Rear Extension Roof

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Stay Safe & Don't Worry About Exact Numbers! There is no need to climb your roof. Simply estimate the dimensions to your best knowledge. I will personally take precise measurements during a free site visit, and the final price will be calculated using the exact same transparent formula.

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The highly recommended choice for long-term peace of mind. I will install a heavy-duty underlay followed by a premium charcoal mineral top sheet. Welding these two layers together creates a thick, impenetrable shield against the harsh British weather. This system is designed to last at least 15 years, keeping your garage bone-dry and secure.
Tecnatorch SBS Torch-On Mineral Felt Charcoal
Tecnatorch SBS Torch-On Mineral Felt Charcoal

Included in Price

Tecnatorch SBS Torch-On Plain Capsheet Sanded
Tecnatorch SBS Torch-On Plain Capsheet Sanded
Tecnatorch SBS

This SBS-modified sanded felt is the core component that upgrades your roof to a high-performance dual-layer system. It provides a robust, elasticated base that perfectly bonds with the final charcoal cap sheet for maximum strength. This foundation layer allows the roof to expand and contract safely during UK weather extremes, preventing cracks and leaks. Choosing this system ensures the highest level of durability and long-term peace of mind.

Included
Felt drip edge (4 sides Hardboard)
Felt drip edge (4 sides Hardboard)

Installing a hardboard trim around the entire roof perimeter to create sharp, rigid drip edges for the new felt. This solid backing allows the felt to be formed into a neat overhang, rather than a weak curve. It effectively directs rainwater away from the fascia boards and stops water from tracking back underneath the roof. This ensures a clean, professional finish and full perimeter protection.

Included
Bitumen Primer: Material Application
Bitumen Primer: Material Application
Cromar

Applying a bitumen primer is a crucial preparation step before installing the new waterproofing system. This liquid coating seals the existing substrate and locks in any remaining dust. Its main purpose is to create the perfect conditions for the first layer of felt to adhere flawlessly. This guarantees a strong, permanent bond between the treated surface and the felt.

Included
Ladder Access Work (Scaffolding Not Included)
Ladder Access Work (Scaffolding Not Included)

This service includes access using standard ladders or a hook ladder. Many repairs can be carried out efficiently without the need for scaffolding, saving both time and cost. However, safety is always our priority. If site conditions do not allow safe ladder use (e.g. limited space or difficult access), scaffolding may be required and is not included in this price.

Included

Optional Add-ons

This service involves a complete restoration of the existing lead flashing. First, the surface is mechanically cleaned using a wire brush to remove oxidation and debris. We then apply Lead Patination Oil, which protects the metal from weathering and prevents white carbonate staining on your roof. Finally, the brickwork joint is cleaned and resealed using a high-quality specialist lead sealant (silicone) to ensure a long-lasting, watertight finish.

Installation of new Code 4 lead flashing (150mm) to create a secure, watertight seal where the roof meets the brickwork. We professionally fit the lead into the masonry joints and apply Lead Patination Oil to the surface. This protective coating prevents natural oxidation and stops white carbonate streaks from staining your new roof. The joints are finished with high-quality specialist lead sealant, which stays flexible and handles temperature changes better than standard silicone. This method ensures a long-lasting lifespan and reliable protection against the weather.

A surcharge for removing old protective stone chippings from the existing roofing felt. Years of weather exposure cause these stones to melt and bond into a solid crust with the underlying bitumen. Removing them requires intensive physical labour to chip and scrape the surface clean. This crucial step ensures the new roof adheres properly.

Replace weakened or failing garage roof joists to ensure the entire structure remains stable and secure. This is necessary when existing timbers are soft from rot, bowing under weight, or compromised by pests. We provide and install new, high-quality timber joists up to 4 metres long. Simply specify the number of joists required; the price is calculated per unit for a complete installation.

Essential when your roof deck is rotten or sagging. We strip all old felt and timber boards back to the bare joists, creating a clean base for reconstruction. Price includes all labour, professional skip hire delivered to your site, and licensed waste disposal fees. No hidden costs for rubbish clearance. Note: This covers removal only; please also add "New OSB Deck Installation" to your estimate to include the new structural boards.

We create a rock-solid foundation using premium 18mm OSB3 moisture-resistant boards, specifically engineered for structural use in the British climate (BS EN 300 standard). These high-performance boards offer superior strength and durability compared to standard chipboard, ensuring a perfectly flat and stable base for your new roof. Each sheet is precision-fitted and securely fastened to the joists to prevent future movement. Note: This service covers new material and installation only.

15 Yrs Warranty 25 Yrs Lifespan

Preparation and Vapour Barrier: A functional vapour barrier is the most critical step. It stops warm, moist indoor air from rising into the insulation, where it would otherwise condense on the colder upper layers of the roof and cause hidden moisture or rot. In many cases, your existing roofing felt can serve as a reliable vapour barrier if it is in good condition. Insulation Laying and Fixing: We install the 100mm insulation boards onto the prepared base. To maintain the integrity of the vapour barrier underneath, we secure the boards using specialized expanding roofing foam. Mechanical fixings are only used in extreme conditions, such as on very tall buildings or highly exposed coastal sites. Edge Reinforcement (Included in Price): Adding insulation raises the overall roof height by 100mm. Therefore, we install new structural timber rafters of the same thickness around the entire perimeter to reinforce the edges. These timbers and their installation are fully included in the price of this item. Fascia Board Adjustment (Not Included): Please note that because the roof profile is raised, new and deeper uPVC fascia boards are almost always required to aesthetically and functionally cover the new height. These boards are not included in this specific item and are quoted separately. Final Waterproofing: Once the insulation is secured, a new protective system is torch-applied directly on top. This consists of a heavy-duty underlay and a final mineral cap sheet that protects the roof from UV radiation.

Installing a hardboard trim around the entire roof perimeter to create sharp, rigid drip edges for the new felt. This solid backing allows the felt to be formed into a neat overhang, rather than a weak curve. It effectively directs rainwater away from the fascia boards and stops water from tracking back underneath the roof. This ensures a clean, professional finish and full perimeter protection.

Cromar

Applying a bitumen primer is a crucial preparation step before installing the new waterproofing system. This liquid coating seals the existing substrate and locks in any remaining dust. Its main purpose is to create the perfect conditions for the first layer of felt to adhere flawlessly. This guarantees a strong, permanent bond between the treated surface and the felt.

This service includes access using standard ladders or a hook ladder. Many repairs can be carried out efficiently without the need for scaffolding, saving both time and cost. However, safety is always our priority. If site conditions do not allow safe ladder use (e.g. limited space or difficult access), scaffolding may be required and is not included in this price.

A surcharge for removing old protective stone chippings from the existing roofing felt. Years of weather exposure cause these stones to melt and bond into a solid crust with the underlying bitumen. Removing them requires intensive physical labour to chip and scrape the surface clean. This crucial step ensures the new roof adheres properly.

Installation of new Code 4 lead flashing (150mm) to create a secure, watertight seal where the roof meets the brickwork. We professionally fit the lead into the masonry joints and apply Lead Patination Oil to the surface. This protective coating prevents natural oxidation and stops white carbonate streaks from staining your new roof. The joints are finished with high-quality specialist lead sealant, which stays flexible and handles temperature changes better than standard silicone. This method ensures a long-lasting lifespan and reliable protection against the weather.

This service involves a complete restoration of the existing lead flashing. First, the surface is mechanically cleaned using a wire brush to remove oxidation and debris. We then apply Lead Patination Oil, which protects the metal from weathering and prevents white carbonate staining on your roof. Finally, the brickwork joint is cleaned and resealed using a high-quality specialist lead sealant (silicone) to ensure a long-lasting, watertight finish.

Estimate Summary

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Rear Extension Flat Roof Cost Calculator | Expert Installation in Sussex

Planning a new kitchen-diner or noticed a leak in your existing back extension? The roof of a rear extension is one of the most critical structural elements of your home, protecting what is often the most used living space. My instant online calculator provides a transparent, itemised estimate for professional flat roof replacement or installation across Brighton, Hove, Worthing, and Sussex. With 18 years of specialized experience, I ensure your extension remains dry, warm, and structurally sound.

Technical Excellence for Extension Roofs

Rear extensions present unique technical challenges, particularly where the new flat roof meets the original masonry of the main house. I use high-performance SBS-modified bitumen torch-on felt systems, which are engineered for maximum flexibility. Unlike cheaper alternatives, these membranes safely accommodate the natural thermal expansion and contraction of your extension without developing fatigue cracks at the wall abutments.

The “Warm Roof” Standard: 100mm Thermal Insulation

Most modern extensions serve as primary living areas, making thermal efficiency a priority. Upgrading to a 100mm “Warm Roof” system is the most effective way to meet building regulations and reduce energy costs. My technical approach includes:

  • Integrated Vapour Control: I install a high-performance vapour barrier to prevent moisture from your home (especially from kitchens) from condensing within the roof structure, eliminating the risk of interstitial rot.
  • Structural Decking: We use 18mm OSB3 moisture-resistant structural boards to provide a rigid, flat base that prevents water pooling.
  • Lead Flashing & Weatherproofing: We ensure a watertight seal at the join between the extension and the main house using Code 4 lead flashing, finished with patination oil to prevent carbonate staining.

Full Transparency via Your Client Portal

I don’t just promise quality; I prove it. When you choose me for your rear extension project, you gain access to a Secure Client Portal. This tool is designed to give you absolute peace of mind through every stage of the build:

  • Real-Time Photo Logs: I document the entire process, from the bare joists to the final mineral cap sheet. You will see the quality of the hidden layers (insulation and underlay) that most contractors don’t show.
  • Live Progress Tracking: Monitor exactly where we are on the project timeline directly from your smartphone.
  • Centralised Documentation: Access your detailed estimates, invoices, and long-term warranties in one secure digital location.

Why Choose a Specialist?

A rear extension roof is more than just a cover; it’s a complex system involving drainage, thermal barriers, and structural integrity. Leveraging 18 years of industry experience, I provide solutions that prioritize longevity over the “quickest fix.” By using the calculator above, you can accurately estimate the cost of a high-spec system tailored to your property’s specific dimensions.

Ready for a precise quote? Enter your extension dimensions above and select your preferred configuration. I will personally verify all measurements during a free site visit to ensure the final price is accurate and fair.