The method of smoke extraction employed was variable depending upon the weather and time of year. On calm days the smoke simply rose and exited through gaps in the roof tiles. This had a slightly antiseptic effect on mosses and liverworts which would normally have grown on the roof with the location of an open hall being clearly visible from the outside by just looking for clean roof tiles. If you look again at the photo for North Cray you will see this effect.
For a clearer explanation on the operation of open halls please check out this link