Principal rafters and their heads.

The correct answer to the question is option 'C', which states that the principal rafters and their heads are housed in the splayed shoulders of a king post truss. Let's break down the components and explain their roles:

1. King Post Truss:
- A king post truss is a type of roof truss that consists of a vertical central post (known as the king post) and two diagonal braces that extend from the post to the top of the truss.
- The king post truss provides support and stability to the roof structure.

2. Principal Rafters:
- Principal rafters are the main load-bearing members of a roof truss.
- They are typically horizontal beams that run along the length of the roof, supporting the weight of the roof covering and transferring it to the walls or other support structures.
- Principal rafters are usually placed at regular intervals across the span of the roof.

3. Heads of Principal Rafters:
- The heads of principal rafters refer to the upper ends of the rafters, where they meet at the apex of the roof.
- The heads of principal rafters are crucial for maintaining the structural integrity of the roof truss.
- They provide support and help distribute the weight of the roof evenly across the truss.

4. Splayed Shoulders:
- The splayed shoulders of a king post truss are the angled portions where the principal rafters and their heads are housed.
- These shoulders are designed to accommodate the shape and position of the rafters, allowing them to fit securely within the truss.
- The splayed shoulders ensure that the principal rafters are properly aligned and supported, preventing any movement or instability.

In summary, the correct answer (option 'C') states that the principal rafters and their heads are housed in the splayed shoulders of a king post truss. This arrangement ensures proper support and alignment of the rafters, contributing to the overall stability and strength of the roof structure.