Sunken Slab

The sunken slab is a type of slab cast at a certain depth ( 200 mm, 300 mm, or any other depending on design) below normal floor level, it is also known as a sunk slab, this extra depth is used for placing pipes, utility ducts, then space is filled with sand or other lightweight materials until the normal floor level.

Here we will learn about sunken slab, advantages & disadvantages of sunken slab.

Introduction to Sunken slab:

This type of slab is used in bathrooms, toilets, washing places or water closets.

The purpose of providing a sunk slab is to prevent water from spilling over from the sunk slab area towards the surrounding slabs.

For the construction of sunk slabs the concert must be mixed with waterproofing material to get a dense watertight concrete.

Material used to fill sunk slab:

Lightly broken CLC blocks are used to fill sunken slabs and these blocks are the best choice for light weight cinder, broken bricks, brick jelly, coal lamps, burn bitter lamps and AAC blocks.

It begins to gain strength over time until some moisture is available in the atmosphere, also requires less maintenance.

Method of construction for sunk slab:

  1. Concrete R.C.C. (Floor and sunken slabs) must be mixed with a waterproof material to get dense underwater concrete.
  2. Then cement and waterproofing materials must be thinned in water and sprayed on RCC sunken slab.
  3. On top, a layer of plaster should be provided using a mortar plasticizer with cement mortar.
  4. The brickwork of the walls and floor must be achieved with cement mortar blended with a mortar plasticizer.
  5. Tile fixing tile for flooring and walls must be non-shrink, waterproofing adhesive to make tiled area waterproof.
  6. Sanitary pipe joints must be sealed with a specially constructed sealant to prevent any water leakage.

Advantages of Sunken Slab:

Sunk slabs have following advantages as follows:

  1. In such slabs, all plumbing pipes are concealed inside the slab.
  2. Special plumbing fixtures in sunken strips are not required to reduce noise in bathroom.
  3. These slabs provide a good plumbing system, also used where the safety system is to be installed.
  4. The sealing design is compromised and creates special ventilation.

Disadvantages of Sunken Slab:

Sunk slabs have following disadvantages as follows:

  1. The height decreases from slab to slab.
  2. Experience or skilled labour is required to fabricate this kind of slabs.
  3. In any such slab, waterproofing and brickbat construction take a longer time for construction.

Application of sunkEN slab:

  • Bathroom/ Toilet / Wash Area Floor: Floor lattices and drainage lines can be moved within the submerged area.
  • Porch Slab: In these places, the beams are inverted so that the beam does not face downward and a plain surface is available.
  • Middle landing on the ladder: The end beam is designed as an inverted beam so that sufficient headroom is available below the landing.
Also read: Post Tension SlabConcrete slab & Flat Slab


A sunken slab is placed approximately 300 mm below normal slab level in the building’s toilet or washroom areas.

The sunken portion is used to hold utilities and then filled with lightweight materials.

Hello, I'm Rahul Patil founder of, I had studied B.E. Civil. This blog provides authentic information regarding civil structures, equipment, materials, tests & much more.

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