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Do I Have to Extend my Roof for EWI?

Last modified: 17.10.2023
A construction worker in high-visibility clothing and a helmet inspecting or installing uPVC soffit boards under a roof overhang. Proper soffit and fascia installation plays a key role in ventilation, moisture protection, and neat finishing on EWI systems.

External Wall Insulation (EWI) is an excellent method for increasing the thermal efficiency of a building. However, one common concern that arises when contemplating EWI is the potential for the insulation to increase the thickness of external walls. The question then arises, do you need to extend your roof to accommodate this additional thickness? Let's delve into the details.

When considering External Wall Insulation (EWI), it's imperative to grasp the extent of thickness it will introduce to your external walls. The added thickness largely depends on the insulation material used, the specific requirements of the property, and the desired energy efficiency outcomes.

  1. Insulation Materials: The choice of insulation material plays a pivotal role in determining thickness. Common materials include:
    • Expanded Polystyrene (EPS): A lightweight and cost-effective option, EPS typically ranges from 50mm to 100mm in thickness.
    • Mineral Wool: Known for its excellent fire resistance and acoustic properties, mineral wool insulation often ranges between 80mm and 120mm.
    • Phenolic Foam: This material offers high thermal performance in thinner profiles, usually between 40mm and 80mm.
  2. Property Requirements: The age, construction type, and current energy efficiency of your property can influence the required EWI thickness. For instance:
    • Older properties with solid walls may benefit from thicker insulation to combat heat loss.
    • Modern properties built with cavities might not require as much added thickness, as they typically have some level of existing insulation.
  3. Desired Energy Efficiency: Your target U-value (a measure of thermal transmittance) will also impact the insulation thickness. Aiming for a lower U-value, indicating better insulation, may necessitate a thicker insulation layer.

It's also worth noting that while increasing the thickness can enhance thermal performance, it might also introduce challenges in terms of aesthetics and structural considerations. Therefore, striking a balance between performance and practicality is crucial.

When adding a substantial thickness to your walls with EWI, the alignment and aesthetic considerations might necessitate an extension of your roofline. Here's a deeper look into what this entails:

  1. Importance of Soffits: Soffits, the underside of the roof overhang, play a crucial role in both the appearance and functionality of a building. When EWI is applied, the wall's outer edge moves outwards, making the soffits appear recessed. Extending the soffits ensures the building retains a balanced and pleasing aesthetic.
  2. Technical Considerations: Extending the roofline isn't just about aesthetics:
    • Water Run-off: Ensuring that rainwater runs off the roof and away from the walls is vital. Without proper extension, there's a risk of water dripping down the insulation, which can lead to dampness or damage over time.
    • Ventilation: Soffits often house ventilation points for the property. When extending the roofline, it's essential to maintain or even enhance this ventilation to prevent moisture build-up in the roof space.
  3. Materials and Styles: The materials and style chosen for the extended roofline should match or complement the existing architecture. Common materials include uPVC, timber, and aluminium. Each has its benefits in terms of durability, maintenance, and appearance.
  4. Installation Process: Extending the roofline may involve:
    • Removing the Existing Fascias and Soffits: This allows for a thorough inspection of the roof edge and ensures that the new installation fits perfectly.
    • Adjusting the Roof Battens: This might be necessary to ensure the new soffits and fascias align correctly with the EWI.
    • Installing the Extended Soffits: This is done in a manner that ensures a tight seal against weather elements, with attention to detail to achieve a seamless look.
    • Sealing and Finishing: Once installed, the extended roofline is sealed to prevent water ingress and finished to match the property's aesthetic.
A close-up of soffit panels being slotted into position under a roof eave. These vented panels help maintain airflow in the roof space and are commonly used in combination with external wall insulation systems.

If extending the roofline seems like an extensive or impractical task for your property, verge trims present an alternative solution. These are designed to seal the edge of the roof and the top of the EWI system, providing a protective and aesthetically pleasing finish.

    • Protection: They protect the insulation, especially at the critical roof-wall junction, from weather elements.
    • Aesthetics: Verge trims offer a neat finish, ensuring a consistent look between the roof and the wall.
    • Cost-Effective: They can be more economical than a full roofline extension, especially for properties where the EWI thickness is moderate.
Dropdown Verge Trims:

These are designed to drop down from the edge of the roof, covering the top part of the EWI system. They provide a seamless transition between the roof tiles and the insulation. Dropdown verge trims are particularly useful when there is a significant thickness of insulation added. They can cover and protect this exposed edge effectively. Upstand Verge Trims: Upstand trims stand upright at the junction between the roof and the wall. They act as a barrier, preventing water from seeping behind the EWI system. These trims are ideal for properties where the insulation thickness is minimal, and you want a trim that subtly blends with the façade. Over Sills: These are specifically designed for windowsills that have been affected by the addition of EWI. Over sills sit on top of the existing sill, extending it outwards to align with the new insulation thickness. They ensure that rainwater drips away from the wall, preventing water ingress and potential damage. Verge trims are typically made of durable materials like aluminium or uPVC. They are designed to withstand external elements and require minimal maintenance. Installing verge trims involves fixing them securely at the roof-wall junction. You must ensure they align correctly with both the roof tiles and the EWI system. Proper sealing is essential to prevent water ingress and ensure longevity. It's crucial to choose verge trims that match or complement your property's aesthetics. While function is vital, the visual appeal shouldn't be compromised. Proper installation is key. While some homeowners might consider it a DIY task, it's advisable to engage professionals, especially when dealing with the complexities introduced by EWI. An installer using a power tool to cut a white plastic or aluminium reveal bead, trim, or soffit section. This kind of profile is often used around windows, doors, or roof eaves in render and insulation systems to create clean, watertight finishes.

From an aesthetic standpoint, extending the roofline offers a seamless and harmonious appearance, staying true to the property's original design. It ensures that the transition between the roof and the walls remains fluid, making it a preferable option for those who prioritise design continuity. Conversely, verge trims, while providing a tidy finish, introduce a distinct element to the property's facade. Their compatibility with the existing design largely depends on the chosen style and material. Protection and durability are paramount when introducing any structural changes. An extended roofline stands out as a comprehensive solution, especially in directing water runoff away from the walls, thereby minimising potential water damage. Verge trims, though effective at the crucial roof-wall junction, vary in efficacy based on their type and the insulation's thickness. Cost and time are practical considerations that cannot be overlooked. Extending the roofline is undoubtedly more labour-intensive and costly, often involving structural nuances that prolong the project. In contrast, verge trims emerge as a more budget-friendly and expedient solution, causing minimal disruption during installation. Furthermore, verge trims offer an added advantage of flexibility. Depending on the property's specific requirements, homeowners can choose between dropdown, upstand, and oversills, ensuring a tailored fit. Maintenance-wise, both options stand on relatively equal ground. While extended rooflines require little upkeep post-installation, verge trims, crafted from durable materials like aluminium or uPVC, demand minimal maintenance but benefit from periodic seal checks.