Last updated: 5 February 2026
Expert Verified by Laura Liu: 7+ Years of Seal Tech Expertise
Struggling with which seal to fit first?
Fitting seals seems straightforward, but mastering the corner joint is where most DIYers come unstuck.
Our expert, Laura Liu, explains how to achieve a professional, watertight finish between your vertical and horizontal seals.
Method 1: The Vertical-First Overlap (Standard Pro)
Best for: Quick, reliable DIY installation.
For a truly watertight barrier, your vertical side seal must overlap and shield the horizontal bottom seal.
This simple logic ensures that any water running down the glass is channeled directly back into the shower tray instead of seeping through the gap.
1. Why Vertical Before Horizontal?
The Rule: Always seal the corner to guide water inwards.
The Mistake: Fitting the bottom seal first creates a small entry point—a "leak magnet" for most bathrooms.
The Fix: The side seal should slightly cover the edge of your bottom shower seal.
A physical overlap forces water to stay exactly where it belongs.
2. The 4-Step Leak-Proof Plan
Step 1: Install Vertical Side Seals
- Prep: Ensure the glass is spotless and bone dry.
- Fit: Slide your new side seal onto the glass edge.
- The Pro Gap: Let the seal protrude 1-3mm past the glass bottom. Don't cut it flush!
Step 2: Precision Trimming of the Bottom Seal
- • Measure: Mark your bottom door seal carefully.
- • The Notch: Cut the hard PVC slightly short so it slots inside the vertical seal's channel.
⚠️ Top Tip: Only trim the rigid PVC. Leave the soft water-fins intact!
Step 3: Threshold Dam (Final Defence)
A threshold dam is vital for stopping water under pressure. Secure your T-Shape threshold seal directly to the tray.
Step 4: Watertight Check
Close the door and check that all three components—the vertical, horizontal, and threshold seals—meet perfectly. No gaps = No leaks!
Method 2: The SIMBA "Notch & Fit" (Pro Secret)
Best for: A high-end, seamless finish without tricky 45° mitre cuts.
Step 1: Align. Use a ruler to ensure minimal gaps without catching the door frame.
Step 2: The Labyrinth Logic.
• Bottom Seal: Trim the inside U-channel (shower side).
• Vertical Seal: Trim the outside U-channel (bathroom side).
Step 3: Interlock. Fitting them this way creates a "Labyrinth Seal"—making it impossible for water to penetrate the joint.
Summary Checklist
✅ Fins pointing inwards?
✅ Gaps kept to a minimum?
✅ Watertight test completed?
FAQ: Shower Seal Fitting
Should horizontal shower seals be fitted before vertical ones? +
The SIMBA Advantage
- Durable Tech: Our replacement sweeps are cycle-tested 30,000 times for long-term flexibility.
- Expert Support: Every order includes professional guides, and Laura Liu is available for free fitting advice on frameless screens.
Find your perfect fit at showerdoorseal.uk