Last Updated: 12 August 2026
Some shower door gaps only look a few millimetres out. Once the wrong seal is fitted, though, the difference becomes much more obvious.
One side of the door may sit almost against the shower tray while the other leaves a visible gap. Water can escape at the higher end, while the lower end presses the bottom seal so firmly that it drags, curls or gradually works loose.
We see this quite often in photos customers send us. One side of the glass sits high, the other low, and the first thought is usually to buy a longer shower door bottom seal.
That can solve one end of the door while making the other worse.
With an uneven gap, the widest point tells you how far the seal needs to reach. The narrowest point tells you how much room the seal has before it starts being compressed. Once you have both measurements, it becomes much easier to judge whether you need a standard shower bottom seal, a side-fin seal, a 5mm threshold bar or some light trimming.
Quick answer: start with the gap difference
For a small difference of around 3mm or less, a standard bottom seal may still work well, provided the fin reaches the wider end without being heavily compressed at the narrower end.
Once the difference becomes more noticeable, a side-extended fin seal can give you more tolerance because the sealing edge does not rely entirely on being pressed directly beneath the glass.
When the maximum gap gets beyond roughly 10mm, it is worth looking at more than fin length alone. A 5mm shower threshold bar can sometimes provide part of the coverage instead of asking a long flexible fin to do all the work.
For a very uneven door, particularly where the smallest and largest gaps differ by around 10mm or more, one fin length may struggle to suit both ends. Careful trimming can then become a practical option.
These figures are useful starting points rather than fixed limits. Fin softness, profile shape, glass thickness and how freely the door moves all affect the final fit.
Why is the gap under my shower door uneven?
A small slope underneath a shower door is fairly common.
The shower tray or tiles may have a slight fall towards the drain. In older bathrooms, the floor or wall may not be perfectly level. A door can also drop slightly after years of use if hinges or rollers have moved.
You do not usually need to diagnose the whole bathroom before choosing a replacement seal.
What matters more is what the door is doing now.
Remove the old seal, close the door normally and look along the bottom edge of the glass. You may find that one end is almost touching the tray while daylight is still visible underneath the other.
For choosing a seal, three measurements are particularly useful: the smallest gap, the largest gap and the glass thickness.
Measure the real gap after removing the old seal
Remove the old bottom seal before measuring. Close the shower door as you normally would and measure from the bottom edge of the glass to the tray, floor or existing threshold surface.
Do not measure from the bottom of the old fin.
Old fins are often bent, stretched or curled after years of use. A fin that appears to be 15mm long does not necessarily mean the original gap was 15mm.
For an uneven door, measure in at least three places:
- left side;
- middle;
- right side.
Suppose you measure 7mm on the left, 10mm in the middle and 15mm on the right.
Calling that “about 10mm” hides the part that matters. The replacement has to reach the 15mm end without being crushed against the tray at the 7mm end.
Glass thickness matters as well. Frameless shower doors can look almost identical while using different glass thicknesses, and the U-channel still needs to grip the glass properly.
Before ordering, check whether the glass is 4mm, 5mm, 6mm, 8mm or 10mm.
If you are not sure where to put the tape measure, our How to Measure Shower Screen Seal guide shows the measurement points in more detail.
Check the widest and narrowest points together
Once you have measured the left, middle and right points, focus on the widest and narrowest gaps.
The widest gap tells you how far the seal needs to reach. The narrowest gap tells you how much room is left before the fin starts being pressed too tightly against the tray.
With the door closed, look at the low-gap end. If the old fin is already curled, flattened or pushed sideways there, choosing a longer fin just because the opposite end is leaking may create more drag.
You want enough reach at the wider end without forcing the fin too hard at the narrower end.
A small slope: can a standard bottom seal still work?
Take a door measuring roughly 7mm at one end and 10mm at the other.
That is a relatively small difference, and a standard shower door bottom seal may still be all you need.
The useful check happens after the seal is fitted.
At the wider end, the fin needs to reach the tray or threshold. At the narrower end, it should bend lightly rather than being flattened underneath the glass.
Open the door slowly. If it moves normally and you do not feel the seal dragging heavily across the tray, the extra contact at the lower end may not be a problem.
A side-extended fin seal is another option where clearance beneath the glass is already tight. Because its sealing edge hangs slightly to the side, it can sometimes deal with a shallow slope without putting as much material directly underneath the door.
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
Now imagine the gap is around 8mm at one end and 13mm at the other.
A standard bottom seal can still reach the wider side, but the longer fin needed to do that may be pressed much harder at the 8mm end.
That is often when you start to notice the practical symptoms: the door feels heavier to open, the same section of fin folds every time, or the seal slowly shifts along the glass.
A side-extended fin can be more forgiving because it changes where the sealing edge sits.
Instead of asking one long fin directly underneath the glass to deal with both heights, part of the sealing edge sits beside the glass. That can reduce the amount of compression needed at the narrow end.
Around 5mm of difference is a useful point at which to start considering this type of profile, but it is not a strict cut-off. Door movement and the amount of space beneath the glass matter just as much as the measurement itself.
A large gap: when to add a 5mm threshold bar
When the widest part of the gap moves beyond roughly 10mm, simply choosing a longer and longer fin becomes less attractive.
The long end may reach the tray, but the same fin can create quite a lot of friction where the gap is smaller.
A threshold bar gives the seal another surface to meet.
A typical clear shower threshold bar is around 5mm high, so it can reduce how far the flexible fin has to reach on its own.
For example, if you are working with a 20mm fin and a 5mm threshold, the theoretical coverage is around 25mm.
That does not mean every 25mm gap can automatically be solved with that combination. The position of the threshold, the flexibility of the fin and the path of the door still need to work together.
The useful question is whether the fin meets the threshold without being heavily compressed at the opposite end.
If you are deciding whether a threshold is suitable, our guide to how to choose a shower threshold bar covers that part in more detail.
Case A: V456S20 + threshold for stronger coverage
Suppose you have 6mm glass, with a 16mm gap at the narrow end and 22mm at the widest point.
A V456S20 has a 20mm fin. Adding a 5mm threshold gives roughly 25mm of available coverage.
That gives the wider end enough reach while leaving some compression at the 16mm side.
The number alone is not the final test.
Once fitted, close the door and watch what the fin actually does. It should meet the threshold without being folded hard underneath the glass, and the door should still move normally.
For this sort of gap, the combination can be worth considering as long as the U-channel suits the 6mm glass and the threshold itself does not sit in the path of the door.
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
When the gap changes sharply from one end of the door to the other, one fin length may not sit comfortably across the full width.
For example, a longer fin may reach the wider end but fold or drag where the glass sits closer to the tray.
In that situation, light trimming can sometimes help the flexible fin follow the slope more closely.
The important part is making sure the replacement profile itself is correct first. If the seal fits the glass properly and the fin simply has more length than you need, our guide to what can be adjusted on a replacement shower door seal explains when trimming makes sense and when a different profile is the better option.
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?
Measure the smallest gap, largest gap and glass thickness first. The replacement needs enough reach for the widest point while still having enough clearance at the narrowest point. A small difference may work with a standard bottom seal, while a more noticeable slope may suit a side-fin seal, threshold bar or careful trimming.
2. Should I choose the seal based on the smallest gap or the largest gap?
Use both. The largest gap tells you how much reach you need. The smallest gap tells you whether that same fin will be compressed too heavily when the door closes.
Choosing only from the largest measurement can leave the seal dragging or folding at the opposite end.
3. When should I use a side-fin shower door seal?
A side-fin seal is worth considering when the gap difference is noticeable and a standard bottom fin would need to bend heavily at the narrow end.
Around 3mm of difference can be a useful reference point, but the way the door moves and the clearance beneath the glass should also be checked.
4. When should I add a 5mm shower threshold bar?
A 5mm threshold bar can help when the fin alone would need to cover a relatively large bottom gap.
It gives the flexible fin a raised surface to meet, which can reduce the amount of fin length needed. Check that the threshold does not interfere with the path of the glass or door.
5. Can I trim a shower door bottom seal to fit an uneven gap?
Yes, particularly when the gap changes enough that one fin length cannot work comfortably across the whole door.
Fit the seal first and look for the area that bends or drags. Mark it, remove the seal and trim the flexible fin gradually with sharp scissors. Test the door again before removing any more material.