Last Updated: 06 Feb 2026
In the world of DIY shower repairs, I hear the same frustrations every day: "Why won't this seal fit?" or "I’ve just fitted a new one, so why is my floor still a swimming pool?"
The truth is, replacing a Shower Door Bottom Gasket isn't the hard part—it’s getting the measurements spot on.
In the UK market, even a 1mm error can be the difference between a dry bathroom and a soggy nightmare.
Today, myself and our founder, James, will walk you through the specialist guide to avoiding those common measurement traps.
1. Your Toolkit: Preparation is Key
Before you start, grab these essentials:
- 📏 Digital Calipers (The Gold Standard): If you have them, use them. They catch the tiny details that a tape measure might miss.
- 🪙 The Coin Trick (Practical & Easy): Tape measures can be a nightmare on curved glass. A few coins can help you bridge the gap (literally).
- 📝 Pen & Paper: Don't trust your memory! UK period properties often hide "surprises" like uneven floors.
2. Measuring Glass Thickness
Laura’s Golden Rule: Measure the glass, never the old seal!
Old seals perish. Over time, contact with shampoo, limescale, and hot water causes them to shrink, crack, or warp. If you measure the old plastic, you’ll likely buy the wrong size.
Standard UK Specs: Most glass in the UK will be 4mm, 5mm, 6mm, 8mm, 10mm, or 12mm.
Pro Tip: The Lego/Coin Method
If the glass edge is tricky, sandwich the glass between two coins or Lego bricks, then measure the distance between the two objects.
3. The Bottom Gap: The Leak-Proof Secret
This is the vertical distance from the very bottom edge of the glass down to the shower tray or threshold.
Why does it matter? A seal is made of a rigid U-channel (to grip the glass) and a flexible fin. The fin's length must be enough to cover the gap comfortably to block the water.
Dealing with Uneven Floors?
Many UK homes have a slight floor sink. Measure the gap at the left, middle, and right of the door. If the gap varies (e.g., 6mm at one end, 10mm at the other), don't go for a standard bottom fin. Choose a seal with a side-deflector fin and go for the average (8mm).
Our SL810DTL is a lifesaver for Uneven gaps.
The Threshold Strip Factor:
If your shower has a metal or plastic bar (threshold) on the floor, measure to the top of that bar, not the floor tray. This is where the seal needs to make contact.
4. Tricky Curved Screens
If you have a Quadrant or curved screen, it's slightly more "fiddly":
- Flexible Tape: Use a tailor's soft tape measure to follow the curve.
- The Peak Point: Gaps are often widest in the centre of the curve. Make sure you measure the "peak" of the arc.
Quick FAQ
What if my shower tray is uneven?
Don't go for a standard bottom-fin seal. Opt for a side-deflector fin. This allows more "give" and handles uneven levels much better.
Still Not Sure? Let Me Help
Every bathroom is unique. If you're staring at these numbers and scratching your head, don't risk a wrong purchase.
"Snap a photo of your shower gap with a ruler next to it for scale, send it over, and I'll personally find the perfect fit for you." — Laura