TECHNICAL
Air bricks to underfloor voids GUIDANCE
6.1/27
– relationship to ground levels
(January 2016) (First issue)
Question
In relation to external ground levels where should air bricks, which provide natural ventilation to
underfloor voids, be positioned?
Considerations
n Ventilation to voids below suspended ground floors is normally installed to prevent condensation but may also form an integral
part of the design for gas protection measures preventing the build-up and aiding dispersal of ground gases, including Radon.
n Air bricks serving underfloor voids should be located and installed to provide good airflow and adequate cross ventilation to
meet NHBC Standards and building regulations. This also applies to any internal/sleeper walls.
n Air bricks should be kept clear of obstructions that could reduce or block the required airflow. Air bricks set at, or close to,
ground level may flood and allow rainwater to enter the under floor void.
n Air bricks located below general ground level with small recesses in front, often formed with raised kerbing/edgings etc., are at
risk of blockage from debris accumulating in the recesses.
Answer
The following guidance relates to sites that do not have any ground gas issues. On gassing sites where the ventilation void will
form part of the gas protection measure, and may have been specifically designed for the gas conditions on the site, the designer
may need to be consulted on any changes to the intended design/construction.
Air bricks may be located either above or below DPC level and where possible on all sides of the building.
Ideally air bricks should be located at least 75 mm above hard and soft landscaped areas to reduce the risk of being obstructed or
flooded (see diagram 1).
Air bricks may be located at or near ground level where measures have been taken to avoid flooding by sloping the ground away
from the air brick and providing drainage where the ground is not free draining (see diagram 2).
Due to potential on-going variations in surface level with gravel and cultivated ground, including the risk of obstruction by
vegetation, air bricks located over such surfaces should not be set at ground level.
Air bricks located below general ground level, where the airflow relies on small recessed enclosures in front, are not acceptable as
regular clearing of debris from the recesses, to maintain ventilation, is unlikely to occur. An acceptable arrangement is to provide a
continuous strip of lowered ground extending at least 600mm in front of the air bricks. Any area of reduced ground level should be
free draining or drained to prevent flooding, a 50-75 mm deep gravel strip may help in this respect ( see diagram 3).
Page 1 of 2
TECHNICAL
Air bricks to underfloor voids GUIDANCE
– relationship to ground levels 6.1/27
(January 2016) (First issue)
The following diagrams illustrate the principles outlined above.
Sections through telescopic under floor vents
Diagram 1
Preferred approach where air brick is
at least 75mm above ground level.
75 mm min
above GL
GL
Ventilated under
floor void
Air bricks between ground level and 75mm
above ground level is acceptable if ground
slopes away from building and is suitably Diagram 2
drained and the ground is hard landscaped. Ground to slope
away from building
and drained if it is
not free draining
GL
Ventilated under
floor void
Air bricks below general ground level are
acceptable with continuous strip of lowered
ground extending at least 600mm away
Diagram 3
from front of air brick(s). Area of reduced
ground level to be free-draining or drained to At least 600mm
avoid flooding, a gravel strip may help. from air brick
General GL
75 mm min above
Ventilated under
reduced GL
floor void
Page 2 of 2
NHBC, NHBC House, Davy Avenue, Knowlhill, Milton Keynes, Bucks MK5 8FP. Tel: 0344 633 1000
NHBC is authorised by the Prudential Regulation Authority and regulated by the Financial Conduct
Authority and the Prudential Regulation Authority.
HB2851 01/20
