Chances are the existing boards need to be replaced because they are at the end of their life and painting is no longer a viable solution. Fixing a thin capping board to an unsound sub structure is a disaster waiting to happen. Any existing rot will spread rapidly as it is sealed in by the plastic without any ventilation. Even if no rot is currently present the existing external timber boards will hold some moisture that will be trapped by the capping board leading to eventual rot.
There are many other problems with the over clad method such as how the finished work will look like if you live in a terraced or semi detached property. There will be a ‘step’ on the boundary between the two properties. The gutters will not be perfectly in line between the two properties leading to a potential leak.. It is also worth noting that many window frames are only a few millimeters below the roof line, over cladding the existing boards could prevent some windows from fully opening – a serious problem should the window be a designated fire escape route.