When replacing a window with sliding doors and adding additional support for the header above the opening, it's essential to ensure that the structure is adequately supported and meets building code requirements. Using three 2x4 posts on each end of the 2x12 header may not provide sufficient support, as it depends on the size of the opening and the load-bearing requirements.
To determine the appropriate support for the header, it's best to consult with a qualified structural engineer or a licensed contractor. They will consider factors such as the span of the header, the weight of the structure above it, the type of sliding doors being installed, and the building code requirements in your area.
In some cases, a larger header or alternative support methods, such as steel beams or laminated veneer lumber (LVL) headers, may be necessary to ensure the structural integrity of the opening. These professionals will also take into account any potential load-bearing walls and the overall structural design of your home.
Failing to provide adequate support when removing a window and installing sliding doors can lead to serious structural issues and safety concerns. Therefore, it's crucial to have a professional assess the situation and design the appropriate support system for your specific project.