The underside of this cottage’s roof covering has been coated with an expanding foam spray. It is marketed as stopping roof slate or tile slippage and improving thermal insulation levels.
In practice, relatively few properties that I see have been treated in this way: maybe a dozen in thirty years. The foam sticks the roof slates or tiles together meaning they are unsuitable for re- use at the time of reroofing making the cost of the job far more expensive.
I also have reservations about the claims for improved thermal insulation. Why would you want to insulate the underside of your roof? Surely it is preferable to conventionally insulate the ceiling within the roof space so that heat loss from the living accommodation is limited and not trapped in the attic.
Proceed with caution if you see and advert for this in your Sunday colour supplement.
There is also a suspicious looking wasps nest in the top left corner of this photo!