If you’re planning to install corrugated bitumen roof sheets, one of the most common questions is whether you need roofing felt underneath. The short answer is: in most cases, no—but there are some important exceptions.
In this guide, we explain when felt is needed, when it isn’t, and what you should consider before starting your roofing project.
What Is Roofing Felt Used For?
Roofing felt is traditionally used as:
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A secondary weatherproof layer
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Protection against condensation
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A backup barrier if the main roof covering fails
It’s commonly used under tiles, slates, and some metal roofing systems.
Do Corrugated Bitumen Roof Sheets Need Felt?
In most cases: NO
Corrugated bitumen sheets are designed to be:
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Fully weatherproof on their own
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Installed directly onto timber roof battens or decking
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Overlapped to shed water effectively
Because of this, roofing felt is not usually required underneath.
Why Felt Is Often Unnecessary
1. Bitumen Sheets Are Waterproof
Corrugated bitumen sheets are manufactured from bitumen-saturated materials, making them naturally resistant to rain and moisture.
2. Ventilation Is Better Without Felt
Installing felt underneath can restrict airflow, which may:
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Trap moisture
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Increase condensation risk
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Reduce the lifespan of timber battens
3. Designed for Simple Installation
Most bitumen roofing systems are intended to be:
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Lightweight
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Easy to install
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Fixed directly to the roof structure
Adding felt can complicate installation without providing much benefit.
When Might You Use Felt Under Bitumen Sheets?
Although it’s not normally required, there are situations where felt can be useful.
1. Heated or Insulated Buildings
If the roof is over a heated space (such as a workshop or garden room), felt can help provide an extra moisture barrier when combined with proper ventilation.
2. Low-Pitch Roofs
On roofs with a very shallow pitch, felt can act as a secondary line of defence in extreme weather.
3. Fully Boarded Roof Decks
If your roof has solid timber decking rather than battens, a breathable membrane or felt may help protect the structure.
4. High-Exposure Locations
In areas with:
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Heavy driving rain
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Strong winds
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Coastal conditions
a secondary layer can offer added reassurance.
Felt vs Breather Membrane
If you do decide to add an underlayer, consider this carefully:
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Traditional felt
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Less breathable
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Can trap condensation if poorly ventilated
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Breathable membrane
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Allows moisture to escape
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Reduces condensation risk
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Often a better modern alternative
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For most projects, a breather membrane is preferable to standard felt.
What’s More Important Than Felt?
Instead of adding felt, focus on these key factors:
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Correct roof pitch (as per manufacturer guidelines)
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Proper sheet overlaps
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Using the correct bitumen roofing fixings
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Adequate ventilation
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Secure fixing to prevent wind lift
These have a much bigger impact on roof performance than adding felt.
Common Mistakes to Avoid
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Installing felt with no ventilation gaps
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Using felt to compensate for insufficient pitch
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Fixing sheets too tightly, restricting movement
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Skipping overlaps to save material
Final Answer: Do You Need Felt?
For most sheds, garages, outbuildings, and agricultural structures:
👉 No, you do not need felt under corrugated bitumen roof sheets.
For insulated, low-pitch, or high-exposure roofs:
👉 A breathable membrane may be beneficial, but it’s still not always essential.
Conclusion
Corrugated bitumen roofing is designed to be a simple, effective, and durable solution without the need for felt underneath. In most cases, correct installation matters far more than adding extra layers.
At bitumenroofing.co.uk, we supply corrugated bitumen roof sheets and fixings suitable for a wide range of projects. If you’re unsure whether your roof needs an underlayer, it’s always best to consider roof pitch, building use, and ventilation before deciding.
