Steel roofing sheets can be installed over your building’s existing tiles, but only if the roof is structurally sound and the installation is carried out correctly.
This method is called over-sheeting and is used to save time and reduce costs. It can be suitable for agricultural buildings, garages, outbuildings and a range of other property and building types, but it’s not always the right solution. In this article, we’ll explain when over-sheeting works, what to check before starting installation and when it’s better to remove the old roof entirely.
What is Over-Sheeting?
Over-sheeting is the process of installing new roofing sheets over an existing roof without removing the original covering. In the case of steel roofing, this means fixing steel sheets above old tiles using a supporting framework such as a timber batten system or lightweight metal purlins. The framework is fixed through the tiles into the rafters. It’s commonly used to extend the life of a roof, reduce installation time and avoid the cost and disruption of a full strip-and-replace job.
When Is Over-Sheeting a Viable Option?
Steel sheets can be fitted over tiled roofs if the tiles are dry, secure and free from damage. The roof structure beneath must be strong enough to support the added weight of the steel sheets and any supporting battens or framework.
This approach is common on farms and industrial sites where speed and cost-efficiency are important. It can also work for domestic buildings, but only after a proper assessment and checks on planning and Building Regulations.
What Are the Benefits of Installing Over Existing Tiles?
Over-sheeting offers practical advantages for those looking to upgrade a roof without the disruption of a full replacement. By installing steel roofing sheets over existing tiles, you can streamline the process and reduce costs, provided the roof is in suitable condition.
- It avoids the need to strip and dispose of old tiles.
- It keeps the building covered during installation.
- It creates space to add insulation between the old and new roof, if the build-up is designed to avoid condensation.
These benefits make it a useful option for buildings that need to stay operational during roofing work.
What to Check Before Installing
Before fitting steel sheets over tiles, you need to carry out the following checks. They confirm the roof can safely support the new installation and will perform as expected over time.
- Structural load: Even though steel roofing is lightweight, adding a second layer increases the load. A structural check should confirm that the rafters and purlins can support the extra weight.
- Tile condition: Tiles must be intact and securely fixed. Cracked, loose or water-damaged tiles can compromise the new roof and lead to future problems.
- Condensation control: The gap between the tiles and the steel sheets must be ventilated, or trapped moisture can damage the timbers. Allow an air path in at the eaves and out at the ridge, and make sure any existing roof vents are not blocked. Steel sheets with anti-condensation backing help stop drips from the underside of the steel, but they do not replace ventilation.
- Fixing method: Steel sheets cannot be fixed directly to tiles. Timber battens or a metal framework must be installed over the tiles, with fixings that reach the structural supports beneath. Steel is light, so these fixings are what stop the wind lifting the roof.
- Roof pitch: Steel roofing requires a minimum pitch to allow water runoff. Most tiled roofs are steep enough, but check the minimum pitch for your chosen sheet profile before ordering.
- Permissions:
- Most re-roofing work needs Building Regulations approval.
- On houses, permitted development allows the new roof to project up to 150mm from the existing slope, as long as it goes no higher than the ridge.
- Flats, commercial buildings, listed buildings and properties in conservation areas may need permission.
- Bats and asbestos:
- Bats are protected by law, and sealing the eaves or verges can block a roost.
- Older roofs may contain asbestos cement, so get a survey before drilling.
How to Install Steel Roofing Sheets Over Existing Tiles
Installing steel sheets over tiles requires careful preparation and the right fixings to ensure a secure, long-lasting result. Here’s a step-by-step guide to help you get it right:
- Inspect the roof. Confirm the tiles are secure and the structure is sound. Any loose or broken tiles should be repaired before work begins.
- Fit the framework. Fix treated timber battens or a lightweight metal framework over the tiled surface. Anchor them into the rafters or purlins, not the tiles themselves, to provide a solid fixing point for the steel sheets. Space the supports no more than 1.2m apart for 0.7mm sheets, or 1m for 0.5mm sheets. We recommend 0.7mm for roofs.
- Keep the void ventilated. Leave an air path in at the eaves and out at the ridge. Use ventilated fillers or mesh so air can move but birds and pests can’t get in.
- Start at the eaves. Begin sheeting from the eaves and work upwards towards the ridge. This ensures proper water runoff and helps maintain alignment.
- Lay with the wind. Lay the sheets so the side laps face away from the prevailing wind, to reduce the risk of wind-driven rain getting under the overlaps.
- Fix the sheets. Secure them using self-drilling Tek screws with washers, driven through the crown of the profile into the battens or framework. Space the fixings according to the sheet profile and span, and don’t overtighten them.
- Seal the laps. Fix both ends of each sheet and along the side laps to ensure a tight, weatherproof seal. Use end laps of at least 150mm on pitches of 10 degrees or more, and 200mm or more, sealed with lap tape, on lower pitches. See our guide to overlapping roof sheets.
- Pre-drill where needed. Pre-drill sheets if necessary, especially when working with multiple layers or thicker profiles, to stop the sheets shifting during installation.
- Finish the edges. Fit flashings and trims to suit the new roof height, and reposition gutters if needed so water still runs into them.
This method allows you to upgrade your roof without removing the existing tiles, provided the structure is suitable and the correct materials are used.
When Should Tiles Be Removed?
In some cases, removing the existing tiles is the better option. This includes situations where:
- the roof shows signs of structural damage or sagging
- tiles are cracked, loose, deteriorating or otherwise not fit for purpose
- the roof has leaks, water damage or rot
- asbestos materials are damaged or deteriorating
- the project requires insulation or vapour barriers directly on the rafters
- building control, planning or listed building rules will not allow an overlay
Removing the tiles lets you inspect and repair the timbers and install the new roof on a sound base.
Conclusion
Installing steel roofing sheets over existing tiles can be a practical and cost-effective solution, but only when the roof is in good condition and the installation is properly planned. Structural checks, ventilation, the right approvals and correct fixing methods are essential to ensure long-term performance.
If you’re unsure whether your roof is suitable for over-sheeting, speak to a roofing professional or contact our team at Cardinal Steels for advice.
Explore our full range of steel roofing sheets, including anti-condensation options and profiles suited for over-sheeting. Get in touch today to find the right solution for your project.
FAQs
1. Can steel roofing sheets be installed directly onto tiles?
No, steel sheets should not be fixed directly to tiles. A timber batten or metal framework must be installed over the tiles and fixed into the rafters to provide a secure fixing point.
2. What pitch is required for steel roofing sheets?
The minimum pitch depends on the sheet profile. Most tiled roofs are steep enough for steel sheets, but check the minimum for your chosen profile with us before ordering.
3. Is over-sheeting suitable for residential roofs?
It can be, but only after a structural assessment and checks on Building Regulations and planning. Over-sheeting is more common on agricultural and industrial buildings, where appearance and insulation standards may differ.
4. Will installing steel sheets over tiles cause condensation?
It can if not properly managed. Ventilate the gap between the tiles and the steel at the eaves and ridge. Steel sheets with anti-condensation backing will also help stop drips from the underside of the sheets.
