The Complete Installation Guide for Polycarbonate Roofing Systems

Glazing bars are one of the most important components of any polycarbonate roofing system. They securely join your polycarbonate sheets, keep the roof watertight, allow for natural thermal expansion, and create a clean, professional finish.

If you’re installing a pergola roof, lean-to roof, carport, canopy, or garden structure, this guide will show you exactly how to install both Snap-Fix and Screw-Down glazing bars the right way.

1. What Are Glazing Bars and Why Do You Need Them?

Glazing bars are long structural profiles designed to:

• Join two polycarbonate sheets together

• Seal the joints from water and wind

• Allow sheets to expand and contract naturally

• Support the sheets evenly

• Improve the appearance of the roof

Using the correct glazing bar ensures your roof is:

✔ Fully watertight
✔ Structurally secure
✔ Protected against wind uplift
✔ Long-lasting
✔ Professional looking

Never attempt to install polycarbonate sheets without glazing bars — leaks and sheet movement are guaranteed.

2. Snap-Fix vs Screw-Down Glazing Bars: What’s the Difference?

Both types form a secure seal, but they function slightly differently:

Snap-Fix Glazing Bars

• Clip together with internal ribs

• No visible screws on the top

• Fast, clean, DIY-friendly installation

• Very popular for pergolas & patio covers

Screw-Down Glazing Bars

• Top cap is screwed into the base

• Excellent strength & compression

• Ideal for bigger spans and exposed locations

• Perfect for lean-to roofs & garden rooms

Both types are suitable for 10mm, 16mm, and 25mm sheets — just ensure you order the bar that matches your sheet thickness.

3. Tools & Materials You’ll Need

Tools

• Drill & bits

• Fine-tooth saw (for trimming bars)

• Tape measure

• Rubber mallet

• Spirit level

• Straight edge

Materials

• Glazing bars (correct thickness)

• Polycarbonate sheets

• Fixing screws

• Breather tape & aluminium tape

• End closures

• Foam gaskets (if applicable)

• Side flashings

4. Preparing Your Polycarbonate Sheets

Before fitting glazing bars, ensure your sheets are prepared:

✔ Keep protective film ON

Prevents scratches and marks during installation.

✔ Tape the sheet ends

• Bottom edge: Breather tape

• Top edge: Aluminium tape

• Add end closures to both ends

✔ Ensure flutes run downhill

The internal flutes must run from top to bottom for drainage.

5. How to Install Snap-Fix Glazing Bars (Step-by-Step)

Snap-Fix bars have two parts:

• Base – fixed to rafters

• Top cap – snaps down to lock sheets in place

Step 1 — Position & Fix the Base

1. Align the base with the rafter or supporting beam.

2. Ensure it is perfectly straight along the entire length.

3. Fix using appropriate screws (usually every 300–400mm).

4. Make sure the base sits flat with no twists.

Pro tip:
Leave a 2–3mm gap at the top and bottom for thermal movement.

Step 2 — Position the Polycarbonate Sheets

1. Place the first sheet so it sits centrally over the glazing bar base edge.

2. Leave a 3–5mm expansion gap between sheets.

3. Check the sheet is fully seated along the base.

Important:
Do not trap the sheet tightly — it must expand and contract.

Step 3 — Snap the Top Cap Into Place

1. Start at one end of the bar.

2. Press firmly down until you hear the first “click”.

3. Work evenly along the full length.

4. Use a rubber mallet for any stubborn areas.

The cap should lock tightly and sit flush the entire way down.

Step 4 — Fit End Caps & Flashings

Finish by installing:

• End caps on the glazing bar

• Side flashings along roof edges

• Wall flashings for lean-to roofs

These protect the joint and complete the weatherproof seal.

6. How to Install Screw-Down Glazing Bars (Step-by-Step)

Screw-down bars are similar but use mechanical compression for extra strength.

Step 1 — Fix the Glazing Bar Base

1. Align base with rafters.

2. Fix down with screws every 300–400mm.

3. Ensure no warping or twisting.

The base must be perfectly straight for a leak-free seal.

Step 2 — Insert the Sheets Into the Base

1. Slide sheets into the base channels.

2. Maintain 3–5mm expansion gaps.

3. Ensure sheets sit flat with no bowing.

Step 3 — Install the Top Cap

1. Lay the cap over the sheets.

2. Start fixing screws at one end.

3. Tighten screws until snug, but do not overtighten.

4. Work along the bar evenly.

Overtightening can crush the sheets, so be careful.

Step 4 — Install End Caps & Flashings

Finish the installation:

• Fit end caps

• Add side and wall flashings

• Seal all edges properly

7. Thermal Expansion: The Golden Rule of Polycarbonate

Polycarbonate roofing expands significantly in hot weather.

✔ Always leave expansion gaps

3–5mm between sheets
5–10mm at edges

✔ Drill oversize holes for fixing buttons

2–3mm larger than screw diameter

✔ Never fully clamp or compress sheets

The roof must be able to move.

Ignoring thermal expansion is the #1 cause of:

• Creaking

• Cracking

• Sheet distortion

• Water leaks

8. Common Mistakes to Avoid

❌ Overtightening screws
❌ Not leaving expansion gaps
❌ Installing glazing bars crooked
❌ Failing to support bars fully
❌ Using the wrong bar for sheet thickness
❌ Forgetting end closures
❌ Not taping sheet ends

Avoid these and your roof will last 10–20 years+.

9. Frequently Asked Questions

1. Do I need glazing bars on every joint?

Yes — every sheet-to-sheet join needs a glazing bar.

2. Can I use one long sheet instead?

You can, but wide roofs still need glazing bars for strength and load distribution.

3. What’s better: Snap-Fix or Screw-Down?

• Snap-Fix: DIY-friendly and clean appearance

• Screw-Down: Better for large spans and exposed areas

4. Should I put sealant between sheets?

No. Never seal polycarbonate sheets together — this stops thermal movement.

5. Are glazing bars universal?

No. They must match your sheet thickness.