Ventilation and control of ventilation is covered under BS 5250: (Code of Practice for control of Condensation in Buildings).
General information relating to traditional cold roof spaces
Breathable underlays work by transferring moisture laden air from inside the building into the batten cavity (the area between the top of the underlay and the underside of the tiles) where it is allowed to ventilate out between the gaps in the roof tiles or slates.
The gaps in between the tiles or slates will vary depending on the manufacturer and therefore additional ventilation may be required at the eaves or ridge line to assist in drawing the air in the batten cavity out of the roof.
Condensation forming on the back of the roof covering without adequate batten cavity ventilation may result 'Lime Staining' on the face of the tile.
In Forticrete's opinion additional ventilation should be provided at high level in the form of a ventilated ridge line. If the selected tile is flat additional ventilation should be used at the eaves.
Please consult with the tile, and underlay manufacturer for further advice.
General information relating to Warm Roof Spaces
Where the insulation is positioned between and on top of the rafter to the appropriate insulation level and is fully sealed is known as a warm roof.
In this instance no additional ventilation is required.