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Ventilation and Condensation
The ventilation of mono-pitched roofs at ceiling level only may allow air to stagnate at the apex of the roof. To prevent this, high level or ridge ventilation, equivalent in total area to that given in the table, should be provided in addition to the ventilation at ceiling level.  
Similarly, air stagnation may occur in duo-pitched roofs of more than 20 degree pitch, or 10.00m span and consideration should be given to the provision of additional high level or ridge ventilation, equivalent to a continuous gap 5mm wide.

When insulation material is close to the roof covering, as at the eaves, or where it is placed at rafter level to form a warm roof space, (as in attic and
raised tie construction),
it is essential to provide an air gap or not less than 50mm between the top of the insulation and the underside of the roof covering or sarking. This gap, which should allow uninterrupted air circulation immediately above the insulation, should be ventilated to the outside at the eaves and, when insulation is placed at the rafter level, also at the ridge. The minimum opening at the eaves to provide
adequate ventilation of the air gap above the insulation in a warm roof space should not be less than that shown in the table for low level ventilation and the ridge ventilation should be not less that that provided by a continuous gap 5mm wide. In normal circumstances, further ventilation of warm roof spaces is not required.