How to Overlap Corrugated Bitumen Sheets (Correct Method + Pitch Guide)

February 9, 2026

Ensure a Watertight, Wind-Resistant Roof With Proper Side & End Overlaps

Overlapping corrugated bitumen roofing sheets correctly is one of the most important steps in achieving a fully waterproof, wind-resistant, and long-lasting roof.
If overlaps are incorrect — too small, facing the wrong direction, or poorly aligned — leaks and wind uplift can occur.

In this guide, you’ll learn:

The correct side lap (1 or 2 corrugations)
The correct end lap (minimum 150–300mm)
How roof pitch affects overlap
How to position sheets for perfect alignment
How to prevent wind-driven rain entering laps

This tutorial applies to sheds, garages, lean-tos, pergolas, workshops and all DIY buildings.

1. Why Overlap Matters for Bitumen Roofing

Correct overlap ensures:

✔ Waterproof protection

✔ Wind uplift resistance

✔ Sheet stability

✔ A clean, professional look

✔ Longevity of 20–25+ years

Incorrect overlap is one of the top causes of leaks in garden buildings.

2. Types of Overlap in Corrugated Bitumen Roofing

There are two types:

A) Side Overlap (Left-to-right overlap)

Where one sheet overlaps the next sheet horizontally.

This overlap follows the corrugation profile.

B) End Overlap (Top-to-bottom overlap)

Where one sheet sits above another sheet vertically.

This is used if sheets aren’t long enough to span the whole roof.

(Whenever possible, avoid end laps by using full-length sheets.)

3. How Roof Pitch Affects Overlap

Your roof pitch determines the correct side lap:

✔ Roof Pitch 10° or Greater

Use 1 corrugation side overlap
(Recommended for most pent and apex sheds.)

✔ Roof Pitch Between 5° and 10°

Use 2 corrugation side overlap

This prevents wind-driven rain from entering the joint.

❌ Roof Pitch Below 5°

Not suitable for bitumen roofing sheets.
You must increase the pitch.

4. The Correct Side Overlap Method

This is the most important overlap in your installation.

Step 1 — Identify the Direction of Prevailing Wind

Your side lap MUST face away from the dominant wind direction.

Example:

If the wind usually blows left to right, overlap sheets right over left.

This prevents rain being forced into the lap.

Step 2 — Position the First Sheet Perfectly Square

The first sheet must be aligned straight along the eaves.

Any misalignment will compound across the roof.

Step 3 — Slide the Next Sheet Into Position

Depending on pitch:

✔ 10°+ pitch → Overlap 1 corrugation

✔ 5–10° pitch → Overlap 2 corrugations

Ensure the corrugated profiles sit neatly together.

Step 4 — Fix Side Laps Correctly

Fix:

Every 2 corrugations along side laps
Through the crest only
Using sealing washers

This locks the sheets together and stops wind uplift.

5. The Correct End Overlap Method (If Required)

It’s always better to use full-length sheets, but if you need an end lap:

✔ Minimum overlap = 150mm (for 10°+ pitch)

✔ Recommended = 200–300mm for 5–10° pitch

✔ Always overlap top sheet over lower sheet

✔ Fix only on battens — never in the middle of the lap

✔ Apply bitumen mastic for low pitches (optional)

DO NOT end-lap if your pitch is below 5°.

6. Overlap Checklist (Essential)

Before fixing, check the following:

✔ Sheets sit tight with no gaps

✔ Overlaps face away from wind

✔ Side lap corrugations align perfectly

✔ End lap is minimum 150mm

✔ No debris trapped between sheets

✔ Fixings placed only on crests

✔ Fixings not overtightened

✔ Ridge and verge pieces cover overlaps correctly

7. Common Overlap Mistakes (Avoid These)

❌ Overlapping on the wrong side (wind-facing)
❌ Using only half a corrugation overlap
❌ Not overlapping enough at low pitch
❌ Misaligned corrugation valleys
❌ Leaving gaps between sheets
❌ Fixing too close to the lap edge
❌ Using sealant incorrectly or excessively

If in doubt, overlap more rather than less.