Last updated: 7 August 2026
When water starts escaping under a shower door, fitting a seal with a longer fin can seem like the obvious fix. If the old fin does not quite cover the gap beneath the glass, a few extra millimetres should give you more protection.
That can help, but only up to a point.
The flexible fin needs enough reach to cover the gap without being pressed down too hard. Too much extra length leaves it bent against the shower tray, floor or threshold. A firmer fin can also make the door noticeably harder to move.
Fin length matters, but so does the way the seal sits once it is fitted. You want enough coverage without putting unnecessary pressure on the flexible part or making the door awkward to use.
The fin is the flexible part of a shower door bottom seal that sits below the glass and helps keep water inside the shower area.
Why Is a Longer Fin Not Always Better?
A bottom fin only needs to cover the space where water could pass beneath the glass. It does not need to fill the whole gap tightly.
With the right length, there is a little extra reach and the flexible edge can sit naturally against the surface below.
Add too much length and that extra material has nowhere useful to go. Instead, it stays compressed against the shower tray, floor or threshold.
A small amount of bending is normal. Constant heavy compression is different. Over time, it puts more strain on the flexible part and can lead to loss of shape and wear. If that wear becomes severe, the fin may eventually crack or split.
A little extra length can be useful. Problems tend to start when there is more fin than the gap actually needs.
How Much Longer Should the Fin Be Than the Bottom Gap?
We would not normally choose a fin that measures exactly the same as the gap under the glass.
A 12 mm fin on a 12 mm gap, for example, may only just reach the surface below. There is very little spare coverage.
From our experience helping customers match bottom seals, allowing up to around 5 mm beyond the measured bottom gap usually gives a sensible amount of extra reach.
For a 12 mm gap, that means looking at suitable profiles with fins up to around 17 mm.
Treat the 5 mm as a guide, not a formula.
Fin shape, thickness and flexibility vary from one seal to another. A softer fin may sit comfortably with a few extra millimetres, while a firmer one can feel tight with less.
Once fitted, it should cover the area beneath the glass without being heavily flattened against the surface below.
Why Is My Shower Door Hard to Open After Fitting a New Bottom Seal?
A door that suddenly feels heavier after a new bottom seal has been fitted usually gives you an obvious clue: there is too much resistance at the bottom.
Extra fin length can cause this. When the flexible edge is pressed firmly against the tray, floor or threshold, it creates more friction as the door moves. You may also hear a rubbing or scraping noise that was not there before.
Length is not always the cause, though.
The fin itself may simply be too stiff.
A soft fin can flex as the door moves across the surface below. A firmer one behaves more like a scraper, staying pressed against the tray and needing more force to move past it.
You may notice this more with some double-fin seals. Where both flexible edges are pressing firmly against the tray, a stiffer material can create more resistance.
The door should not need a noticeable extra push every time you open or close it. If it does, the contact is probably too heavy.
How Can Too Much Friction Make a Bottom Seal Come Loose?
Most push-on bottom seals are held on the glass by the channel at the top.
The flexible part can touch the surface below, but the main body of the seal should stay in place.
When the fin is pressed down too firmly, friction at the bottom pulls on the seal each time the door moves. You may not notice anything at first.
After repeated use, one end can begin to sit lower than it did when fitted, or the whole seal may slowly shift along the glass.
Keep pulling on it in the same way and the seal may become easier to move, eventually slipping off altogether.
This is why a seal can feel secure when first installed but need pushing back into place a few weeks later. Bottom friction can be part of the problem, while glass thickness, channel size and other fitting issues can also cause a shower door seal to keep slipping or coming off.
Why Can a Longer Bottom Fin Still Leave You With a Leak?
If you have already fitted a longer bottom seal and there is still water on the bathroom floor, going longer again may not help.
First, check where the water actually appears.
Water found beside the bottom of the door does not always start there. It can escape at the side or through another joint, run down the glass and only then reach the floor.
Dry the door and shower tray before testing, then watch for the first sign of water rather than the place where it eventually collects.
If the first leak appears somewhere else, adding more fin length will not address it. The reason water appears around the bottom of a shower door may be elsewhere in the enclosure.
When Does a Slightly Longer Fin Make Sense?
If two suitable shower door bottom seals both cover the measured gap, the slightly longer option can give you a little more coverage as long as the door still moves freely.
That can be useful when you would rather have some spare reach than have the fin only just touch the area below the glass.
There is no need to use every available millimetre. A 12 mm gap does not automatically make a 17 mm fin a better choice than a 14 mm or 15 mm one.
What matters is how it sits after fitting. A little extra length is helpful when the fin covers the gap naturally and the door still opens and closes as it should.
Once the fin is heavily compressed or the door starts to feel noticeably harder to move, the extra length is no longer doing you any favours.
If the replacement profile is otherwise correct and the flexible fin is simply longer than you need, you can also check what can be adjusted on a replacement shower door seal before choosing another size.
If you already know your gap but are unsure which profile suits the door, choosing the right shower door bottom seal also means checking the glass thickness, fin style and how the seal will sit once fitted.
A suitable fin covers the gap without being so long or so firm that it puts unnecessary strain on the seal or makes the door difficult to use.
FAQ
Are longer shower door bottom fins better at stopping water?
Not always. Extra length can give useful coverage, but too much leaves the fin heavily compressed and under more strain.
How much longer should the fin be than the bottom gap?
From our usual fitting experience, up to around 5 mm beyond the measured gap is a useful guide. It is not a fixed rule, as fin shape and flexibility also affect how the seal sits.
Why is my shower door harder to open after fitting a new bottom seal?
The fin may be too long or too stiff. Resistance can also be more noticeable on some double-fin seals when both flexible edges are pressed firmly against the shower tray.
Should a shower door bottom fin touch the shower tray?
Some contact is normal. It should not be heavily flattened or make the door noticeably harder to open. If the resistance increases after fitting, check both the fin length and its flexibility.
About the Author: Laura
Laura joined SIMBA in 2017 and works closely with shower door seals, product fitting questions and customer enquiries. She uses that day-to-day experience to create practical guidance for choosing and fitting replacement seals.