Whilst there is no specific offence relating to snow on a vehicle's roof, it is advisable to remove any loose snow before commencing your journey. If it slips over the windscreen, or flies into the path of another car, it could cause a hazard for you or another road user and could leave you open to being penalised for a number of offences, such as 'driving without due consideration' or 'using a motor vehicle in a dangerous condition'.