Guide
Overcladding versus strip and replace: which one your roof can take
Updated
This decision moves the price more than any material choice. It is also the one most often made on price alone, when the deciding vote actually belongs to the structure.
What each option actually is
- Overclad or overroof
- A new outer sheet and insulation are built over the existing covering on a spacer or bar and bracket system. The old roof stays where it is and keeps the building weathertight during the works.
- Strip and replace
- The existing covering, insulation and liner come off and a new system goes on. The frame is exposed, inspected and can be repaired, and the finished build up is a clean, single specification.
When overcladding wins
- The building cannot be opened up. Live production, stored stock, food or pharma environments, or anywhere a temporary weathertight opening is unacceptable.
- Disposal is the expensive part. Not stripping means not disposing, and on a large roof that is a substantial line.
- The existing sheet is sound but tired. Coating failure and minor leaks on an otherwise intact roof are a good candidate.
- You want the insulation upgrade without the disruption. The new layer carries the insulation, so the thermal improvement comes with the overclad.
When stripping is the right call
- The structure will not take the load. An overclad adds dead load and changes wind uplift paths. This needs a structural engineer's view on the purlins and frame, not a roofer's opinion, and it is the most common reason an overclad is ruled out.
- The liner or insulation is wet or failed. Building over saturated insulation traps the problem and hides it from view.
- Condensation risk. Sealing a new layer over an old build up changes where the dew point sits. This needs designing, not assuming.
- There is asbestos and you want it gone. Overcladding can leave asbestos cement sheets in place and managed, which is sometimes right, but it leaves you with a long term management duty rather than a resolved building.
The asbestos angle, either way
If asbestos cement sheets stay in place under an overclad, they are still in your building. HSE's duty to manage guidance states that the duty covers all non-domestic premises such as factories or shops, and requires the dutyholder to keep an up to date record of the location and condition of asbestos containing materials, assess the risk, write a management plan, review it every 12 months or sooner if necessary, and provide information on the location and condition of those materials to anyone who may work on or disturb them, including the emergency services (HSE). An overclad does not end that duty. It just puts a lid on it.
Get both options priced by the same contractor on the same visit, and ask for the structural basis of the overclad answer in writing. Get quotes and say you want both routes costed.