Moss is becoming more of a problem on thatched properties recently. This is due to the change in the climate we are seeing. Years ago, you could depend on a solid four seasons, with harsh winters and hot summers. But, with this a particularly mild winter (how many times have you had to defrost the car in the mornings this year) and, unfortunately, rainy summers we now seem to get, conditions are ripe for moss growth.
However, moss doesn’t necessarily damage thatch. The lighter green moss (lichens) often indicates a healthy thatched roof underneath. The darker green moss does hold water in and can cause damage, but your roof could still be solid under the surface. As with any concern, contact a thatcher as each case is different. Indeed, you could cause more damage by removing the moss from your thatch. Even if the moss is removed, the roots are likely to remain, and the moss will grow again in a few years. Some sprays are available but have not had the test of time, and I would recommend putting as little chemicals on your thatch as possible.