Sunk Slab in Bathrooms: Depth and Waterproofing
Sunk slab in bathrooms: a sunk (sunken or depressed) slab is the part of a floor slab under a bathroom or toilet that is cast lower than the rest of the floor. The drainage pipes from the WC, floor trap and basin run inside this depression and are buried in filling, so the bathroom floor finishes level with, or just below, the rooms around it. Depth is set by the pipe layout: commonly about 200–300 mm for western WCs and floor traps, and more for Indian-style pans with traps below. Because water sits in this box, it needs careful waterproofing, a pond test, and a way to drain any water that gets into the filling.
A leaking bathroom is one of the most common and expensive problems in Indian houses and flats: damp patches on the ceiling below, peeling paint on the next room’s wall, and a smell from the filling. Most of these leaks come from the sunk slab. This guide explains what a sunk slab is, how deep it should be, the right waterproofing sequence, and the mistakes that cause leaks.
Last updated: 25 October 2026
What is a sunk slab?
In a normal RCC floor, the slab is flat and the floor finish sits on top. In bathrooms, toilets and sometimes kitchens, the waste pipes from the WC, floor trap, shower and basin need to run horizontally, with a slope, to the vertical stack outside. A sunk slab provides a box in the floor for those pipes. After the pipes are laid and tested, the box is filled, screeded and tiled.
The sunk portion is part of the structural slab and is designed with it; the beams around a bathroom are often arranged to frame the depression. For slab behaviour in general, see one way vs two way slab.
How deep should a sunk slab be?
There is no single standard depth. It is worked out from the pipe sizes, the length of the horizontal runs and the slope they need, plus the trap heights and the screed and tile thickness on top. Common ranges in Indian practice:
| Bathroom layout | Typical sunk depth |
|---|---|
| Western WC with floor trap and short pipe runs | About 200–300 mm |
| Longer pipe runs or several fixtures | Deeper, as the slope requires |
| Indian-style (squatting) pan with trap below | Often about 400–450 mm |
| Wall-hung WC with concealed cistern and wall outlets | Can be shallower, or no sunk slab at all |
Decide the fixture types and positions before the slab is cast. The plumber, architect and structural engineer should agree the depth and the outlet positions together, because changing them after casting means breaking concrete.
Alternatives to a sunk slab
- Pipes below the slab: the bathroom slab stays flat and the pipes run under it, hidden by a false ceiling in the room below. No filling means less to leak into, but pipe access is from the floor below.
- Raised bathroom floor: the bathroom floor is built up above the room floor, with a step at the door. Simple, but the step is a tripping hazard.
- Wall-hung fittings: wall-hung WCs and wall outlets reduce the horizontal pipework, so a shallow sunk or none may be enough.
Waterproofing a sunk slab: step by step
- Prepare the surface. Remove debris and loose concrete, repair honeycombing, and form rounded fillets (coving) at the junctions of the base and the walls.
- Seal the pipe sleeves. Grout around every pipe passing through the slab or walls with non-shrink grout and treat it with the waterproofing.
- Apply the first waterproofing layer over the whole base and up the sides of the sunk area, continuing up the bathroom walls by about 300 mm above finished floor level. Cementitious polymer coatings, liquid membranes and sheet membranes are all used; follow the product’s coats and curing.
- Pond test. Fill the sunk area with water and leave it for at least a day or two (some specifications ask for 72 hours), checking the ceiling below for damp. Fix any leak and repeat.
- Protect the layer with a thin screed so pipes and filling do not damage it.
- Lay and test the pipes. Fix the drainage pipes at the right slope with proper joints, then fill and pressure- or water-test them before any filling goes in.
- Provide a sunk drain (sometimes called a weep pipe): a small outlet at the lowest point of the sunk area, connected to the outside drain, so any water that reaches the filling can escape instead of soaking it.
- Fill the depression with a lightweight filling in layers. Heavy filling such as brick bats adds load to the slab and holds water.
- Screed to slope towards the floor trap.
- Apply the second waterproofing layer over the screed and up the walls, higher in the shower zone, then tile with a good adhesive and joint grout. Epoxy grout resists water better than cement grout.
Concrete and screeds in the sunk area should be cured properly; see curing of concrete.
Why sunk slabs leak
- Waterproofing only on the base, not up the sides, so water runs around it at the wall junction.
- No pond test before filling.
- Pipe joints that were never tested leak into the filling.
- No sunk drain, so the filling becomes a water tank that slowly soaks through.
- Broken waterproofing around pipe sleeves and the floor trap.
- Cracked or missing tile grout letting shower water into the screed.
- Nails or drilling for fittings punching through the waterproofing.
Damp in walls next to a bathroom can also come from rising damp at the base of the wall; see damp proof course (DPC). For wall finishes in wet areas, use cement plaster rather than gypsum; see cement vs gypsum plaster.
Hindi terms used on site
On site it is called “sunk slab”, “sunken” or “sunk portion”. The filling is भराई (bharai), waterproofing is “waterproofing”, the floor trap is simply “floor trap”, and the pond test is often called “pani bhar ke test” (a test by filling water). Seepage is सीलन (seelan) or रिसाव (risaav).
FAQ
What is a sunk slab?
A portion of the floor slab under a bathroom or toilet that is cast lower than the rest of the floor, so drainage pipes can run inside it and be concealed under the finished floor.
What is the depth of a sunk slab in a toilet?
Commonly about 200–300 mm for western WCs with floor traps, and around 400–450 mm for Indian-style pans with traps below. The actual depth comes from the pipe layout.
How do you waterproof a sunk slab?
Waterproof the base and sides, pond test it, protect it with a screed, lay and test the pipes, add a sunk drain, fill with lightweight material, screed to slope, then waterproof again under the tiles.
How long should a pond test be?
At least a day or two, and up to 72 hours if specified, while checking the ceiling below for damp.
Is a sunk slab necessary?
No. Pipes can run below the slab behind a false ceiling, or a raised bathroom floor can be used, and wall-hung fittings reduce the need for depth.
Related guides: Damp proof course (DPC) · Curing of concrete · One way vs two way slab · Bathroom door designs · All construction guides
Disclaimer: Educational only. Sunk depth, slab design, drainage layout and waterproofing system must be designed by a qualified engineer and plumber and applied to the waterproofing manufacturer’s instructions. Disclaimer
Tanmay Arora, is an Architect with an experience of 13+ years. He is a Management Grad with a post Graduate Diploma in Building Construction and Materials. Tanmay has worked with renowned firms. He has graduated in Bachelor of Architecture. His deep interest in Architecture and Civil Engineering and his experience has made him one of the best architects a client can think of.
