What is the purpose of installing a spreader board between the side jambs?

The purpose of installing a spreader board between the side jambs is primarily to maintain the correct opening width. This component is critical in ensuring that the elevator door operates smoothly and aligns properly with the door frame. A properly set opening width is essential for the functionality and safety of the elevator door system, as it prevents issues like misalignment or obstruction that could lead to operational failures or safety hazards.

While the other options mention aspects like aesthetics, insulation, and soundproofing, these are secondary considerations and not the primary function of the spreader board. The focus is on structural integrity and functionality, making the maintenance of the correct opening width vital in elevator door installations.