Last Updated: 20 April 2026
In the UK, whether you're in a heritage Victorian terrace or a sleek modern flat, many homeowners face the same bit of a nightmare: a uneven gap under bathroom door.
One side might be a tiny 7mm, while the other stretches to a massive 15mm.
Standard bottom seals are usually one size all the way across, which leaves you with a leak on one end or a door that jams on the other. Not ideal.
In this guide, we’ll show you the "Golden Combination": using a threshold bar and the right seal to sort out those sloping gaps for good.
Why is my shower door gap uneven?
- Floor Gradient: Most UK shower trays or tiles slope toward the drain to stop water pooling.
- Building Settlement: In older properties, floors and walls are rarely "spirit-level" perfect.
- Hinge Sag: Over time, gravity can cause the door to drop slightly on one side.
Before attempting a fix, it’s vital to understand the basic mechanics of your sweep. Consult our Bottom Seal Sweep & Gasket Masterclass to identify your profile's 'Molecular Memory' and limits.
Solution 1: A Slight Slope (Gap difference <3mm)
The Scenario: One side is 7mm, the other is 10mm.
The Fix: A Standard Bottom Seal or a Side-Extended Fin Seal.
How it works: If the slope is minor, high-quality virgin PVC is your best friend. It’s flexible enough to bend at the narrow end and still reach the tray at the wider end.
SIMBA Pro Tip: Always go for virgin PVC. Cheap, low-grade plastic won’t "spring back" after bending; it just goes brittle and cracks.
Advanced Choice: We recommend the Side-Extended Fin Seal. Because the fin hangs from the side, it doesn't get crushed directly under the glass, so it lasts much longer.
Note: For a proper watertight finish, use these with a Waterproof Threshold Bar.
Standard Bottom Seal
Side-Extended Fin Seal
Solution 2: Noticeable Slopes (Difference >3mm)
The Fix: Side-Extended Fin Seals.
This is the most professional way to handle sloping gaps. The seal has an extra "curtain" that hangs down like a skirt. As the gap changes, the side skirt continues to block the spray effortlessly.
Best for gaps under 10mm. Must be used with a threshold seal for a 100% dry floor.
Alternatively, you can watch this video to see how to trim the bottom seal to fit uneven gaps.
If you accidentally cut too much, you can refer to the method shown in the image below to cover the gaps.
Continue reading for further instructions.
Solution 3: The Ultimate Fix (Large Gaps >10mm)
If your gap is too big for a side-extended seal, you need to "bridge the gap" from the floor up.
The Strategy: Raise the Floor with a Threshold Bar
Our transparent threshold bars are 5mm high. By fitting one, you effectively raise the floor level, cancelling out part of the gap and giving the seal a level surface to meet.
Case Study:
The Data: 6mm glass. Gap is 16mm on one side and 22mm on the other (6mm difference).
The Math: Seal Fin + Threshold (5mm) should be ≥ Max Gap (22mm).
Selection: Use V456S20 (20mm fin) + Threshold (5mm) = 25mm Total Coverage.
Result: 25mm easily covers the 22mm gap. The narrow end (16mm) will compress by 4mm, which is well within the seal's comfort zone.
Pro Tip: Avoid Too Much Squeeze
If you think a 20mm fin is too long, you might try a 16mm fin (V456S16). With the 5mm threshold, that gives you 21mm total—just 1mm shy of the 22mm gap.
The Trick: When fitting, don’t push the U-channel all the way up. Leave a tiny 2-3mm gap at the top. This lets the seal reach the floor without extra friction.
Note: This method is ideal for gaps where the total height (seal + bar) is within 3mm of the max gap.
Solution 4: Extreme Slopes (10mm+ Difference)
The Scenario: 6mm glass. Max gap is 20mm, but the min gap is only 9mm. That's a massive 11mm difference!
The Problem: A 20mm fin will fit the big end, but it will be crushed flat at the 9mm end, causing it to tear. As we show in our video guide, over-compression kills seals.
The Professional Fix: Precision Trimming
For a bespoke fit, follow these DIY steps:
- Initial Fit: Push the seal onto the glass.
- Mark It: Use a pen to mark where the fin is bending or crushing too much.
- Safety Buffer: Don't worry about being 100% perfect. The 5mm threshold bar gives you a buffer—as long as your cut is within that 5mm range, it won't leak.
- Trim: Remove the seal and use sharp scissors to snip away the excess fin along your line.
The Result: A seal that perfectly contours to your sloping floor. Job done.
Quick Help
Still feeling a bit confused? +
Why Choose SIMBA UK?
- Anti-UV Material: Specially engineered for our UK climate. Our seals won't turn yellow or go brittle in the sun.
- 9,000+ Profiles: We have a solution for every "weird" gap in the country.
- Expert Support: Real UK-based advice from people who know shower doors inside out.