I assume that it's due to the expanding nature of the foam -- they are afraid that the foam might continue to push against the roof timber and crack it I think
No they specifically mentioned condensation and how it affects properties in colder climates more. I think they're afraid the improperly installed insulation will allow condensation to build up below the roof timber which will cause the timber to become wet and degrade.
>A report published in March by the government’s Health and Safety Executive found that condensation could cause 25% of roof timber to decay within five years if spray foam is applied directly to roof tiles, or certain underlays. The general risks increase the further north the property is, because of colder climates.
