Dirty roof vs Clean roof

Many houses suffer from damp ingress via the roof tiles but a re-roof is not possible due to funds. It is possible for us to clean the moss and lichen from the roof and spray with sealer. This stops water backing up and soaking through the tiles. The secondary barrier of felt should normally be sufficient to drain off water but most roofs have various holes in the felt of in bad cases the felt has saturated and then fails leaving tell-tale run marks down the roof trusses or rafters.