Getting the design of car park ventilation systems right is essential 
to the safety of a car park, as they must provide both day to day ventilation for the people using it and protection against fire, as required by Building Regulations. However, it’s not as straightforward as you might think.
Simple guidance on "free areas"...
Guidance on the ventilation requirements for car parks to meet the Building Regulations’ requirements is available in Approved Document B (ADB) and Approved Document F (ADF) for England, and their equivalents in Wales, Scotland and Northern Ireland.
For open sided car parks to ADB and naturally ventilated car parks to ADF, the requirement is for permanent wall openings on each level, equal to 5% of the plan area, arranged to provide cross ventilation. Of this, at least 1.25% should be well distributed along one side and 1.25% well distributed along the opposite side. It is intended that this should provide sufficient ventilation for both day to day and smoke clearance requirements across the whole area, since the assumption is that a cross flow of air across the whole car park will be encouraged.
… but what if the car park isn’t rectangular?
An issue here is that this guidance is simple to apply if a car park is rectangular, but how many are? In this case, it becomes a matter of interpretation at what point sides cease to be “opposing” and what constitutes a “well distributed” layout. What the guidance should say is that the ventilation layout should avoid the creation of any potentially stagnant areas.
Avoid potential stagnant areas in your design from the very beginning
It’s quite common for designers or Building Control bodies to decide, often quite late on, that the proposed ventilation openings are not suitably located to avoid such stagnant areas.
Providing last minute additional ventilation (whether in the form of additional natural ventilation or mechanical ventilation) is likely to be difficult and costly. Often the best solution is to enhance the natural ventilation with strategically located impulse fans to ensure that otherwise stagnant areas are ventilated. These may sometimes need to be reversible to ensure that the system works efficiently regardless of wind direction.
Defining correctly the ‘free area’ is far from straightforward
Another issue is that ADB and ADF define “free area” differently, and ADF’s definition of it as “aggregate equivalent area” is quite opaque and not well understood. To calculate the equivalent area of an opening it is essential to know the aerodynamic coefficient for the opening; simple for an open hole or a ventilator or louvre for which a coefficient test has been carried out, but otherwise challenging. The architectural treatments often given to these openings can make it very difficult for designers to assess the equivalent area and for Building Control to check that the building achieves what is set out in the Regulations.
Paul Compton is a Technical Director for Colt, experienced in smoke control, HVAC, solar shading and louvre systems.
