Historic England’s maintenance context
Historic England notes that ageing thatch can attract moss, algae and fungi and that this may be worse where overhanging trees restrict airflow. It describes raking down growth and debris as part of maintenance, but that does not mean every owner should start working on the roof themselves.
Record where the growth is
Take dated photographs and note the elevation, nearby trees and whether the area remains wet longer than the rest of the roof. Compare with earlier photographs if available.
Ask a competent thatcher before aggressive treatment
A specialist can distinguish superficial growth from a roof condition that needs work and advise on an appropriate method. Avoid assuming chemical treatment or pressure washing is suitable for traditional thatch.
Record vegetation work too
If trees or vegetation are reduced to improve airflow, keep that change in the property history with photographs. It may explain later changes in roof condition.
Monitor the same viewpoint
Return to the same photographs after the professional-recommended interval. Consistent evidence is more useful than a one-off note saying “moss removed”.
Source guidance can change. Check the current version and obtain property-specific advice where needed.