WHAT ARE THE DIFFERENT LAYERS WITHIN A ROOF?
Within a pitched slate or tiled roof there are usually one or two layers.
Roofs constructed pre 1950 were usually laid without any felt or insulation underneath the rafters, meaning that the only layer is the tiles / slates themselves. This often leads to the loft space being unusable as dirt, dust and damp gets underneath the tiles / slates into the roof space ruining anything stored there.
Roofs constructed between the 1950s and 1980s were usually constructed with a non breathable membrane underneath the roof tiles / slates forming two layers. This formed an effective barrier to seal the roof from dust and dirt, but can, in severe winter weather conditions, trap moisture leading to condensation and damp issues.
It was rare for roofs of this age to be constructed with any insulation, although many will have had insulation added since.
Roofs constructed after the 1980s will usually have been constructed using a breathable felt underneath the tiles / slates, sealing the roof against dust and dirt whilst still allowing it to breathe, thus preventing most condensation issues.
Insulation will often have been added as well - although this will often have been laid on top of the celiling of the rooms below the roof, meaning that the roof will still have two layers as such.