Guide to the Construction of Sunken Slabs | RCC Construction





Construction of Sunken Slabs | Building Construction

Sunken slabs are used in the toilets, bathrooms and washing place where we have our washing machines. The purpose of having a sunken slab is to conceal all the pipes below the floor. Since the pipes that carry water are concealed below the floor, care has to be taken to avoid leakage problems.

It is seen that people are not much aware of the idea of waterproofing the Sunken slab before the floor finish is done. In this article, we will discuss the method of the construction of Sunken slab and waterproofing technique.

Method of construction of a Sunken Slab

  • The concrete of the R.C.C. (floor and sunken slab) should mixed with a waterproofing material to get a denser, watertight concrete.
  • Then cement and waterproofing material should be diluted in water and splashed onto the RCC sunken slab. Over that a layer of plaster should be provided using a mortar plasticizer with the cement mortar.
  • Brick laying of walls and plastering (prior to tiling) of the walls and floor should be done with cement mortar mixed with a mortar plasticizer.
  • Tile fixing for the floor and walls tiles should be done with non-shrink, waterproof tile adhesives to make the tiled area waterproof.
  • Sanitary pipe joints should be sealed with sealants specially manufactured for Sealing Sanitary joints firmly so that no water can leak through.


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  1. #1 by Russell Davison on March 16, 2011 - 8:25 am

    I think that there should be mention of bitumen. A bitumen or waterproof sheet is usually laid between the ground and the sunken slab to stop moisture rising from the ground through the sunken slab. Am I right?

  2. #2 by ARUN on August 25, 2011 - 3:45 am

    why are we using sunken slab

    • #3 by BenzuJK on August 27, 2011 - 4:00 am

      Hello Arun,
      Sunken slabs are used in bathrooms. The sunk which is designed for a toilet or bathroom is to hide all the pipes that are to be laid beneath the flooring of the bathroom.

      I hope I have it clear as to why we need sunken slabs in the bathrooms. Let me know if you have any further questions.
      Cheers:)

      • #4 by SK Shahi on September 21, 2011 - 8:23 am

        Dear ben,
        i am fm lko, india and is planing to construct a house on plot area 33X66 ft. i spoke to architect too but needed some more assistance to know how to go ahead bout this. i am not frmthisfield in any way. I have been following u on this site for quit some time and found u quit knowldgable. can u help me out..

        • #5 by Afzal on August 28, 2012 - 3:05 am

          The sunken slab arrangement used in bathrooms and toilet helps in the proper fixing of the water closets. Also, it assists in the smooth drainage of water or moisture through weepholes provided in the cavity. Also, the provision of sunken slab ensures that no water from the bathroom floor drains onto the bedroom or living room..Hope it answers ur query arun.

      • #6 by narayanaswamy on February 24, 2012 - 6:06 pm

        please tell me how to proceed for analysis of sunken slab in the design.

  3. #7 by Rintu amity on July 13, 2012 - 4:44 am

    Dear benz

    Pls tell me how to casted sunken portion,its casted simulteneously with natural floor level or not,and sunken potion how to reinforcement are placed ,reinforcement also up& down or not.

  4. #8 by raj on December 20, 2012 - 10:49 am

    Russell Davison :
    I think that there should be mention of bitumen. A bitumen or waterproof sheet is usually laid between the ground and the sunken slab to stop moisture rising from the ground through the sunken slab. Am I right?

    Thanks Russel. You sure are about the bitumen

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