Fitting a shower tray upstairs is one of those jobs that looks simple on the surface — until you lift the floorboards and discover a maze of joists, pipes, and structural constraints. Whether you’re a DIY renovator or a professional installer, the success of an upstairs shower installation almost always comes down to one thing:
The waste location.
Get the waste position right, and the job flows smoothly. Get it wrong, and you’re into joist notching, tray repositioning, awkward pipe runs, or worst of all — a tray that simply can’t be installed where you planned.
This guide explains exactly how to fit a shower tray upstairs, what to check before you start, and why the waste location is the single most important factor in the whole installation.
🛠️ 1. Start With the Basics: Choosing the Right Shower Tray
Before you even think about lifting floorboards, make sure you’ve selected a tray that suits the space:
Stone resin trays (the most common choice)
Strong, rigid, and ideal for upstairs installations where movement must be minimised.
Low‑profile trays
Great for modern bathrooms, but require careful planning of waste depth and fall.
Quadrant, offset, rectangular, square
The shape determines where the waste can be positioned — some trays have fixed waste points, others offer more flexibility.
Tip: If your waste position is restricted by joists, a tray with a central waste is often easier to work with than one with a corner waste.
🪚 2. Lift the Floorboards and Identify the Joist Layout
Upstairs bathrooms are almost always built on timber joists, and these dictate everything about your waste route.
You need to identify:
- Joist direction
- Joist spacing
- Joist depth
- Any existing notches or holes
- Any structural beams or doubled‑up joists
- Existing pipework (hot, cold, heating, soil)
This is where most installations go wrong. Installers often position the tray first, only to discover the waste outlet sits directly above a joist.
You cannot cut through a joist to make a waste fit.
Notching rules are strict, and for good reason — joists carry the structural load of the floor.
🚫 3. Why Waste Location Is Critical Upstairs
Unlike ground‑floor installations where you can run the waste under the floor or into a screed, upstairs trays rely on the space between joists.
Your waste trap needs:
- Enough vertical depth
- Enough horizontal run
- A clear path to the soil stack
- A fall of around 18–25mm per metre
- Space for the trap body and compression fittings
If the waste outlet lands directly on a joist, you have three options:
Move the tray
Often the simplest solution, but not always possible in tight bathrooms.
Use a riser kit
Lifts the tray above floor level, allowing the waste to run over the joists instead of through them.
Choose a tray with a different waste position
Sometimes the only structurally safe option.
What you cannot do is cut a joist to make the waste fit.
Building regulations are very clear:
- Notches must be in the top third of the joist
- Holes must be in the middle third
- You cannot notch near supports
- You cannot notch deep enough to weaken the joist
A shower waste trap is far too deep to sit in a legal notch.
🧰 4. Fitting the Tray: Step‑by‑Step
Step 1 — Dry‑fit the tray
Position the tray exactly where you want it and mark the waste location on the floor.
Step 2 — Check the waste alignment
Lift the floorboards and confirm whether the waste falls between joists.
If it doesn’t, stop here and reassess — this is the decision point that determines the whole installation.
Step 3 — Install the waste trap
Choose a slimline trap if space is tight.
Ensure the trap is accessible for future maintenance.
Step 4 — Create the fall
Run the waste pipe with a consistent fall towards the soil stack.
Avoid excessive bends — every bend increases the risk of blockages.
Step 5 — Secure the tray
Use a full bed of flexible adhesive or mortar (depending on tray type).
Stone resin trays must be fully supported to prevent movement.
Step 6 — Seal the tray
Allow the tray to settle for 24 hours before applying silicone.
Movement during curing is the number one cause of failed seals.
🧱 5. When You Should Use a Riser Kit Upstairs
A riser kit is often the best solution when:
- The waste outlet clashes with a joist
- The joists are unusually deep
- The waste needs to run across multiple joists
- You want easier access to the trap
- You’re installing a low‑profile tray but don’t have the depth for the trap
Riser kits lift the tray by around 80–120mm, giving you a clear void for pipework without compromising the structure of the floor.
🧩 6. Planning Ahead: How to Avoid Waste Problems
The smartest installers always:
- Check joists before buying the tray
- Choose trays with waste positions that suit the joist layout
- Avoid corner‑waste trays in tight upstairs bathrooms
- Use slim traps where depth is limited
- Keep waste runs short and direct
- Avoid unnecessary bends
- Ensure the fall is consistent and compliant
A little planning saves hours of rework.
🏁 Final Thoughts
Fitting a shower tray upstairs isn’t difficult — but it is technical.
The tray itself is rarely the problem.
The waste location almost always is.
By understanding your joist layout, planning the waste route early, and choosing the right tray and trap combination, you can achieve a solid, leak‑free installation that lasts for years.
If you need help choosing the right tray for an upstairs bathroom — especially with tricky waste positions — ShowerTrayStore.co.uk can recommend the best options for your layout.