The owner of this property has been persuaded by someone that it would be a good idea to spray the underside of his roof slates with an expanding foam material. The purpose is to stop slate slippage and keep the rain out.
The down side is that when reroofing is eventually needed the existing slates cannot be used, adding to the cost of the work. In the meantime, the foam greatly restricts ventilation into the roof space and this could cause the build up of potentially damaging condensation.
So, not recommended as a course of action but if you do proceed on this basis be aware of the limitations of what you are doing.