Generally speaking, around 25% of the heat generated by your main heat source, be it a boiler or alternative solution, will be lost via the roof of your home.
For context, about 10% of heat will exit through the floor, with 35% of generated heat escaping the property through the walls, or in gaps surrounding windows and doors.
To gain a more detailed insight into the energy efficient operations of a household, take a look at our blog, How Can I Make My Home More Energy Efficient?