Guide to the Construction of Sunken Slabs | RCC Construction

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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.

16 thoughts on “Guide to the Construction of Sunken Slabs | RCC Construction”

  1. 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?

    • 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:)

      • 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..

        • 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.

  3. 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. 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

  5. Dear Sir,

    We are constructing a 4 storey flat and during excavation process we found 25×25 well in the corner near and as per design there were about 6 columns are coming.

    Please suggests need to have additional columns or rafting will solve the problem or need to rework on entire structural disign.

    Thanks and Regards
    M.Sundaram

    • For a 4 storey flat, 6 columns will not be sufficient. I cant tell you exactly where the changes will need to be made without seeing the drawings. But you will have rework on the structural design. If you are not a structural engineer, I suggest you get it designed by a structural engineer. I hope this helps.

  8. Sir,
    I have a problem with the sunken area in my villa which is under construction. My builder has by mistake not made the sunken portion provision and has fasted the roof on a single slab. This results in the pipes being laid on the surface of the bathroom , and later covered by concrete and tile. I want to correct the situation and bring in the sunken portion in the bathroom. How can this be done. One recommendation is to break that portion of the slab and reconstruct the sunken portion. Other suggestion is to drill a hole in the slab and bring down the pipes below the slab of the bathroom. This results in bathroom pipes being visible on the room of the bedroom. So please what best solution can I have to build the sunken portion?

  10. hey….i am an archi student…..bt during my training prgm i jst sw the construction of sunken slab..but it is a storey above the cellar…. ( ground or first floor ??)
    thats y…..i jst seen that the sunken portion is just below the floor level according to design it may vary usually 20-30 cm depth is allawable the floor height frm cellar is different at the bottom of sunken and bottom of other rooms slab (i recntly visit the construction of a shoping complex)

    but here i have a doubt weather it is built in the ground floor of normal building ie.,for recidential purpose…..i think the toilets at the ground floor also require the same kind also i thnk it is from a depth of 30 cm below the floor level (ie., 45-30=15 frm the ground level(45 cm is the plinth height))…but what we call it for
    sunken slab……??

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