A Step-by-Step Guide for Pergolas, Lean-To Roofs, Carports & Canopies

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One of the most common questions people have when planning a polycarbonate roofing project is:

“How many sheets do I need?”

Whether you're building a pergola roof, lean-to canopy, carport, walkway, or garden shelter, calculating the correct number of sheets is simple when you follow the right method.

This guide shows you exactly how to work it out — including:

• Measuring your roof

• Understanding sheet widths

• Planning glazing bar spacing

• Allowing for overhangs and expansion gaps

• Worked examples for common roof types

Use this as your go-to calculator when ordering from polycarbonatex.co.uk.

1. What You Need Before You Start

You only need three basic measurements:

✔ Total roof width

Side to side (left beam to right beam)

✔ Total roof length

Top to bottom (wall plate to gutter/front beam)

✔ Sheet width you plan to use

Usually:

• 700mm

• 1050mm

• 2100mm

(Availability depends on thickness: 10mm, 16mm, 25mm)

Once you have these, the calculation is easy.

2. Understanding How Polycarbonate Sheets Are Laid Out

Polycarbonate sheets always run vertically, following the slope of the roof.

This means:

• Sheet width = horizontal measurement

• Sheet length = vertical slope measurement

• Glazing bars run vertically at every join

So you calculate sheet quantity based on width, not length.

3. The Basic Formula

Here is the simple formula to calculate the number of sheets:

Number of Sheets = Roof Width ÷ Sheet Width

Example:
Roof width: 3200mm
Sheet width: 1050mm

3200 ÷ 1050 = 3.04 sheets

Since you cannot use a partial sheet:

✔ Always round UP

→ You need 4 sheets

4. Don’t Forget the Glazing Bars

Glazing bars are required between every sheet, so include them in your layout.

Standard rule:

• If you use 3 sheets, you need 2 glazing bars

• If you use 4 sheets, you need 3 glazing bars

Optional: Edge Bars

Some installers also use glazing bars along both edges for a cleaner finish, which adds:

• 2 extra bars (left and right)

5. Allowing for Expansion Gaps

Polycarbonate expands significantly in hot weather.

You MUST allow for:

• 3–5mm gap between sheets

• 5–10mm gap at outer edges

This gap does not affect sheet quantity, but it prevents cracking and warping.

6. Allowing for Overhang (Length)

Although sheet quantity is based on width, sheet length must include the overhang:

✔ Typically 50–100mm beyond the front beam
✔ Ensures water drips cleanly into the gutter or away from pergola
✔ Choose the next longest sheet and trim if needed

7. Full Worked Examples

Example 1 — Pergola Roof

Roof size:

• Width: 3150mm

• Length: 3000mm

• Sheet width: 1050mm

Calculation:

3150 ÷ 1050 = 3 sheets exactly

✔ Sheets required: 3
✔ Glazing bars required: 2
✔ Sheet length: 3000–3100mm (with overhang)

Example 2 — Lean-To Roof

Roof size:

• Width: 2800mm

• Length: 2500mm

• Sheet width: 700mm

2800 ÷ 700 = 4 sheets exactly

✔ Sheets required: 4
✔ Glazing bars required: 3
✔ Sheet length: 2500–2600mm

Example 3 — Larger Carport or Canopy

Roof size:

• Width: 4400mm

• Length: 4000mm

• Sheet width: 1050mm

4400 ÷ 1050 = 4.19 → round up to 5 sheets

✔ Sheets required: 5
✔ Glazing bars required: 4
✔ Length: 4000–4100mm

Example 4 — Using 2100mm Wide Sheets

For very wide roofs, 2100mm sheets reduce the number of joins.

Roof size:

• Width: 4200mm

• Sheet width: 2100mm

4200 ÷ 2100 = 2 sheets

✔ Sheets required: 2
✔ Glazing bars required: 1
✔ Length: matching roof slope

8. Sheet Coverage Table

This table makes rough planning quick and simple.

9. Polycarbonate Sheet Sizes Available

At polycarbonatex.co.uk, we offer sheets in:

Widths:

• 700mm

• 1050mm

• 2100mm

Thicknesses:

• 10mm

• 16mm

• 25mm

Lengths:

• 2m

• 2.5m

• 3m

• 4m

• 5m

• 6m

You’ll always find a close match without excessive cutting.

10. Common Mistakes When Calculating Sheet Quantity

Avoid these to save time and money:

❌ Forgetting glazing bar spacing
❌ Forgetting expansion gaps
❌ Measuring vertically instead of along roof slope
❌ Not accounting for overhang
❌ Choosing wrong sheet width
❌ Trying to force sheets into a space

If in doubt — round up and trim.

11. Frequently Asked Questions

1. Should I leave gaps between sheets?

Yes — 3–5mm is essential for thermal expansion.

2. Do glazing bars affect the number of sheets?

They don’t change sheet quantity, but they are required for each join.

3. Can I mix sheet widths?

Not recommended — leads to uneven spacing.

4. Should I cut sheets or choose closest size?

Choose the closest size and trim if required.

5. Do sheets run horizontally or vertically?

Always vertically, along the slope.

Order Polycarbonate Sheets in the Perfect Size

At polycarbonatex.co.uk, we supply:

• British-made polycarbonate sheets

• All major widths and lengths

• Snap-Fix & screw-down glazing bars

• Breather & aluminium tape

• End closures & flashings

• Fixings

• Fast UK delivery

• Cheap prices

• Unbeatable service

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