Sunk Slab
A recessed portion of floor slab beneath a bathroom or toilet, dropped to accommodate drainage pipework and later filled and screeded to fall.
On an actual building
It is where wet-area waterproofing is applied and where every pipe penetration occurs. Once it is filled and tiled, none of it can be inspected without demolition, which is why the pond test before filling is not optional.
What we most often have to correct
That the tiling protects it. Tiling and grout are permeable; the barrier under the fill is what stops water reaching the flat below.
Where this comes up in our work
Terms that come up alongside this one
- Upturn (Skirting)
The vertical continuation of a waterproofing layer up an adjoining wall or parapet, terminated mechanically at the top..
- Ponding / Flood Test
Plugging the outlets, flooding a completed waterproofed area and holding the water — typically 48 to 72 hours — while the space below is observed for any sign of ingress..
- Fillet
A curved or angled cement mortar profile formed at an internal angle so that a waterproofing layer turns the corner on a curve rather than a sharp right angle..
More building elements terms
Sunk Slab — questions we get asked
What does sunk slab mean in waterproofing?
A recessed portion of floor slab beneath a bathroom or toilet, dropped to accommodate drainage pipework and later filled and screeded to fall.
Why does sunk slab matter on a real project?
It is where wet-area waterproofing is applied and where every pipe penetration occurs. Once it is filled and tiled, none of it can be inspected without demolition, which is why the pond test before filling is not optional.
What do people commonly get wrong about sunk slab?
That the tiling protects it. Tiling and grout are permeable; the barrier under the fill is what stops water reaching the flat below.
Have a building showing this?
If a term on this page describes something you are looking at, a site inspection will tell you how far it has gone and what it needs. It costs nothing.
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