Last Updated: 26 June 2026
Some shower door gaps only look a few millimetres out. Once the wrong seal is fitted, though, the problem becomes much more obvious.
One side of the door may sit almost against the shower tray, while the other leaves a visible gap. The wider end may let water escape. The narrow end may press the bottom seal so tightly that it drags, curls, or slowly works loose.
We see this quite often in photos customers send to us: one side of the shower door sits high, the other sits low. The first thought is usually to buy a longer shower door bottom seal.
That is not always the safest fix.
An uneven gap is not just about whether the seal is “long enough”. The widest gap tells you what the seal has to reach. The narrowest gap tells you how much room is left once the door closes. Once you know those two numbers, it is much easier to decide whether you need a standard shower bottom seal, a side-fin seal, a 5mm threshold bar, or light trimming.
Quick answer: start with the gap difference
If the gap difference is under 3mm, start by looking at a standard bottom seal or a side-extended fin seal.
If the gap difference is over 3mm, a side-extended fin seal is usually the safer option because it does not rely entirely on the fin directly under the glass being compressed.
If the maximum gap is over 10mm, do not look at the seal fin length on its own. Include the height of a 5mm shower threshold bar in the total coverage.
If the smallest and largest gaps differ by more than 10mm, a full-length fin may struggle to suit both ends. In that case, careful trimming may be needed.
Why is the gap under my shower door uneven?
This is not unusual in UK bathrooms, especially in older properties, converted shower areas, or glass doors that have been in use for several years.
There are usually three common reasons.
The shower tray or floor tiles may have a slight fall towards the drain. As the surface slopes, the distance between the bottom of the glass and the tray can change across the width of the door.
The floor or wall may not be perfectly level. This is especially common in Victorian terraces and older houses, where not every surface is spirit-level perfect.
The door itself may also have dropped slightly. Loose hinges, worn rollers, or years of use can cause one side of the door to sit lower than the other.
For choosing a seal, the most useful thing is not finding out exactly which part of the bathroom is uneven. It is getting three measurements right: the smallest gap, the largest gap, and the glass thickness.
Measure the real gap after removing the old seal
Before choosing a new seal, remove the old bottom seal and close the shower door as you normally would. Measure from the bottom edge of the glass to the shower tray, floor, or existing threshold surface. Do not measure from the bottom of the old seal.
This is where many sizing mistakes start. An old fin may already be bent, stretched, or curled at one end. If you measure the old seal directly, it may look like 15mm, while the real gap from glass to tray is only 10mm or 12mm.
For an uneven gap, do not only measure the leaking end. Use a tape measure at three points:
left side of the door;
middle of the door;
right side of the door.
For example, you might get 7mm on the left, 10mm in the middle, and 15mm on the right. In that case, the useful answer is not “about 10mm”. You need the smallest gap, 7mm, and the largest gap, 15mm. Those two numbers will guide whether you choose a seal, a side-fin style, or a threshold bar.
Glass thickness matters too. Many frameless shower doors look similar, but 4mm and 8mm glass need different U-channels. The fin may be the right length, but if the channel does not grip the glass properly, the seal can still move. Before ordering, check whether your glass is 4mm, 5mm, 6mm, 8mm, or 10mm.
If you are not sure where to place the tape measure, read our How to Measure Shower Screen Seal guide first, then come back to compare your smallest gap, largest gap, and glass thickness.
Check the widest and narrowest points together
Once you have measured the left, middle, and right points, do not jump straight to a product. Circle two numbers first: the widest gap and the narrowest gap.
The widest gap shows whether the seal can reach the tray or threshold. The narrowest gap shows whether the door still has enough room to close without forcing the seal too tightly.
Look at the low-gap end with the door closed. If the glass is already very close to the tray, or the old seal is curled, whitened, or pushed to one side there, do not choose a long fin only because the other end is leaking.
The aim is simple: the wider end should be covered, while the narrow end only bends slightly. The door should not scrape along the tray every time it opens.
If the old seal has slipped, curled, or been pulled loose at one end, it is also worth checking the common reasons why shower door seals fall off. Sometimes the issue is not just the seal length, but the wrong glass thickness, a loose U-channel fit, or long-term pressure at the narrow end.
A small slope: can a standard bottom seal still work?
If your measurements are something like 7mm on one side and 10mm on the other, this is usually a slight uneven gap. The door is not perfectly level, but it may not need a complicated fix.
With this type of door, picture how the seal will sit once fitted. The wider end needs to meet the tray or threshold. The narrower end should only bend lightly, not be crushed out of shape. If the door opens and closes without obvious scraping, a standard shower door bottom seal will usually be enough.
If you want the door to move a little more smoothly, or the low-gap end is already close to the tray, a side-extended fin seal can be a better option. Its sealing edge hangs down from the side rather than sitting directly under the glass, so it gives a little more tolerance on a shallow slope.
Standard Bottom Seal: suitable when the gap difference is small and the door moves smoothly.
Side-Extended Fin Seal: useful when you want to reduce bottom friction, or when you do not want the seal to sit directly under the glass.
A noticeable slope: when to use a side-fin seal
If the gap difference is over 3mm, for example 8mm on one side and 13mm on the other, a standard bottom seal starts to become less predictable.
To reach the 13mm end, it needs a longer fin. At the 8mm end, that same fin may be pressed too tightly. The door may still close, but you might notice it dragging slightly on the tray, or one part of the seal folding in the same place.
This is where a side-extended fin seal is often a better fit.
Its advantage is not simply that it is “longer”. It seals in a different position. A standard bottom seal does most of its work under the glass. A side-fin seal gives you another sealing edge, hanging slightly to the side like a small skirt, so water moving outwards is blocked before everything depends on the fin being compressed underneath the glass.
When the floor has a slight slope, the tray sits higher on one side, or the bottom edge of the glass is not quite parallel with the tray, a side-fin seal is usually more forgiving than forcing a longer standard bottom seal into the gap.
This type of fix suits doors where the gap difference is noticeable, but the maximum gap is still around 10mm or less. If the maximum gap is over 10mm, do not rely on the side-fin seal alone. The next step is to include a 5mm threshold bar in the calculation.
A large gap: when to add a 5mm threshold bar
If the maximum gap is over 10mm, using a longer seal fin on its own can become less stable. The longer the fin, the more friction it may create at the low-gap end. If water is running along the tray surface, a soft fin alone may also struggle to form a clean stopping line.
At this point, it helps to stop asking only, “How long should the seal be?” and start asking, “Is the total coverage enough?”
Total coverage = seal fin length + threshold bar height.
For example, a clear shower threshold bar is usually 5mm high. Once fitted, it raises the contact point by 5mm, so the seal does not have to cover the whole gap on its own. For larger bottom gaps such as 12mm, 16mm, or 20mm, this is often easier to control than simply choosing a longer fin.
If you are not sure which threshold bar is suitable, read our guide on how to choose a shower threshold bar before deciding whether to use one with your bottom seal.
A threshold bar does not replace the seal. It gives the seal a clearer edge to meet. As long as the door can still open and close normally, and the bar does not sit in the path of the glass, this combination is usually more stable than relying on a long fin alone.
Case A: V456S20 + threshold for stronger coverage
Suppose your largest gap is 22mm, your smallest gap is 16mm, and the glass is 6mm thick. If you use a bottom seal alone, the fin needs to be long enough to reach the 22mm end, but it also needs to avoid being over-compressed at the 16mm end.
A more reliable way to look at it is to include the threshold bar first.
If you use V456S20 with a 20mm fin and add a 5mm shower threshold bar, the total coverage is:
20mm fin + 5mm threshold bar = 25mm
That 25mm coverage is enough for the 22mm maximum gap. At the 16mm end, the seal will compress slightly, but it should not be flattened completely. For a door with this kind of 6mm difference, this combination is easier to control than using a long fin on its own.
In this case, V456S20 + a 5mm shower threshold bar is worth considering. Before ordering, still check two things: that the glass is 6mm thick, and that the threshold bar will not block the door’s movement when the door is closed.
Case B: V456S16 + threshold for less friction
Now use the same measurements: 6mm glass, with a 16mm gap on one side and a 22mm gap on the other.
The previous option used a 20mm fin with a 5mm threshold bar, giving 25mm total coverage. That covers the widest gap well. But if you are more concerned about friction at the low-gap end, you may prefer a shorter fin, such as V456S16.
The total coverage would be:
16mm fin + 5mm threshold bar = 21mm
On paper, that is 1mm short of the 22mm maximum gap. A small difference like this can sometimes be handled during fitting.
At the wider-gap end, do not push the U-channel fully onto the glass as far as it will go. Leave around 1–3mm of space, so the fin sits slightly lower. This can help it meet the threshold while reducing friction at the low-gap end when the door opens and closes.
This should only be used for small 1–3mm adjustments. The seal must still match 6mm glass, and the U-channel must still have enough depth to grip the glass securely. Do not fit the whole seal too shallow just to make up a larger missing measurement. That can make the seal loose or cause it to move out of position.
A very uneven gap: when trimming may be needed
Trimming is not usually the first option. It is only worth considering when the widest and narrowest gaps are so different that a full-length fin will be wrong at one end, whichever size you choose.
For example, if you have 6mm glass, with a 9mm gap at one end and a 20mm gap at the other, that is no longer a small slope.
In that situation, simply choosing a longer fin can cause problems. A 20mm fin can reach the wider end, but at the 9mm end it may be bent too far, or scrape the tray every time the door opens.
You can then consider lightly trimming the fin so it follows the slope more closely. Keep the process simple:
fit the seal onto the glass and close the door to see where it is being pressed most;
make a light mark on the fin;
remove the seal and trim gradually with sharp scissors;
refit it and check that the door still opens and closes smoothly.
Do not cut too much at the start. It is better to trim a little twice than to remove too much and lose the water seal.
If you are not sure how to cut it, watch the trim shower door bottom seal video guide below before working on your own seal.
Still unsure? Send a photo with a tape measure
If your gap does not match the examples above, you can also read our shower seal selection guides to compare different door types, glass thicknesses, and bottom gap sizes.
You can also send SIMBA UK a photo of the bottom of your shower door with a tape measure in place. Try to show the bottom edge of the glass, the tray or threshold, and the positions of the smallest and largest gaps. This helps us check whether you need a bottom seal, a side-fin seal, a threshold bar, or light trimming.
FAQ: Uneven gap under shower door
1. How do I choose a shower door seal for an uneven bottom gap?
For an uneven bottom gap, measure the smallest gap, the largest gap, and the glass thickness first. Choose a seal that can reach the largest gap, then check that the narrowest end will not be over-compressed. A small gap difference may work with a standard bottom seal, while a larger difference may need a side-fin seal, a 5mm threshold bar, or careful trimming.
2. Should I choose the seal based on the smallest gap or the largest gap?
Use both measurements. The largest gap tells you how much coverage the seal needs. The smallest gap tells you whether the seal will be too tight when the door closes. Choosing only by the largest gap can make the seal drag, fold, or come loose at the narrow end.
3. When should I use a side-fin shower door seal?
A side-fin shower door seal is usually useful when the gap difference is noticeable, but the maximum gap is still around 10mm or less. The side fin hangs down beside the glass, so it can block spray without relying only on the fin directly under the glass being compressed against the tray.
4. When should I add a 5mm shower threshold bar?
Add a 5mm shower threshold bar when the bottom gap is too large for the seal fin alone, especially when the maximum gap is over 10mm. The total coverage should be calculated as seal fin length plus threshold bar height. The threshold bar also gives the seal a clearer contact line, as long as it does not block the door from opening or closing.
5. Can I trim a shower door bottom seal to fit an uneven gap?
Yes, but trimming should usually be a last option for very uneven gaps. If the smallest and largest gap differ by around 10mm or more, a full-length fin may be too long at one end and too short at the other. Fit the seal first, mark the area that bends or drags, then remove it and trim the fin gradually with sharp scissors.