Substrate Preparation and Rough-In
What this domain tests
Rough-in is where most receptor failures are born. Candidates must understand drain positioning (level, centered, correctly seated in the subfloor), subfloor flatness and deflection, framing and blocking for curbs and seats, and the sequencing of the preslope relative to the drain flange.
Key concepts
- The drain flange must be set so the membrane clamps over it and the weep holes remain open.
- Weep holes (or weep channels) let water that reaches the membrane escape to the drain. They must not be blocked by mortar or sealant.
- The preslope is a mortar bed sloped to the drain, installed before the membrane, so water on the membrane flows out rather than ponding.
- Framing must be stable and square; blocking is needed at curb and seat locations and at membrane terminations.
Traps
- A drain set too high or too low relative to the finished tile - questions often describe a drain that cannot accept the required thickness.
- Blocked weep holes caused by mortar squeezed under the flange during preslope placement.
- Subfloor deflection or out-of-level framing that makes a uniform slope impossible.
- Skipping the preslope because "the membrane is sloped by the top bed" - the top bed does not protect the membrane.
Rule of thumb
Water must always have a downhill path to the drain at every layer. If any layer slopes away from or is flat to the drain, the assembly is wrong.
Worked example
A job shows ponding on the membrane after a flood test. The preslope was omitted and the membrane laid flat on the subfloor. Correct diagnosis: the membrane has no slope to the drain, so water sits on it and migrates laterally. Remedy is a properly sloped preslope, not more thinset on top.