Last Updated: 20 April 2026
In the UK, whether you live in a Victorian terrace or a modern flat, an uneven gap under a shower door is a common problem. One side may sit close to the tray, while the other side leaves enough space for water to escape.
One side might be as little as 7mm, while the other side could be 15mm or more. A standard bottom seal is the same height across the whole door, which can leave one end leaking and the other end dragging or jamming.
The good news is that most uneven gaps can be fixed with the right combination of shower door seal, threshold bar, and, in some cases, careful trimming.
Quick answer: how to choose the right fix
- For a slight slope where the gap difference is under 3mm, a standard bottom seal or side-extended fin seal may be enough.
- For a noticeable slope where the gap difference is over 3mm, a side-extended fin seal is usually more reliable.
- For gaps over 10mm, combine the seal fin length with a 5mm threshold bar to increase total coverage.
- For extreme uneven gaps, trim the seal carefully so it follows the slope without being crushed at the narrow end.
Why is my shower door gap uneven?
In most cases, an uneven shower door gap is caused by one of three things:
- Floor Gradient: Most UK shower trays or tiles slope toward the drain to stop water pooling.
- Building Settlement: In older properties, floors and walls are rarely "spirit-level" perfect.
- Hinge Sag: Over time, gravity can cause the door to drop slightly on one side.
Before choosing a seal, measure both the smallest and largest gap under the door, as well as the glass thickness.
A simple rule: always choose your seal based on the largest gap, then check that the narrowest end will not be over-compressed. If the gap is large or uneven, adding a 5mm threshold bar can give the seal a flatter surface to meet.
Once you have your measurements, you can match the gap to the right type of fix.
| Gap condition | Recommended fix | Why it works |
|---|---|---|
| Gap difference <3mm | Standard bottom seal or side-extended fin seal | The seal can flex enough to cover a minor slope. |
| Gap difference >3mm | Side-extended fin seal | The side skirt blocks spray without being crushed directly under the glass. |
| Gaps >10mm | Seal fin + 5mm threshold bar | The threshold bar raises the floor level and increases total coverage. |
| 10mm+ difference between smallest and largest gap | Precision trimming | The seal follows the sloping floor without tearing from over-compression. |
Solution 1: A Slight Slope (Gap difference <3mm)
The Scenario: One side is 7mm, the other is 10mm.
The Fix: For a small difference in height, a standard bottom seal may be enough. If the gap changes across the width of the door, a side-extended fin seal can give a little more coverage without being crushed directly under the glass.
How it works: If the slope is minor, high-quality shower door bottom seal is your best friend. It’s flexible enough to bend at the narrow end and still reach the tray at the wider end.
Both a standard bottom seal and a side-extended fin seal are good options for slight uneven gaps. For the smoothest opening and closing, we recommend the Side-Extended Fin Seal. Because the fin hangs from the side rather than being crushed directly under the glass, it is less likely to jam, tear, or wear out quickly.
Note: For a proper watertight finish, use these with a shower threshold bar.
Standard Bottom Seal
Side-Extended Fin Seal
Solution 2: Noticeable Slopes (Gap Difference >3mm)
If the gap difference is more noticeable, a standard bottom seal may start to struggle. It may be compressed too much at the narrow end while still not reaching properly at the wider end.
The Fix: Side-Extended Fin Seals.
This is the most professional way to handle sloping gaps. The seal has an extra "curtain" that hangs down like a skirt. As the gap changes, the side skirt continues to block the spray effortlessly. Best for gaps under 10mm.
For customers dealing with uneven gaps of around 10mm, our side-fin shower door seals offer a more reliable option than forcing a standard bottom seal to over-compress. For better water control, especially on uneven floors, pair it with a shower threshold bar.
Solution 3: The Ultimate Fix (Your Gaps >10mm)
If your gap is too big for a side-extended seal, you need to “bridge the gap” from the floor up.
Instead of thinking only about the seal fin length, think about the total coverage: the seal fin plus the height of the threshold bar.
The Strategy: Raise the Floor with a Threshold Bar
Our transparent threshold bars are 5mm high. By fitting one, you effectively raise the floor level, cancelling out part of the gap and giving the seal a level surface to meet.
Case Study
Option 1
The Data: 6mm glass. Gap is 16mm on one side and 22mm on the other (6mm difference).
The Math: Seal Fin + Threshold (5mm) should be ≥ Max Gap (22mm).
Selection: Use V456S20 (20mm fin) + Threshold (5mm) = 25mm Total Coverage.
Result: 25mm easily covers the 22mm gap. The narrow end (16mm) will compress by 4mm, which is well within the seal's comfort zone.
Option 2: Using a Shorter Fin Seal
If you feel that a 20mm fin seal may be too long, you can try a 16mm fin seal instead, such as the V456S16. When paired with a 5mm threshold bar, this gives you a total coverage of 21mm.
At first, this may seem 1mm short for a maximum gap of 22mm. However, that small difference can usually be covered during fitting.
When installing the seal, do not push the U-channel fully onto the glass at the wider-gap end. Instead, leave a small 1–3mm space at the top of the U-channel. This allows the fin to sit slightly lower, helping it reach the tray or threshold bar more comfortably while also reducing unnecessary friction when the door opens and closes.
Please note: this adjustment should only be used within a small range. For our bottom shower seals, the U-channel depth is over 10mm, so allowing a 1–3mm fitting adjustment will not normally affect the grip or cause the seal to come loose, as long as the seal is correctly matched to the glass thickness.
Solution 4: Extreme Slopes (10mm+ Difference)
If the difference between the smallest and largest gap is very large, simply choosing a longer fin is not always the best answer.
The Scenario: 6mm glass. Max gap is 20mm, but the min gap is only 9mm. That's a massive 11mm difference!
The Problem: A 20mm fin will fit the big end, but it will be crushed flat at the 9mm end, causing it to tear. As we show in our video guide, over-compression kills seals.
If the uneven gap is already letting water onto the floor, check whether it is also a shower door leaking at the bottom issue.
The Professional Fix: Precision Trimming
For a bespoke fit, follow these DIY steps:
- Initial Fit: Push the seal onto the glass.
- Mark It: Use a pen to mark where the fin is bending or crushing too much.
- Safety Buffer: Don't worry about being 100% perfect. The 5mm threshold bar gives you a buffer—as long as your cut is within that 5mm range, it won't leak.
- Trim: Remove the seal and If you need to trim the fin, use sharp scissors and follow a clean cutting line.
The Result: A seal that perfectly contours to your sloping floor. Job done.
Alternatively, you can watch this video to see how to trim the bottom seal to fit uneven gaps.
Watch the video guideStill feeling a bit confused?
For other gap types and seal choices, see our shower seal selection guides.
Don’t worry! Contact the SIMBA UK team. Send us a photo of your door with a tape measure held against the gap, and we’ll find your perfect match.
FAQ: Uneven gap under shower door
What should I measure before buying a shower door seal?
Measure both the smallest and largest gap under the door, as well as the glass thickness.
Should I choose the seal based on the smallest or largest gap?
A simple rule: always choose your seal based on the largest gap, then check that the narrowest end will not be over-compressed.
When should I use a shower threshold bar?
If the gap is large or uneven, adding a 5mm threshold bar can give the seal a flatter surface to meet.
What can I do if one end of the seal is being crushed?
For extreme uneven gaps, remove the seal and trim the fin carefully so it follows the sloping floor without tearing from over-compression.