What is sunk slab in construction?

A sunk sab, also known as a sunken slab, is a part of construction in underflows where the secondary foundation for pipes, wires, etc., is placed. Sunk slabs are primarily used in the construction of bathrooms, washrooms, and laundry areas where drainage and water supply are needed the most. 

A sunk slab is constructed to keep the pipes and drainage systems hidden from the human eye to maintain the look and appeal of the area above it. Sunk slabs are also used to place ducts and are usually placed 30 cm below the conventional floor level for easy access in case anything needs looking after. 

Coal, sand, gravel, brick pieces, whatever construction leftovers consist of, are placed in the space between the sunk slab and the floor above it. These materials should be lightweight and easily removable to reduce maintenance and keep the pipes protected. A sunk sab has to be made with strong materials to prevent it from hazards like water damage and moisture. 

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Sunk slab: Setting it up

Setting up the foundation for a sunk slab is easy; just go about how you’d make a normal flooring and leave about 300mm of space above it for light materials to protect the sunk slab. 

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1. In the space where the sunken slab is supposed to go, lay out the framework and TMT. 
2. Mix the concrete with waterproofing material to make it denser and water resistant. Pour it into the mould. Cure it as needed and make sure it is densely accumulated. 
3. On top of the reinforced concrete sunken slab, spray a mixture of waterproofing material and cement. 
4. On top of the first layer, put a layer of plaster with cement mortar. 
5. Use waterproofing and anti-shrink adhesive to place and fit the tiles for flooring. 
6. To avoid water leakage, seal the joints of sanitary pipes with waterproof sealant. 

Sunk slab: Uses

A sunken slab is a very important part of constructing properties. It is primarily used in apartments to have more space between floors and to protect the floor underneath in the event of a leakage.

Sunken slabs are primarily used in the following: 

• Places with tons of pipes like toilets, bathrooms, washrooms and kitchens. 
• Mid-landing staircases with inverted end beams. 
• Porches. 

Sunk slab: Advantages

A sunk slab is a wonderful piece of civil engineering that has saved and improved the lives of many. Apart from it being so easy to build, user-friendly, and cost-cutting, sunk slabs have a few more advantages over traditional flooring. 

1. It helps hide all the pipes and does not hinder the building’s architectural design. 
2. Reduces the chances of plumbing failures. And if plumbing fails, it reduces the chances of hazards or damage to the floors. 
3. Economically sustainable.

Sunk slab: Disadvantages

1. It raises the height of the slab.
2. Waterproofing is less economical and time-consuming. 
3. Requires skilled, experienced labour workers to put in hours of their work. 

Sunk slab: Alternatives

There are alternative approaches to sunken slabs, each with its own set of advantages and disadvantages. These alternatives include:

Raised toilet floor

In this option, the toilet slab is constructed at the same level as other slabs, while the floor is elevated to conceal the drainage system. The benefits of a raised toilet floor include the elimination of visible pipes and the reduction of noise generated during toilet usage, all without requiring specialised plumbing. However, it comes with the drawback of decreased toilet headroom, making it less convenient for elderly occupants.

Underslung plumbing

In the case of underslung plumbing, the toilet slab is built at the same level as adjacent slabs, and holes are punctured into the slab wherever pipes need to pass through. These pipes are affixed to the slab’s bottom above a false ceiling, effectively concealing the drainage system. For maintenance purposes, the false ceiling is equipped with a door trap.

The advantages of underslung plumbing include easy maintenance of drainage systems without affecting the overall structure and the elimination of the cost associated with filler material required in sunken slabs. However, a notable drawback is the necessity for specialised plumbing fixtures to address noise issues during bathroom usage, leading to an increased overall cost for underslung plumbing

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