Calculating the correct free area is a vital consideration when designing an architectural façade.
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In a nutshell:
• Free area is the open space in a structure allowing airflow, light, or visibility and is typically expressed as a percentage.
• It plays an important role in car park façades by ventilating fumes and reducing light pollution.
• Building Regulations stipulate that car parks must have at least 5% open wall area for ventilation.
• Too much free area weakens structural integrity, increasing the risk of bending or failure under wind loads.
• Too little free area results in poor ventilation and small perforations may cause whistling in windy conditions.
In construction, ‘free area’ refers to the open space within a material or structure that allows for airflow, light, or visibility. It is commonly expressed as a percentage and is crucial in architectural systems like façades.
For expanded mesh or perforated panel façades, calculating free area helps to balance airflow requirements with visual aesthetics. For example, car park façades are required to have a certain percentage of free area to ventilate exhaust fumes and reduce the impact of light pollution.
Regulations
Regulations on free area for car park façades can be found in Document F of the Building Regulations. The regulations state that naturally ventilated car parks must have at least 5% of the total wall area open for ventilation and that the openings should be distributed to ensure proper air circulation.
Free area is also a consideration when assessing fire safety, as the BS 7346-7 and BS EN 12101 stipulate that the free area percentage of façades allow for smoke dispersion in case of a fire. Free area calculations also help determine if additional smoke ventilation is required such as mechanical ventilation systems.
Risks of too much free area
If there is too much free area in an architectural façade, it can lead to issues such as reduced structural integrity. A higher percentage of open space weakens the panel, making it more prone to bending, warping, or failing under wind loads. If a greater percentage of free area is required, additional fastenings will be required.
Too little free area
If the percentage of free area in a façade is too low, this inhibits airflow, leading to a build-up of exhaust fumes, heat and moisture. Also, if the perforation holes are too small, this could also risk whistling during windy conditions. This is particularly a risk for picture-perforated façades where an arrangement of different perforation sizes projects a bespoke image along the elevation of a building.
Ultimately, free area is dependent on the nature of the project. If you have any questions about calculating the correct free area percentage for your design, a façade specialist can guide you through the different considerations.