Sunken Slab – Uses, Advantages & Disadvantages

What is Sunken Slab?

It is a type of slab provided below the standard floor level in toilets, bathrooms, and laundry where a drainage system is installed, known as a sunken slab or sunk.

A sunken slab is provided to conceal all the pipe below the Standard floor level to maintain the space’s aesthetic appeal.

Generally, the slab is constructed at a 20 to 30 cm depth from the standard floor level to install pipes and utility ducts.

For protecting pipes and less maintenance, the level difference between the standard floor and sunken slab is covered by broken bricks, sand, coal, or other lightweight material to protect the pipe.

Since the area where this slab is provided is vulnerable to dampness and water leakages hence special attention is needed while casting the sunken slab.

Why do we use sunken slabs?

This slab is used to conceal the pipes of the drainage system and keep the bathroom level down from the standard floor level to avoid water splashing. Sometimes it is needed to maintain the building’s architectural aesthetic.

If we don’t use a sunken slab, we have to use its alternatives like raised toilet floor and underslung plumbing, which have their set of advantages and disadvantages.

Construction Procedure of Sunken Slab

Sunken Slab
  1. Install the centring and shuttering, but the formwork level of the sunken slab will be down to the standard slab formwork level.
  2. Then place the reinforcement and tie them according to the design.
  3. Now, cast the slab; the concrete should be dense and watertight then cure it.
  4. A mixture of cement and waterproof material should spray on RCC sunken slab.
  5. A layer of plaster of cement mortar with mortar plasticizer should be applied.
  6. For improving the tile space, waterproofing, non-shrinkage, waterproofing adhesive should be used for fixing wall and floor tiles.
  7. Special sealant should be used to joint sanitary pipes to avoid leakage of water.

Filling Material For Sunken Slab

The surrounding area of this slab requires filling with suitable lightweight material to protect the pipe of the drainage system from any damage.

Sunken Slab

AAC Blocks, broken CLC blocks, brick jelly, broken bricks, lumps of burnt tar, coal lumps, etc. are suitable for filling the space of the sunken slab

Use of Sunken Slab

  1. It is idle for toilets, bathrooms, and wash areas.
  2. In porch, to provide the plan ceiling surface and to avoid any downward due to beam.
  3. It is suitable for mid-landing of the staircase to get adequate headroom below the landing.
  4. It is used for some special architectural design where a sanitary system is required.


  1. It helps to conceal all plumbing pipes inside it and maintain architectural design.
  2. Avoid the requirement of special plumbing fixtures needed to reduce the noise during usage.
  3. Help to keep the bathroom level down from room level and avoid water splashing.


  1. It decreases the slab’s height.
  2. It requires brickbat work and waterproofing, which is a time-consuming process.
  3. For the construction of a sunken skilled and experienced worker needed.

Alternative to Sunken Slab

Raised Floor Level

In this method, the toilet slab is constructed to the standard floor level, but the toilet floor is raised above the slab by creating a step to conceal the pipes of the drainage system. 

Sunken Slab – Uses, Advantages & Disadvantages

It is suitable to conceal the pipes and reduces noise generated during toilet uses without using a particular plumbing fixture.

But it decreases the toilet headroom and not suitable for older occupants.

Underslung Plumbing

In this method, a toilet slab is constructed at the adjacent slab level, and wherever drainage system pipe needs to pass through a hole or sleeves is punched. 

Sunken Slab

The pipes are fastened to the slab’s bottom and concealed under the false ceiling to hide the drainage system. False ceiling is installed with a door trap for ease of maintenance.

This system avoids the filler material required in sunken slabs and allows easy drainage system maintenance without harming the structure.

But this system requires a particular plumbing fixture to reduce the noise generated during toilet uses, which increases its cost.

Final Words

I hope now you have essential information about the sunken slab.

We advise you not to use raised toilet floors in houses since they are unsuitable and aesthetically not pleasing.

You can make an informed decision between the sunken slab and under-slung plumbing based on the abovementioned pros and cons. 

Please share it.


Leave a Comment