Look upwards inside many UK lofts and you will see a dark sheet running behind the roof battens and tiles.
Homeowners often call it roofing felt, although newer roofs may use a modern roof underlay or membrane instead.
It is an important part of the roof build-up, but its job is very different from the insulation sitting lower down in the loft.
Quick answer: The felt or underlay beneath roof tiles forms part of the roof’s weather-management system. It provides a secondary layer beneath the main roof covering, helping manage moisture that gets past the tiles or forms within the roof build-up. It is not the loft insulation and should not be treated as one.
A pitched roof is made from several layers
From inside the loft, it is easy to think of the roof as simply timber on one side and tiles on the other.
In reality, several components work together.
Tiles or slates
The primary external roof covering that receives rain, sunlight and wind.
Battens
Horizontal timber sections that support and position the roof covering.
Underlay or felt
A secondary weather-management layer beneath the tiles or slates.
Rafters
The main sloping structural timbers supporting the pitched roof.
Ventilation
Some roof constructions use intentional airflow to help manage moisture within the roof space.
Insulation
The thermal layer that resists heat transfer between occupied rooms and the spaces around them.
The tiles are the first line of weather protection
Roof tiles and slates are arranged so rainwater is directed down the roof and towards the gutters.
They are the primary weather-facing layer.
But pitched roofs are not usually designed on the assumption that absolutely nothing will ever find its way beneath the outer covering.
Wind-driven moisture, small amounts of water or condensation within parts of the roof build-up may still need to be managed.
This is where the underlay comes in
The felt or membrane beneath the tiles provides a secondary layer within the roof construction.
It can help direct incidental moisture downwards rather than allowing it to fall directly into the loft.
Its presence also helps protect the roof space during construction and forms part of the complete weather-management strategy.
Key point: Roofing felt or underlay is part of the roof’s weather-management system, not the thermal insulation layer.
Why do older roofs often have black felt?
Many older UK roofs use traditional bituminous roofing felt beneath the tiles.
From inside the loft, this often looks dark, relatively heavy and sometimes slightly brittle with age.
Newer roof constructions may use lighter modern membranes instead.
The appearance can therefore give some indication of the type of underlay present, but it does not by itself tell you whether the roof is performing correctly.
Modern membranes can behave differently
Modern roofing underlays can have different vapour and airflow characteristics from older traditional felt.
That means the ventilation strategy suitable for one roof is not automatically the same as another.
Our guide to the types of loft ventilation explains why roof construction should be understood before ventilation changes are made.
Why does roofing felt overlap?
Look carefully from inside some roofs and you can see horizontal overlaps where one sheet of underlay passes over another.
Those overlaps allow water reaching the underlay to continue travelling downwards towards the eaves rather than being directed behind the sheet below.
It is similar in principle to the way roof tiles themselves overlap.
Those overlaps can also be useful for ventilation in some roofs
In suitable traditional felted roofs, lap vents can be inserted at particular underlay overlaps to create additional airflow into the roof space.
They are designed for specific situations rather than being something every roof automatically needs.
If moisture is present in a loft, the cause and existing ventilation arrangement should be understood before deciding that more airflow is the answer.
Roofing felt is not insulation
This is probably the most important distinction.
Felt sits much closer to the external roof covering and is primarily involved with weather and moisture management.
In a conventional cold loft, the thermal layer usually sits across the ceiling level much further below.
These two layers therefore occupy different parts of the roof and perform different jobs.
What does the insulation do instead?
Suitable traditional loft insulation resists heat transfer between the rooms below and the unheated roof space above.
It does not waterproof the roof.
If water is entering through a roof defect, adding more insulation does not fix the weatherproofing problem.
Likewise, replacing roofing felt does not replace the need for an appropriate thermal layer.
Why can you sometimes see the back of the tiles?
Some older roofs may not contain an underlay at all, meaning the underside of the tiles or slates can be visible directly from inside the loft.
That does not automatically mean somebody has removed the felt.
The roof may simply have been built using a different traditional construction.
Again, the age and design of the roof matter.
What if the felt is torn?
A small tear or damaged section deserves context rather than panic.
The importance depends on:
- where the damage is located
- whether water ingress is occurring
- the condition of the outer roof covering
- the extent of the damage
- the wider roof construction
A roof underlay is only one part of the complete roof.
Visible damage should therefore be assessed alongside the tiles, flashings and any evidence of water entry.
What if there is moisture on the underside?
Moisture on roofing felt is not automatically a leak.
Condensation can form when moisture-containing air reaches sufficiently cold surfaces within the roof space.
On the other hand, a highly localised wet patch appearing after rainfall may suggest a different source.
The location, timing and pattern help distinguish between possibilities.
| Roof element | Its main job | What it does not do |
|---|---|---|
| Tiles or slates | Primary external weather covering | Provide the main thermal insulation layer |
| Roofing felt or membrane | Secondary weather and moisture management | Replace loft insulation |
| Roof ventilation | Provide appropriate airflow where the construction requires it | Repair a leaking roof covering |
| Loft insulation | Resist heat transfer across the thermal boundary | Waterproof the roof |
| Gutters and downpipes | Carry collected rainwater away from the roof | Control loft temperature |
Each layer should be allowed to do its own job
Problems often become confusing when one roof component is expected to solve another component’s problem.
For example:
- more insulation does not repair torn flashing
- more ventilation does not repair a leaking tile
- roofing felt does not replace insulation
- insulation should not block ventilation routes the roof needs
The roof works because those separate roles complement one another.
If you are looking at the felt beneath your roof tiles, consider:
- Whether it appears to be older felt or a modern membrane
- Whether the material is broadly continuous
- Whether any visible damage is local or extensive
- Whether staining appears after rainfall
- Whether moisture is widespread or concentrated in one area
- How the roof space is ventilated
- Whether lap vents are already present
- Whether the insulation remains separate from required airflow routes
The felt is one layer in a much bigger system
Looking up from a loft hatch, the roofing underlay can appear to be the inside surface of the roof.
In reality, it sits between several other components, each doing something different.
The tiles manage the weather outside.
The underlay provides a secondary layer beneath them.
Ventilation manages airflow where required.
And insulation provides the thermal boundary much lower down.
Understanding that arrangement also helps explain another thing homeowners often notice around the roof edge: visible daylight.
Frequently asked questions
What is the felt under roof tiles for?
Roofing felt or underlay provides a secondary weather-management layer beneath the main tiles or slates and forms part of the overall roof construction.
Is roofing felt the same as insulation?
No. Felt is primarily part of the roof’s weather-management system, while insulation is used to resist heat transfer through the thermal envelope.
Why does roofing felt have overlapping sections?
The sheets overlap so incidental moisture can continue travelling down the roof rather than passing behind the layer below.
Can lap vents be fitted into roofing felt?
In suitable felted roof constructions, lap vents can be fitted at overlaps to provide additional airflow. They are not automatically appropriate for every roof.
Does torn roofing felt mean the roof is leaking?
Not necessarily. The significance depends on the extent and location of the damage, the condition of the main roof covering and whether there is evidence of actual water ingress.
In the next article: Look towards the lower edge of the roof and you may notice small areas of daylight. Sometimes that is exactly what the roof needs — and sometimes it deserves a closer look.
