A Practical Guide to Diagnosing & Repairing Leaks in Corrugated Bitumen Roofing Sheets
Corrugated bitumen roofing sheets are designed to be highly waterproof, but even the best roof can develop leaks if something has been installed incorrectly, worn over time, or become damaged.
The good news?
Bitumen roof leaks are usually easy to diagnose and simple to fix.
In this guide, you’ll learn:
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How to identify the source of a leak
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The most common causes of bitumen roof leaks
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How to fix each problem properly
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What products and methods work best
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How to prevent leaks from returning
Let’s get started.
1. Common Causes of Leaks in Bitumen Roofing Sheets
Nearly all bitumen roof leaks come from installation errors, not the sheet material. Here are the top issues:
✔ 1. Fixings in the Wrong Place (Valley Fixings)
Fixing through the valley allows water to pool around the fixing → this WILL leak over time.
Fix:
Replace fixings and re-fix through the crest of the corrugation using sealed fixings.
✔ 2. Too Few Fixings Used
If the roof can lift or flex, water is forced under the laps.
Fix:
Add more fixings following the correct fixing pattern.
(Eaves + ridge every corrugation, intermediate every 2.)
✔ 3. Incorrect Overlaps
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Too small overlap
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Wrong direction (facing the wind)
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Gaps due to poor alignment
Fix:
Re-align sheets and ensure:
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1 corrugation overlap (10°+ pitch)
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2 corrugations overlap (5–10° pitch)
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Overlaps face away from the prevailing wind
✔ 4. Poor Ridge or Verge Installation
Leaks often occur at the roof’s edges, not in the middle.
Fix:
Reseal by:
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Fixing ridge/verge every corrugation
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Overlapping ridge pieces by 150mm
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Ensuring verge trims sit tight
✔ 5. Damaged or Cracked Sheets
Occasionally caused by:
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Impact damage
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Overtightened fixings
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Heavy snow load
Fix:
Replace the damaged sheet and re-fix using correct technique.
✔ 6. Missing or Worn Sealing Washers
If washers perish, water will enter around fixings.
Fix:
Remove old fixings and install new sealed fixings with colour-matched caps.
2. How to Find the Source of a Bitumen Roof Leak
Leaks sometimes appear far from where they start. Here’s how to track them down:
Step 1 — Look for Stains or Damp Patches Inside
Follow the wet trail upwards.
If water is running down timber, check the point where it starts.
Step 2 — Check Fixings First
These are the most common leak points.
Look for:
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Loose nails
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Missing caps
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Cracked washers
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Fixings in the valleys
Step 3 — Check Overlaps
Run your hand along the overlap.
If you feel a gap or lifted section, that’s likely your leak.
Step 4 — Inspect the Ridge
Wind-driven rain often enters the top.
Look for:
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Gaps
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Lifting ridge pieces
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Incorrect overlap direction
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Missing fixings
Step 5 — Check the Verge
Outer edges are vulnerable to wind uplift.
Step 6 — Hose Test (Optional)
Spray water gently on small areas and check inside.
Do NOT pressure-wash — it forces water under sheets.
3. How to Fix Each Type of Bitumen Roof Leak
Below are the specific fixes for the most common leak issues.
A) Fixing Leaks Caused by Incorrect Fixing Placement
Symptoms
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Water dripping from fixing holes
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Rusty nails, missing caps, or exposed washers
Solution
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Remove the incorrect fixing
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Fill the hole with a roof sealant compatible with bitumen
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Install a new fixing on the crest
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Ensure the washer compresses properly
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Close the coloured cap
B) Fixing Side Lap Leaks
Symptoms
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Drips along corrugation join
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Damp marks near sheet edges
Solution
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Add extra fixings every 2 corrugations
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Apply bitumen mastic if needed for low pitches
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Ensure overlaps face away from wind
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Confirm overlap is correct (1 or 2 corrugations)
C) Fixing Ridge Leaks
Symptoms
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Drips near highest part of roof
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Wet patches just below ridge line
Solution
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Remove loose or poorly aligned ridge piece
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Reinstall ensuring 150–200mm overlap
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Fix every corrugation on both sides
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Apply bitumen mastic under overlap for extra protection
D) Fixing Verge Leaks
Symptoms
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Water running down side of building
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Damp at inner edges of roof
Solution
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Install or re-align verge trims
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Fix every corrugation
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Ensure no overhanging sheet without support
E) Fixing Cracked or Damaged Sheets
Solution
Replace the sheet.
Bitumen sheets cannot be patched reliably for long-term use.
4. Sealants That Can Help (If Needed)
While bitumen roofs shouldn’t rely on sealants, they can help with repairs.
Compatible Sealants:
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Bitumen mastic
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Rubberised roof repair sealant
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MS polymer sealants
Avoid:
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Silicone (doesn’t adhere properly long-term)
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Acrylic caulk
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Incompatible solvent-based products
5. How to Prevent Leaks in the Future
✔ Use the correct fixing pattern
✔ Fix at every corrugation at eaves and ridge
✔ Maintain the minimum roof pitch (5°)
✔ Use proper sealed fixings
✔ Replace old washers when worn
✔ Keep ridge overlaps large and wind-facing side correct
✔ Don’t overtighten fixings
These simple steps guarantee long-term waterproofing.
6. Frequently Asked Questions
1. Can you walk on a bitumen roof to repair leaks?
No — sheets are not load-bearing.
Use crawl boards if necessary.
2. Will painting the roof help waterproof it?
Not required, but bitumen-compatible coatings can extend lifespan.
3. Can leaks occur years later?
Yes, if washers perish or ridge/verge pieces loosen.
4. Should I seal the sheet overlaps?
Only for very low pitches (5–8°) using bitumen mastic.
5. Is replacing the whole roof necessary?
Rarely — individual sheets or fixings normally solve the issue.
Order Bitumen Roofing Sheets & Accessories
At bitumenroofing.co.uk, we supply:
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Corrugated bitumen roof sheets
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Colour-matched fixings
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Ridge pieces & verge trims
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Eaves fillers
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Repair sealants
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Fast UK delivery
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Cheap prices
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Unbeatable service
👉 Shop Bitumen Roofing Sheets
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