The Complete Step-by-Step Measuring Guide for Pergolas, Lean-Tos, Carports & Canopies

Accurate measuring is one of the most important stages of any polycarbonate roofing project. Whether you're building a pergola roof, lean-to canopy, porch cover, garden room, or carport, measuring correctly ensures you:

• Order the right size sheets

• Avoid costly mistakes

• Minimise cutting

• Ensure a watertight fit

• Save time on installation

This complete guide walks you through exactly how to measure a roof for multiwall polycarbonate sheets, including width, length, sheet coverage, glazing bar spacing, overhang, pitch requirements, and expansion allowance.

1. Tools You’ll Need for Measuring

You don’t need anything complex — just:

• Tape measure (5m or 8m)

• Ladder

• Pencil

• Paper or notepad

• Spirit level (optional)

• Helper (for large roofs)

2. Identify the Type of Roof You’re Measuring

Polycarbonate roofs usually fall into one of these styles:

✔ Pergola Roof

Open-sided structure with timber rafters.

✔ Lean-To Roof

Attached to a house wall with pitch sloping outward.

✔ Carport or Canopy Roof

✔ Garden Room or Patio Cover

Each type requires the same measuring method — but lean-tos require wall flashing allowance.

3. Understand the Direction of Polycarbonate Sheets

Before measuring, remember:

✔ Flutes MUST run downhill

From the highest point (house wall or pergola ridge)
→ to the lowest point (gutter or front beam)

This means:

• Sheets run vertically

• Glazing bars run vertically

• Measuring width = measuring horizontally across rafters

With that in mind, let’s begin.

4. How to Measure the Width of the Roof

This determines how many sheets you need.

Step-by-Step Width Measuring:

1. Measure from the left outer edge to the right outer edge of the supporting beams/rafters.

2. Record the total width in millimetres (mm).

3. Subtract any required spacing or framing allowances.

4. Compare with available sheet widths.

Typical Sheet Widths Include:

• 700mm

• 1050mm

• 2100mm

(Availability varies — check polycarbonatex.co.uk)

Calculate How Many Sheets You Need

Formula:

Roof width ÷ Sheet width = Number of sheets needed

Example:
Roof width = 3150mm
Sheet width = 1050mm

3150 ÷ 1050 = 3 sheets

Always Round Up

If your measurement is slightly over, never force sheets to flex — order the next full sheet.

5. Measuring for Glazing Bars

Glazing bars go between sheets, so you must allow for them.

Glazing Bar Widths:

• 45mm

• 50mm

• 60mm

Layout Example:

For 3 sheets:

• Sheet

• Bar

• Sheet

• Bar

• Sheet

This means:
Number of glazing bars = Number of sheets + 1 (if using edge bars)
or
Number of sheet-to-sheet bars = Number of sheets – 1

6. How to Measure the Length of the Roof

The length is the slope from top to bottom.

Steps:

1. Measure from the highest point (wall plate or top beam)

2. Down to the lowest point (front beam/gutter)

3. Along the angle of the roof (not vertically)

4. Add your desired overhang (usually 50–100mm)

✔ Add Extra for Overhang

Polycarbonate sheets should extend slightly beyond the front beam for drip clearance.

Examples of Sheet Lengths:

Sheets are commonly available in:

• 2m

• 2.5m

• 3m

• 4m

• 5m

• 6m

Always choose the next longest sheet and cut down if needed.

7. Measure Roof Pitch (Essential for Drainage)

Polycarbonate roofs MUST have a pitch of at least 5° (100mm drop per metre).

How to Measure Pitch:

1. Use a tape measure to find the height of the back and front beams.

2. Subtract the lower height from the higher height.

3. Divide by the length of the roof.

Example:
Roof rise = 200mm
Roof length = 3m

Pitch = 200 ÷ 3000 = 6.6°

✔ Suitable pitch
❌ If under 5°, add angled rails to increase slope

8. Important Allowances When Measuring

✔ Expansion Gap Between Sheets

3–5mm between sheets
5–10mm at roof edges

✔ Overhang

50–100mm at the gutter end

✔ Wall Flashing Overlap (Lean-Tos)

Allow 20–40mm for flashing to overlap onto sheet.

✔ Glazing Bar Width

Add 45–60mm for each join.

✔ End Closures

These add a few millimetres but do NOT affect sheet size.

9. Worked Example: How to Measure a Pergola Roof

Pergola Measurements:

• Width: 3200mm

• Length: 3000mm

• Using 1050mm sheets

Step 1: Calculate Sheets

3200 ÷ 1050 = 3.04 → require 4 sheets
(or 3 sheets if you trim width; depends on design)

Step 2: Overhang

3000mm roof → choose 3m or 3.5m sheet length

Step 3: Glazing Bars

4 sheets → 3 sheet-to-sheet bars
(or 5 if using two edge bars)

10. Worked Example: How to Measure a Lean-To Roof

Lean-To Measurements:

• Width: 2700mm

• Length: 2500mm

• Using 700mm sheets

Step 1: Calculate Sheets

2700 ÷ 700 = 3.85 → require 4 sheets

Step 2: Add Flashing Allowance

Add 20–40mm at wall end.

Step 3: Choose Length

2500mm roof → choose 2500mm or 3000mm sheet length.

11. Common Measuring Mistakes to Avoid

❌ Measuring vertically instead of along the slope
❌ Forgetting glazing bar widths
❌ Not allowing for sheet expansion
❌ Choosing sheets too short for overhang
❌ Incorrect flute direction
❌ Underestimating sheet quantity

Avoid these and your installation will be smooth and trouble-free.

12. Frequently Asked Questions

1. Should I cut sheets or order exact sizes?

Order closest size and cut only if needed.

2. Do flutes always run vertically?

Yes — from wall to gutter or back beam to front beam.

3. Should sheets be tight together?

Never — always leave expansion gaps.

4. How much overhang do I need?

50–100mm depending on gutter and design.

5. How do I measure if my roof is uneven?

Measure multiple points and use the largest measurement.

Buy Polycarbonate Sheets in the Right Sizes

At polycarbonatex.co.uk, we supply:

• 10mm, 16mm & 25mm multiwall polycarbonate sheets

• British-manufactured for long-lasting performance

• Full range of lengths (2m–6m)

• Multiple widths (700mm, 1050mm, 2100mm)

• Matching glazing bars, tapes, flashings & fixings

• Fast UK delivery

• Cheap prices

• Unbeatable service

👉 Shop Polycarbonate Roofing Sheets
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