Most linear drain specification errors happen before the product is chosen — in the sequence of decisions. Get the order right and the choice becomes straightforward; get it wrong and you end up reducing the floor slope to make the drain fit, which is where the problems start.
Step 1: Measure the available floor build-up first
Before considering length, finish or brand, measure from the structural slab to the finished tile surface at the drain position. This is a hard constraint: if the build-up is 45 mm, a 90 mm body does not fit regardless of anything else.
| Build-up available | What fits |
|---|---|
| Under 40 mm | Ultra-thin body (~28 mm), side outlet |
| 40–60 mm | Low-profile body (~50 mm) |
| 60–90 mm | Standard body |
| 90 mm+ | Full range including high-capacity bodies |
Measure at the drain, not at the door. Screed depth varies across a room, often by 10 mm or more, and the drain sits in the deepest part of the build-up, not the shallowest.
Step 2: Confirm the outlet route before anything else
The outlet — horizontal, vertical or side — determines whether the pipe can physically run from the drain to the stack. On timber floors this means checking joist direction and depth. On concrete it means checking reinforcement and existing services.
This step is skipped more often than any other, and it is the most expensive to get wrong. Discovering after the floor is opened that the pipe route is blocked turns a one-visit job into three.
Step 3: Size the drain against the shower, not the room
Two numbers matter:
- Length — match the channel to the shower opening width so the floor can fall in a single plane at a manageable slope. A drain far shorter than the opening forces water to travel further and the slope to increase.
- Flow rate — size above the total output of the shower, not equal to it. Add up every outlet: rain head, hand shower, body jets.
In level-access showers, allow extra margin. With no threshold, water spreads across the whole floor and the drain has to keep pace with the shower rather than a tray containing it.
Step 4: Match the grate to the floor finish
| Floor finish | Grate choice | Why |
|---|---|---|
| Large-format tile | Tile-insert or slim steel grate | Keeps the pattern continuous |
| Natural stone | Tile-insert cut from the same stone | A metal grate breaks the pattern and never matches |
| Small ceramic tile | Steel grate | Cutting many small tiles into an insert tray is impractical |
| Vinyl / LVT | Steel grate with sealing flange | Sheet material cannot be cut into a tray |
Note the flow trade-off: a tile-insert grate lets water in around the edges of the insert rather than through a wide grille, so its effective flow rate is lower than an open grate of the same nominal size.
Step 5: Specify the material grade for the environment
- 304 stainless — standard indoor bathrooms away from the coast
- 316 stainless — coastal locations, swimming pools, chlorinated water, aggressive cleaning regimes, marine and offshore
The two look identical. Grade is confirmed by material certificate, not appearance. If the project is within a few hundred metres of open sea, 316 is a specification decision rather than an upgrade.
Step 6: Check the details that decide long-term performance
These matter more over the life of the building than appearance or brand:
- Waterproofing flange — can the membrane be bonded directly over it? This is the single biggest determinant of whether the installation leaks.
- Body rigidity — a body that flexes under load cracks tile and breaks the waterproofing joint.
- Frame adjustment range — must accept the specified tile or stone thickness with room for on-site variance.
- Trap access — can it be cleared without tools, from above?
- Spares availability — will a matching replacement grate be available in three years?
Step 7: Decide wall position or centre
Wall position is the right answer in the large majority of cases: single-plane fall, shallower slope, drain out of the walking area, and a simpler waterproofing detail. Centre placement is justified where the shower is open on two sides, where the structure dictates the outlet position, or where a very large shower needs more than one collection point.
The order matters
Work through the steps in sequence: build-up, outlet, size, grate, material, details, position. Each step constrains the next, and the early steps — build-up and outlet — cannot be worked around later.
Most drainage failures are not product failures. They are specification and installation failures that began with a drain chosen before the constraints were understood.
Send us your floor build-up, outlet route and shower dimensions and we will confirm a specification and quote it. Request a Quote