Last updated: 5 February 2026
Expert Verified by Laura Liu: 7+ Years of Seal Tech Expertise
Frameless shower doors are popular for a reason. They look clean, modern and make a bathroom feel more open. But because there is no bulky metal frame to help hold water back, the seals have to do more of the work.
That is why the order of fitting matters.
If you are replacing the seals on a frameless shower door, the usual rule is simple: fit the vertical seal first, then measure and trim the bottom seal afterwards.
The vertical seal helps set where the door closes. Once that side position is right, the bottom seal can be cut to meet it neatly. This gives the lower corner a cleaner finish and helps reduce the small gaps where leaks often start.
Why the Fitting Order Matters
A leaking frameless shower door does not always mean the seal is worn out. Quite often, the problem is where two seals meet.
The lower corner of the door is the most common trouble spot. Water runs down the glass, reaches the bottom edge, and then looks for the easiest way out. If the vertical seal and bottom seal do not meet properly, even a small gap can let water escape.
This is where fitting the bottom seal first can cause problems. Once the vertical seal is added, it may press against the bottom strip, push it out of line, or create a small opening at the corner. Everything may look installed, but the frameless shower door leaks once it is closed and in use.
Fitting the vertical seal first gives you a fixed side line to work from. The bottom seal can then be trimmed to suit the actual door position, rather than guessed from the old seal.
Installation Video
If this is your first time replacing frameless shower door seals, watch the fitting video before making any cuts.
It is much easier to understand the corner detail when you can see how the vertical seal and bottom seal should meet. The video also helps you judge how much material to leave when trimming.
Step-by-Step Fitting Guide
This method works for most frameless shower doors found in UK homes, including doors using F seals, H seals or magnetic vertical seals.
The goal is not to force the thickest seal into the tightest gap. A good fit should let the door close smoothly while guiding water back into the shower tray.
A slight overlap at the corner helps stop water from travelling through the joint.
A physical overlap forces water to stay exactly where it belongs.
Step 1: Fit the Vertical Shower Seal First
Start with the side seal. Push the vertical shower seal onto the glass edge, making sure the glass is clean and completely dry before you begin.
Any limescale, soap residue or moisture on the glass can stop the seal from sitting properly. This is especially common in hard water areas, where old seals often leave marks along the glass edge.
Choose the correct vertical shower door seal for your door. F seals, H seals and magnetic seals do slightly different jobs, but they all need to help the door close in the right position.
Once the vertical seal is in place, close the door gently. It should meet the frame, fixed panel or opposite seal without force. If the door feels tight, catches, or pushes the seal out of shape, adjust it before moving on.
At the bottom, keep the gap between the vertical seal and the shower tray or threshold as small as possible, as long as the door still opens and closes freely. Avoid cutting the vertical seal too short. In many cases, leaving a very slight drop at the lower end helps it cover the edge of the bottom seal more effectively.
Step 2: Trim the Bottom Shower Seal
Once the vertical seal is sitting correctly, move on to the bottom seal.
Do not use the old seal as your only guide. Old shower seals can shrink, bend or may have been cut slightly wrong when first installed. Instead, hold the new bottom shower seal against the bottom edge of the glass and mark where it meets the vertical seal.
When trimming, concentrate on the rigid PVC section. This is the part that needs to sit neatly against the vertical seal. The soft water fins should be left intact where possible, because they help direct water back into the shower tray.
Take your time here. If you are unsure, cut a little less first and test the fit. You can always trim a little more, but you cannot put material back once it has been cut away.
The finished joint should look neat and relaxed. The two seals should meet or slightly overlap, but they should not be crushed together.
Top tip: trim the rigid PVC section only where needed. Keep the soft water fins intact wherever possible.
Step 3: Add a Threshold Seal If Needed
After the bottom seal is fitted, take a look at the shower tray itself.
Some trays have a good raised edge that naturally helps hold water in. Others are flatter, or may not have any threshold strip at all. In those cases, water can still be pushed out under the door, especially when the shower spray is aimed towards the glass.
If the door continues to leak from the bottom even after the vertical and bottom seals have been fitted properly, the answer may not be to keep trimming the seal or choosing a thicker one. The shower may need an extra water barrier at tray level.
This is where a T-shaped threshold seal, also called a threshold dam, can help. It is usually fitted directly to the shower tray and works with the bottom seal to stop water being driven out under pressure.
A threshold seal is particularly useful when the lower corner keeps leaking, the shower tray has no clear raised edge, or water regularly escapes from beneath the door.
That said, it is not something to add blindly. A threshold seal should not stop the door from opening, and it should not squash the bottom seal too tightly. The best setup is one where the door still closes smoothly and the bottom seal sits lightly against, or just above, the threshold.
If you are not sure whether your shower tray needs one, check our threshold seal selection guide. For ongoing leaks, you can also send us a message. We can help you decide based on your shower door style, tray edge height and where the water is escaping.
Step 4: Check the Door and Test for Leaks
Once everything is fitted, do not finish straight away.
Open and close the shower door a few times. The movement should feel smooth. The bottom seal should not drag heavily across the tray, and the vertical seal should not twist or pull out of place.
Next, check the meeting point between the vertical seal, bottom seal and threshold seal if you have fitted one. There should be no obvious gap, but the seals should not be forced together either.
Then do a simple water test. Close the door and run a small amount of water down the inside of the glass. Watch where the water goes.
If it runs back into the shower tray, the seals are doing their job. If it escapes from the lower corner, check the alignment before buying a thicker replacement strip. The problem is often the meeting point, not the size of the seal.
When Might the Bottom Seal Go First?
Fitting the vertical seal first is the safest approach for most frameless shower doors, but there are exceptions.
If your shower door has a manufacturer-designed bottom water guide, or the shower tray has a high raised edge, the bottom seal may need to line up with the threshold first. In that case, follow the manufacturer’s fitting instructions.
The order can also be more flexible if the vertical seal does not run all the way down to the bottom of the door, or if the hinge design means the side seal does not affect the lower corner.
A simple rule is this: follow the manufacturer’s instructions where available. If there are no clear instructions, fit the vertical seal first and trim the bottom seal afterwards.
Summary Checklist
Before you call the job finished, check these points:
Are the soft water fins facing inwards, so water is directed back into the shower tray?
Do the bottom seal and vertical seal meet neatly, without a visible gap?
If you have fitted a threshold seal, can the door still open and close smoothly?
Does the door close without catching, scraping or feeling over-compressed?
After a simple water test, does the water stay inside the shower area?
If yes, the seals are likely to be fitted correctly.
For more practical fitting and maintenance advice, see our shower seal fitting and maintenance guides.
FAQ: Shower Seal Fitting
Should horizontal shower seals be fitted before vertical ones?
Ideally, no. For most frameless shower doors, fit the vertical side seal first. This allows you to trim the bottom seal to meet it neatly and reduce the risk of corner leaks.
Does the bottom shower seal need to overlap the vertical seal?
A slight overlap or close meeting point is helpful, but the seals should not be forced tightly together.
If the two seals press too hard against each other, the door may become difficult to close. The bottom seal may also lift, twist or lose its shape.
Which matters more: the bottom seal or the vertical seal?
They do different jobs. The bottom seal helps stop water from escaping under the door. The vertical seal helps control side gaps and spray.
If one seal is new but the other is worn, warped or badly positioned, the lower corner may still leak. Good sealing comes from both parts working together.
Should I replace both seals at the same time?
If both seals are yellowed, hard, cracked, mouldy or no longer lining up properly, replacing them together is usually the better option.
If only the soft fin on the bottom seal is damaged and the vertical seal still closes well, you may only need a new bottom seal. Even then, measure against the current door position rather than copying the old strip exactly.
Do shower seals need silicone?
Most replacement shower seals are push-fit and do not need silicone.
Only use silicone if the manufacturer recommends it or if your shower door design specifically requires it. Unnecessary silicone can make future replacement harder and may stop the seal from sitting correctly.
Do I need a threshold seal if the door still leaks?
If the bottom and vertical seals are fitted correctly but water still escapes under the door, especially on a flat shower tray with no raised edge, a T-shaped threshold seal may help.
Before fitting one, make sure it will not interfere with the door movement or compress the bottom seal too much. If you are unsure, check our threshold seal selection guide or contact us for advice.
Final Advice: Dry Fit First, Then Cut
Do not rush the cut. With frameless shower door seals, a few millimetres can make a real difference.
For most doors, fit the vertical seal first, check the door movement, and then trim the bottom seal to suit that position. If water still escapes underneath, look at the shower tray and consider whether a threshold seal is needed.
A good seal is not about forcing everything tight. It is about letting the door close properly and giving water a clear route back into the shower tray.
When the order, size and cut are right, many lower-door leaks can be solved without replacing the whole shower screen.