If you have a flat roof the chances are that you will need to make sure it is sealed. To start with, remember that it is not a true flat roof. It is in fact a slightly domed area with or with an imperceptive incline down to one corner. It is also true to say that a slanted roof could be with some sealant attention as well. Nevertheless whatever its shape it needs to be checked and Roof sealant applied. In the past simple felt would have been used but sealant is a much better option and lasts for a longer period of time.
If the roof is showing signs of raising tiles or increased moss growth there is a chance that moisture is getting in and underneath. Moisture is by far the worst thing that can happen to a roof as it will cause no end of issues later on. If water gets in it will start to affect the ceiling. This increased moisture can promote mould growth when combined with the warmth of the home.
Sealant is a covering that is sprayed over the roof to give it a waterproof skin. When it hits the roof the water will just run off the surface and into the waiting gutters.