Pitched roof spaces should have ventilation openings at eaves level to promote cross ventilation. These openings should have an area at least equal to continuous ventilation running the full length of the eaves and 10mm wide.

A Pitched roof space which has a single slope and abuts a wall should have ventilation openings at eaves level and at high level. The ventilation at high level may be arranged at the junction of the roof and the wall or thought the roof covering. If it is through the roof covering it should be placed as high as practical.

Roof spaces should have ventilation openings in two
opposite sides to promote cross ventilation. These openings should have an area at least equal to continuous ventilation running the full length of the eaves and 25mm wide.

A pitched roof space which has a single slope and abuts awall should  have ventilation openings at eaves level and at high level. The ventilation at high level may be arranged at the junction of the roof and the wall or through the roof covering. If it is through the roof covering it should be placed as high as practicable.

Roof spaces should have ventilation openings in two opposite side to promote cross ventilation. These openings should have an area at least equal to continuous ventilation running the full length of the eaves and 25mm wide.

UNDER FLOOR VENTILATION 1985 BUILDING REGULATIONS C1
Each external wall should have ventilation openings placed so that the ventilating air will have a free path between opposite sides and to all parts. The openings should be large enough to give an actual opening of at least equivalent to 3000mm² for each meter run of wall. Any pipes needed to carry ventilating air should have a diameter of at least 100mm.

In accordance with current practice the recommendations of both the NHBC and British Board of Agreement suggest ventilation of under floor voids by providing  the equivalent of a continuous gap along the perimeter of exterior walls.