Last Updated: 14 January 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.
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.
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.