How Many Corrugated Bitumen Sheets Do I Need?

January 4, 2026

A Simple Calculator Guide for DIY Roofing Projects

One of the most common questions we hear is:
“How many corrugated bitumen sheets do I need for my roof?”

Ordering too few sheets delays your project. Ordering too many wastes money.
This simple guide will show you exactly how to calculate the number of corrugated bitumen roofing sheets you need, whether you’re roofing a shed, garage, pergola, or agricultural building.

Step 1: Measure Your Roof Area

Start by measuring your roof in metres.

You’ll need:

Roof length (eaves to ridge)
Roof width (side to side)

👉 Multiply them together:

Roof area (m²) = Length × Width

Example:

Length: 4.0m
Width: 3.0m

Total roof area = 12m²

Step 2: Know Your Sheet Size

Our corrugated bitumen sheets sold in the UK are approximately:

Length: 2000mm (2.0m)
Width: 950mm (0.95m)

However, you cannot use the full sheet width due to overlaps.

Step 3: Allow for Overlaps (Very Important!)

Corrugated bitumen sheets must overlap to remain waterproof.

Typical overlaps:

Side overlap: 1 corrugation
End overlap: 150–200mm (depending on roof pitch)

Effective sheet coverage (after overlaps):

Effective width: ~850mm (0.85m)
Effective length: ~1.8m

👉 Effective coverage per sheet ≈ 1.53m²

Step 4: Use the Simple Sheet Calculator

Now divide your roof area by the effective coverage per sheet.

Formula:

Number of sheets = Roof area ÷ 1.53

Example:

Roof area: 12m²
12 ÷ 1.53 = 7.84

👉 Always round up
You will need 8 sheets

Quick Reference Calculator Table

Roof Size (m²)	Sheets Needed
6m²	4 sheets
9m²	6 sheets
12m²	8 sheets
15m²	10 sheets
20m²	14 sheets
25m²	17 sheets

(Figures include standard overlaps)

Step 5: Add a Safety Margin

We strongly recommend adding 1 extra sheet for:

Cutting errors
Future repairs
Awkward roof shapes

👉 For an 8-sheet roof, order 9 sheets for peace of mind.

Don’t Forget Fixings & Accessories

Many DIY projects fail due to missing accessories.

You’ll also need:

Corrugated bitumen fixings (with waterproof caps) 20 per sheet
Ridge pieces (for apex roofs) cover length 850mm
Verge flashings (for exposed sides) cover length 850mm
Eaves fillers (to prevent wind uplift)

💡 Rule of thumb:
20 fixings per sheet

Common Mistakes to Avoid

❌ Measuring the roof but forgetting overlaps
❌ Ordering based on full sheet size
❌ Not allowing for roof pitch
❌ Forgetting ridge and verge pieces
❌ Ordering the exact number with no spare