Thatch advice please

[Anonymous]

My daughter's C16 timberframe house is thatched. It has a very steep roof pitch and therefore a good sized attic, which she is about to convert into a room.
The underside of the thatch has all been plastered in lime between the rafters (some time ago). It has been suggested to me that this might not be a good thing, because it prevents air circulation around the thatch.
My daughter's plan was to leave the lime plaster - cleaning and smoothing it a bit with another thin skim of lime plaster on top of it.
Have any thatching experts got a view on this please?

 

[Anonymous]

As a thatcher i have seen this on older properties with no ill effects. The airflow from the outside goes up and around the stems therefore preventing decay

 

[Anonymous]

There was research into the degradation of thatch some years ago and in particular the impact of linings under the thatch.

From recollection it was found that impervious modern linings (plastic, foil, bitumen felt, etc) resulted in an increase in the risk of condensation to the underside of thatch and this led to increased rates of degradation. If I remember correctly there was some debate about what some called the 'heat pump' effect on south facing slopes, which seemed to force moisture into the thatch.
Whatever the technical ins and outs, the resulting advice was that impervious linings under the thatch (that is where the thatch is directly on the lining without a ventilation gap between the lining and thatch) increased the risk of early failure of the thatch.

With regard to traditional lime plaster under the thatch, as a surveyor I have not yet come across a problem, my findings are as Nick has stated. I suspect that the breathability of lime plaster means that the risk of condensation is remote and does not cause the degradation of the underside of the thatch. I suspect that provided traditional lime plaster ceilings are retained, repaired, reinstated, etc there should be no problem.

 

[Anonymous]

Many thanks to Nick and Stephen - I couldn't wish for more authoritative advice. I'm very grateful.