Walls, ceilings, windows and doors have to be built with the appropriate materials and assemblies in order to meet fire separation requirements. Materials such as wood, carpet, textiles, and many other things have to meet flammability and fire resistance ratings. And designers need to know what ratings apply in what situations.
Corridors, aisles, and doors have to meet appropriate lengths, widths, and clearances that adhere to stringent building codes in order to allow for proper evacuation in the event of a fire.
So, unlike structural engineers, interior designers are usually not responsible in designing load bearing, walls, or structures, but we still have to be responsible for the material and structural integrity of many things, such as knowing where to put deflection channels and expansion joints in materials. Knowing where to put additional support and blocking for wall-mounted or ceiling-mounted fixtures such as lighting, cabinets, and heavy equipment. And where to install additional bracing and support for things like suspended ceilings and demountable partitions in certain seismic zones.
When it comes to finishes, interior designers need to have extensive knowledge in standardized testing of materials, and industry requirements. Sound absorbency or transmission levels of products in order to meet acoustic needs. Coefficients of friction, what they mean, and how to meet floor slip resistance requirements. And if the materials emit volatile organic compounds or toxic chemicals that could adversely affect indoor quality.
For instance, to you it may look like the interior designer just picked out a nice looking window covering, but really he or she may had also had to consider factors like light control, thermal heat gain/heat loss, privacy needs, acoustic transmissions, operability, air circulation, material quality, durability, flammability, and fire resistance. While at the same time balancing those factors with the clients’ budget and personal requirements.
Every step of the way, designers also need to make sure the spaces are inclusive of people with disabilities.