing process fast.

Step: 15 Remove Forms Properly

Remove concrete forms properly without damaging the slab. Repair any cavity left out in concrete with rich cement mortar mix.

Advantages of Floating Slabs

Floating Slabs have the following advantages,

Floating Slab Track

- The major advantage of using floating slabs is that they can be constructed on low-bearing capacity soil. Also, where there is money investment for soil treatment to increase soil bearing capacity is too expensive.

- These slabs have the property of spreading the vertical load or the stress on it to be distributed over a large area.

- The foloating slab are best suited when there is a need of extension of house area. It is very esy and cost effective in case of floating slab foundation to extend the house without distubing the existing structure of house foundation.

- The floating slab foundation does essential need to have footer excavation trench. These slab can be filled with concrete only by digging and treaching in foundation.

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- The floating slabs act as a barrier to the entry of moisture coming from the ground. It acts as a barrier between the superstructure and the ground.

- It protects against water seepage and frost. For this purpose, the slab can be insulating for frost or moisture. This led to their widespread use in cold field construction.

- The layer of soil lying below the floating slab does not disturb nor will it interfere with the quality.

- In case, where there are changes of soil layers settlement due to heavy moisture content in soil, the floating foundation provides best soilutin.

Disadvantages of Floating Slabs

Along with advantages Floating Slab also has some disadvantages as listed below,

- The floating slab does not provide underground access for the connection lines facing the utilities.

- It’s a primitive technology.

- Floating slab foundations have low design resonance.

Uses of Floating Slab

Floating Slab can be used for the following conditions,

- The primary use of floating slabs is as a base for industrial workshops, sheds and garages.

- It is very cost effective in areas where need foundation is not required.

FAQs

What Is a Floating Slab? Floating Slabs are concrete slabs that are laid on the ground without any anchoring, such that it simply sits and floats on them. The floating slab, as the name suggests, looks like a plate, which is only placed on top of the water, with no connection between them.

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