In a concrete slab, the drain body is set during the slab or screed stage with the outlet cast in position, then the membrane is bonded over the flange and the screed is formed to a single-plane fall. The key is getting the body level to the finished tile line before the screed sets.
Two common approaches
| Approach | Description | Best for |
|---|---|---|
| Cast-in during slab pour | Body positioned and boxed out before concrete is poured | New build, coordinated with the structural programme |
| Set into screed after slab | Opening formed in the screed; body set and levelled, then screeded around | Most projects — easier to adjust on site |
Setting into the screed is generally more practical because it allows the drain to be levelled to the finished tile line after the structure has settled.
Sequence
- Confirm the drain position and outlet route before forming the opening
- Form the opening in the screed, sized to the drain body
- Position the body and set it level with the finished tile line
- Connect and support the outlet
- Apply primer and bond the membrane over the flange
- Form the screed to a single-plane fall toward the channel
- Test before tiling
- Tile, keeping the grate protected
Points specific to concrete
- Level reference — work from the finished tile line, marked on the walls, not from the slab
- Shrinkage — new screed shrinks; allow for it and check levels after curing
- Slab penetration — confirm the outlet route does not conflict with reinforcement or services
- Waterproofing bond — concrete dust must be removed and primer applied for the membrane to bond to the flange
- Support — the body must not be supported only by fresh screed; pack under it if the opening is deep
Related: waterproofing around a drain · slope requirements