What Is the Purpose of Felt Under Roof Tiles: Ensuring Durability and Protection

Felt under-roof tiles serve as an essential layer of protection for residential and commercial properties. It acts as a secondary barrier against moisture, preventing water ingress that could lead to structural damage and damp issues within a property.

While roof tiles are designed to shed water effectively, the underlay provides additional defence by helping to channel any water that does get through the tiles safely away from the roof structure and into the gutter system.

The placement of felt under roof tiles also plays a critical role in the longevity of the roof. By defending against the elements, this felt helps to maintain the integrity of the roof deck and reduce the likelihood of rot and mould growth.

Moreover, it provides an extra layer of insulation, contributing to thermal efficiency by helping to keep heat within a home during colder months.

In areas with frequent high winds, the felt under the tiles can prevent wind-driven rain and snow from penetrating the roof space. It also supports the tiles, helping to distribute the load evenly across the roof structure, which can prevent sagging and warping over time.

The felt ensures that even if a tile breaks or becomes dislodged, the roof is still protected, thus maintaining the security and comfort of the building’s occupants.

Importance of Roof Felt Under Tiles

Roofing felt serves as an integral part of a roof’s structure, primarily offering enhanced protection and longevity to the roofing system. This component acts as a secondary defence layer beneath the tiles, which is crucial in safeguarding the roof.

Protection from Water Damage

Roof felt, also known as a moisture barrier, plays a pivotal role in preventing water ingress into a building’s interior. It adds a protective layer that lies immediately beneath the roof tiles, thereby ensuring a supplementary shield against water entry.

Even if a tile is damaged or displaced, the felt restricts water from reaching the roof deck and causing potential damage to the structure.

  • Key Functions:
    • Acts as a secondary waterproof barrier: In the event of tile breakage.
    • Prevents wind-driven rain entry: Coverage under the tiles minimises moisture ingress.

Improving Roof Longevity and Performance

The inclusion of roofing felt under tiles significantly contributes to the overall longevity and performance of the roof. The felt acts as a stabilising layer which helps maintain the position of tiles and, in some instances, provides an additional layer of insulation contributing to thermal efficiency.

  • Performance Enhancements:
    • Stabilises roof tiles: Helps to keep tiles securely in place.
    • Insulates the roof: Adds to the thermal efficiency by minimising heat transfer.

Types of Roof Underlayment

Roof membranes are a critical layer of material placed under roof tiles to provide a secondary barrier against the elements. It is typically available as standard felt or synthetic options, with variations in breathability.

Traditional Felt Versus Synthetic Underlayments

under-roof generally made from organic materials and saturated with asphalt, has been the standard for decades. This type of underlayment is valued for its reliability and cost-effectiveness.

It typically comes in two thicknesses: 15-pound and 30-pound felt, with the latter being more durable.

Synthetic underlayment is a newer innovation made from woven or spun polyethylene or polypropylene. It is lighter and stronger than traditional felt and often incorporates additional features such as improved resistance to tearing and water penetration.

Synthetics can inhibit mould growth and last longer than felt, making it a popular choice for many builders.

Breathable Versus Non-Breathable Membranes

Breathable membranes allow for ventilation, enabling moisture from inside the building to escape while still protecting against external water penetration. This can greatly reduce the risk of condensation within the roof framework, thus protecting the longevity of the roof.

Contrastingly, non-breathable underlays do not permit air movement through the membrane. They require adequate venting to be included in the roof design to prevent condensation.

Non-breathable underlays are generally more water-resistant and can be an advisable choice in regions with high rainfall.

Installation and Maintenance of Roof Felt

Roof felt serves as a protective layer, and its proper installation and maintenance are essential for the longevity of a roof framework. Ensuring correct procedures and recognising wear signs can prevent costly repairs for the homeowner.

Effective Installation Techniques

The process of installing roof felt commences by selecting high-quality materials that comply with British Standards.

Roofers should adhere to the following steps to ensure effective installation:

  1. Prepare the Roof: Clean the roof surface of all debris and check for any structural issues that may affect the felt application.
  2. Measure and Cut: Measure the roof and cut the felt to size, allowing for adequate overlaps between each strip.
  3. Apply the Felt: Start from the bottom of the roof, laying the first felt strip parallel to the eaves. Secure it with clout nails spaced at regular intervals.
  4. Seam Sealing: Overlap subsequent felt strips by at least 100mm and seal the seams securely to prevent water penetration.

Consistency in nail placement and overlap precision are crucial to prevent future leakage or felt degradation.

Signs That Your Roof Felt Needs Replacing

It is vital for homeowners to recognise when the roof felt needs replacing to protect their property from water damage.

  • Visual Inspection: Look for tears, bald patches on the felt, or an excess of daylight visible from the loft, this will require checking under the slate roof.
  • Moisture Damage Signs: Brown stains or drips on the underside of the roof or in the attic space.
  • Sagging or Bulging: Indicates trapped moisture and deterioration of felt which can’t be ignored.

Regular checks, especially after severe weather conditions, can help in identifying any issues promptly, aiding in timely maintenance and avoiding large-scale roof replacement.

Choosing the Right Underlayment for Your Project

Selecting the appropriate underlayment is crucial for the longevity and performance of your roof. It acts as a safeguard against water intrusion and enhances the overall effectiveness of the roofing products.

Factors Influencing Underlayment Selection

The type of roofing is a significant factor when choosing underlayment. Materials such as clay tiles, or shingles each have compatible underlayment types that ensure optimal performance and durability.

For instance:

  • Asphalt shingles: Synthetic or felt underlayment
  • Metal roofing: Synthetic with noise-reducing properties
  • Clay tiles: High-grade felt or self-adhering membrane

Another aspect to consider is the climate where the roofing will be installed. In areas with high rainfall or snow, a waterproof or water-resistant underlayment can provide added protection.

The roof pitch also plays a vital role. Steeper roofs may require different underlayment than low-sloped roofs due to the varying water runoff rates.

Local building codes often specify certain types of underlayment to conform with regional safety and performance standards, which must not be overlooked during selection.

Consulting with Roofing Professionals

Before finalising your underlayment choice, getting in touch with roofing professionals is advisable.

They offer a wealth of knowledge and can recommend the underlayment that is right for your project, considering the specific roofing material and local environmental conditions.

A roofing expert will consider aspects such as:

  • Compatibility with the chosen roofing products.
  • Installation requirements, since some underlayments may require special installation techniques.
  • Warranty implications, as some roofing product warranties might be void if an incompatible underlayment is used.

Roofing experts are also up-to-date with current trends in roofing underlay, including new materials and technologies that could benefit your project.

It is essential to ensure that the professional’s qualifications and experience align with the type of roofing project at hand.

Compliance and Standards in Roofing

Within the domain of roofing, adherence to regulatory standards ensures that the quality and safety of materials employed contribute to the longevity of the structure. Compliance with building codes is non-negotiable for property owners and construction professionals.

Building Regulations for Roofing Materials

Building regulations in the UK mandate certain criteria that roofing products must meet to be deemed suitable for use.

Quality is closely scrutinised to ensure that roofing tiles, including those laid over felt, maintain resilience over time.

Roofing products are expected to conform to standards such as BS 5534, which outlines specifications for slating and tiling. These regulations cover aspects like:

  • Load-bearing capacity
  • Durability against weather conditions
  • Fire resistance

Material selection ties directly to the lifespan of a roof, a key factor in the overall integrity of a building.

Ensuring Quality and Safety

Maintaining safety is paramount in roofing.

Regulations are in place to ensure that risks to both the installers and inhabitants are minimised. Safety considerations include:

  • Slip resistance of tiles and felt underlay
  • Stability and resistance to wind uplift
  • Adequacy of ventilation
  • Thermal barrier properties

Materials used must also pass strict quality control measures.

Organisations such as the National Federation of Roofing Contractors (NFRC) provide guidelines and support to roofing experts. They promote high standards within the industry.

Regular audits and inspections by accredited bodies act as a mechanism to uphold these standards. They also support the long-term safety and performance of roofing systems.


In conclusion, the integrity and longevity of a roof hinge upon careful selection, installation, and maintenance of roofing products.

Recognising signs of wear in the roof felt, choosing the appropriate underlayment for specific roofing types and environmental conditions, and adhering to regulatory standards are crucial components.

By consulting with roofing experts and adhering to building guidelines, property owners can ensure the durability, safety, and performance of their roofing systems.

Regular inspections and timely replacement of worn materials are vital practices that help protect the property from potential moisture damage and structural deterioration.

Frequently Asked Questions

Q: What is the purpose of felt under tiles?

A: Roofing felt, also known as roofing membrane, is a crucial moisture-proof material used as a vapour barrier to protect your home from water infiltration. It provides an extra layer of protection between the roof covering and the roof framework.

Q: Why is roofing felt replaced when getting a new roof?

A: Roofing felt is often replaced when getting a new roof to ensure that the underlayment is in good condition and can continue to serve as an effective vapour barrier. This helps maintain the integrity of the roof and protects the home from leaks.

Q: How do I know if felt under my tiles needs to be replaced?

A: Signs that the roofing felt under your tiles needs replacement include water leaks, visible damage or wear on the material, or if a roofer recommends it during a roof inspection.

Q: Is roofing felt the same as tar paper?

A: Yes, roofing felt is sometimes referred to as tar paper. It is a bitumen-saturated paper material used as an underlayment in roofing applications to provide weatherproofing and protection.

Q: What material is used in roofing underlay?

A: The roofing industry commonly uses materials such as bitumen membranes or breathable underlays for roofing underlayment. These materials serve as vapour barriers and help protect the roof framework.

Q: How does felt underlayment protect your home?

A: Felt underlayment acts as a waterproof barrier, preventing water from seeping into the roof framework. It helps protect the home from moisture damage, leaks, and other issues that can arise from water infiltration.

Q: When is felt underlay right for your project?

A: Felt underlay is suitable for various roofing projects, including those involving flat roofs, roof shingles, slates or tiles, and chimney flashing. Consulting with a professional roofer can help determine if felt underlay is the best option for your specific project.