When replacing an old shower door seal, the easiest option is to find a replacement that fits without any trimming.
That is not always possible. Shower seals come in a wide range of profiles and sizes, and the old seal in your bathroom may already have been trimmed when it was first fitted. Years of use can also leave the fin worn, bent or slightly distorted.
A few millimetres of difference do not necessarily mean you have found the wrong replacement.
Flexible fins often leave some room for adjustment. Small differences in rigid PVC can sometimes be dealt with as well, provided the basic profile is right.
The first job is to work out whether the seal itself is suitable. From there, you can decide which dimensions need to match and which can be adjusted to suit the actual fitting.
Check the Structure and Sealing Range First
Start by comparing the basic structure of the old and new seals. The sealing section also needs enough reach to cover the gap it is there to protect.
These are the same basic checks involved in choosing the right shower door seal: the fitting method, glass thickness and gap all need to work together.
A U-channel bottom seal, for example, needs to suit the thickness of the glass. Its flexible fin must also be long enough to cover the space between the bottom of the glass and the shower tray, bath rim or threshold.
With a T-shaped seal, check that the main profile and sealing position match the original arrangement.
Once those points line up, a slightly longer flexible fin is usually easier to adjust than one that is already too short.
Once a fin is too short to cover the gap, trimming cannot solve that problem. A different fixing structure is a different problem altogether.
A Slightly Longer Flexible Fin Is Usually Easier to Adjust
The flexible fin is there to cover or contact the gap that needs sealing.
So after finding the right profile, the new fin does not always need to be exactly the same length as the old one. What matters more is whether it has enough reach to do the same job.
Side seals tend to allow a little more tolerance.
On an H-type seal, a fin that extends slightly beyond the side gap can often be left alone. Provided it reaches the sealing area, clears the hinges and does not interfere with the door, the extra length may only affect how it looks.
Bottom seals need more care.
Their fins usually sit directly against, or very close to, the tray, bath rim or threshold. Too much extra length can leave the material heavily compressed and may create noticeable drag as the door moves.
That is why a longer fin is not always better on a shower door bottom seal.
The fin needs enough reach to cover the gap. Any extra length should then be judged by how the door actually moves.
Use the Actual Gap to Decide How Much Fin to Keep
An old seal that worked properly and did not leak can be a useful reference. In that case, copying the old fin length is perfectly reasonable.
A worn, badly bent or previously modified seal is less reliable as a template. Measuring the actual gap gives you a better starting point.
For a bottom seal, measure from the lower edge of the glass to the tray, bath rim or threshold.
The flexible fin can be slightly longer than this gap. As a practical starting point, around 5mm of extra length is often enough to leave some adjustment without creating unnecessary contact.
For example, with a 12mm bottom gap, you might start with a fin of up to roughly 17mm and then check how it behaves on the door.
The 5mm figure is only a guide. Fin softness, profile shape and the space beneath the glass all affect how much contact you actually get.
Side H-type seals do not need to follow the same rule.
For a side gap, around 3mm of overlap can be a useful starting point, provided the fin still clears the hinges and lets the door move freely.
A T-shaped seal with a flexible bottom fin can be judged in much the same way as another bottom seal. Look at the actual bottom gap rather than relying only on the length left on the old seal.
How to Trim a Longer Flexible Fin
Flexible fins are normally easy to cut.
A small pair of scissors or a sharp utility knife is usually enough.
Mark the length you want to keep, then remove a small amount first rather than cutting straight to the shortest possible size.
Fit the seal back onto the door and move the door slowly.
With a bottom seal, look for two things: whether the fin is being heavily folded and whether there is obvious drag as the door opens and closes.
Still too long? Take off a little more and test it again.
Once the door moves normally and the fin still covers the gap, there is no reason to keep trimming.
The same method works well for shower doors with an uneven bottom gap.
One end of the glass may sit closer to the tray than the other. In that case, the flexible fin can be trimmed to follow the changing gap rather than being cut to exactly the same height from one end to the other.
A Slightly Wide T-Shaped Fitting Can Sometimes Be Trimmed
Framed shower doors can present a different problem.
The replacement T-shaped seal may have the correct profile, sealing position and general dimensions, but the fitting section is just slightly wider than the original and feels tight in the aluminium track.
That does not always mean you need another profile.
Once you are confident that the T-shaped fitting is the correct type, a very small amount of rigid PVC can sometimes be removed from the edge that is stopping it from entering the track.
A sharp knife or suitable scissors can be used to shave away a little material.
The aim is simply to remove the part that is preventing the seal from fitting.
This needs more care than trimming a flexible fin. The rigid PVC helps hold the seal in place, so only minor adjustment makes sense.
Once you are changing the basic shape of the fitting, it is probably the wrong profile.
When Should You Stop Trimming?
Trimming is useful for small differences in specification. It cannot correct the wrong type of seal.
A U-channel that does not suit the glass thickness will not be fixed by shortening the fin.
The same applies when the rigid section is being blocked by a hinge, roller or bracket. At that point, the issue is more likely to be the profile shape or the position of the hardware.
Our guide to choosing a bottom seal around shower door hinges and hardware covers that type of fitting problem in more detail.
The finished fit also tells you when to stop.
A seal that lets the door move freely but no longer reaches the gap is not suitable for that fitting.
Likewise, a T-shaped section that needs a large amount of rigid PVC removed is probably not the right size or profile.
Small adjustments are useful when the profile is right and a dimension is slightly off. They are not a way to turn one seal design into another.
Still Not Sure? SIMBA Can Help Check the Fit
Older shower enclosures can be difficult to match, especially when the original model is no longer available and the old seal has already been cut or worn.
A few clear photos are often more useful than one measurement.
Send SIMBA a photo of the old seal profile, where it fits on the door and the gap it needs to cover. For seals that clip onto the glass, include the glass thickness. With a T-shaped seal, a clear photo of the original track is useful too.
We can then help work out whether an existing size can be used as supplied, whether a small adjustment makes sense, or whether another profile would be a better fit.
For some shower door seals, where the structure is correct but the available size still needs a more regular adjustment, factory trimming may also be possible before the seals travel with our next bulk shipment to the UK.
Factory trimming can mean a wait of around two months, so it is better suited to customers who are not in a hurry and cannot find a workable stock size.
A ready-made size that fits straight away is still the easiest option. But when the profile is right and only a small dimension is different, a careful adjustment can be more practical than searching indefinitely for an exact copy of an old seal.