Last updated: 17 June 2026
It is easy to assume a new shower door seal is fine once it clips onto the glass and the door closes. The problem often appears later, when the door starts to drag, squeak, or the seal works loose from one end.
We saw this recently with a customer whose shower door had an 11mm gap at the bottom. She had fitted an 18mm bottom seal bought from Amazon. A deeper seal sounded like the safer option, but in use it made the door drag against the shower tray every time it opened.
The seal was not simply falling off. It was being pulled away by the movement of the door.
A vertical shower door seal may appear to grip the glass when first fitted, but then slowly slips down after the door has been opened and closed a few times. It has not been washed off by water. It usually means the seal is not gripping the glass firmly enough.
Before replacing the seal again, check how it is failing. A bottom seal is often dragged off by the door. With a vertical seal, the better clue is whether the channel is holding the glass firmly enough.
Is the Seal Being Dragged Off or Sliding Down?
Start by looking at the way the seal moves when the door opens.
At the bottom of the door, the signs are usually practical: the door feels tighter than before, the bottom edge rubs on the tray, the fin bends or curls, or the seal starts working loose from one end. In that situation, the seal is probably too deep for the space under the glass.
A side seal behaves differently. The bottom of the door may feel normal, with no scraping at all. Instead, the vertical seal slowly slips down the glass. You push it back into place, it seems fine for a while, and then it moves again after a few door movements.
For a bottom seal, check the gap under the glass. For a side seal, check whether the channel is holding the glass firmly enough. That is why replacing like-for-like can repeat the same problem.
Bottom Seal: Deeper Does Not Mean Better
A bottom shower door seal should touch the shower tray lightly enough to help control water, but not so tightly that it stops the door moving freely.
If the real gap is 11mm and you fit an 18mm bottom seal, the extra depth has nowhere useful to go. The fin is forced to bend, press against the tray and drag every time the door moves. At first, this may only show up as a squeak or a slightly stiff door. Over time, the seal can curl, shift, or come away from the glass.
That is not better sealing. It is resistance caused by the wrong size.
When a bottom seal keeps coming off, do not start by pushing it on harder. Check whether the seal depth is too much for the actual gap.
If you want to understand how bottom fin depth should match different gaps, see our dedicated guide: Why Longer Fins Are Not Always Better for Shower Door Bottom Seals.
Vertical Seal: Fitting Over the Glass Is Not the Same as Holding It
A vertical shower door seal relies on the U-shaped channel gripping the glass edge. It may look tidy when fitted, but that does not always mean it is secure.
A common issue is a channel that is slightly too wide. For example, if the glass is 6mm and the seal is better suited to thicker glass, it may slide on easily but fail to hold. After a few door movements, or after being touched during cleaning, it can begin to move down the glass.
This is where many “universal” seals disappoint. They may slide on easily, but that does not mean they are the right fit.
If a vertical seal keeps slipping, check the glass thickness, U-channel size and seal profile. Fixed panels, hinged doors, closing edges and glass-to-glass gaps may require different profiles, even when the seals look similar at first glance.
Grip Force: A Good Seal Should Not Just Hang on the Glass
For a push-on vertical seal, grip is what keeps it in place.
When the glass thickness is correct, a stable push-on seal should usually resist more than 50N of pull force before it starts to move. In practical terms, that is roughly the force from a 5kg load.
Under test conditions, using compatible glass thickness and a dry glass edge, some Simba push-on seals resisted around 90N before they began to slide.
The figure is not a promise for every installation. It will vary by seal model, glass thickness and surface condition. It does, however, give a useful reference for what a secure push-on fit should feel like.
A side seal needs to hold the glass, not just look as though it fits. A vertical seal that moves with a light push is unlikely to be a good match for that glass.
Do Not Copy the Old Seal Blindly
The old seal is not always a reliable guide. It may have hardened, stretched, flattened or distorted over time. In some cases, the old seal was the wrong size from the beginning.
Before ordering a replacement, confirm at least two measurements.
Glass thickness matters for the U-channel; the bottom gap matters for seal depth.
For bottom seals, measure the gap with the door closed. Check the left side, centre and right side rather than relying on one point only. Older shower doors can drop slightly, and a shower tray may not be perfectly level.
If you are unsure how to measure, see our guide: How to Measure a Shower Screen Seal.
Glue Is Usually Not the Right Fix
When a shower seal keeps falling off, silicone or glue may seem like a quick answer. If the size is wrong, it only hides the problem for a while.
A bottom seal that is too deep will still drag against the tray.
A vertical seal with a loose channel still will not grip the glass properly.
A push-on seal should stay in place because it fits the glass properly, not because it has been glued into position. If it only stays on with adhesive, the seal is usually not the right fit.
When to Choose a New Bottom Seal
Check the bottom seal first if the seal comes off from the base of the glass and the door feels stiff, rubs against the tray, squeaks, curls the fin, or pulls the seal loose from one end.
The main question is not whether the seal can be pushed on more firmly. It is whether the seal depth matches the real gap under the glass.
Once you have measured the glass thickness and bottom gap, you can compare suitable options here: Shower Door Bottom Seals.
When to Choose a New Vertical Seal
Check the vertical seal if the seal slides down the side of the glass or gets pushed out when the door closes.
This usually comes down to glass thickness, U-channel size and seal profile. A seal can slide onto the glass and still be too loose for long-term use.
If you have confirmed that the side seal is the issue, see: Vertical Shower Door Seal Strips.
What to Check Before Replacing the Seal Again
If a shower door seal keeps falling off, look at the way it is failing before replacing it.
When it is being dragged off by the door, measure the real bottom gap.
When it is sliding down the glass, check the glass thickness and U-channel grip.
An 11mm gap fitted with an 18mm bottom seal may look more secure, but it can make the door drag and gradually pull the seal away. A vertical seal that slips down usually needs a better match to the glass thickness and a stronger grip.
If you are not sure which seal to order, send us a clear photo of the glass edge and the gap under the door before buying.
For more measuring, fitting and care advice, see our Fitting & Care guides.
About Simba
Simba was founded in 1998 and focuses on the manufacture, testing and replacement support of shower door glass seals. This article is based on customer feedback, product measurements and selected grip-force testing.