The correct answer is C.
The cost of a roof truss is determined by its size, span, and type of construction. The cost of purlins is determined by their length, width, and thickness. The ratio of the cost of truss to the cost of purlins will vary depending on the specific design of the roof. However, in general, the ratio should be between 2 and 3. This is because the truss is the main structural element of the roof, and it must be strong enough to support the weight of the roof deck, the insulation, and the roofing material. The purlins are secondary structural elements, and they are only required to support the weight of the roof deck. Therefore, the cost of the truss should be greater than the cost of the purlins.
Option A is incorrect because the ratio of the cost of truss to the cost of purlins should be greater than 1. This is because the truss is the main structural element of the roof, and it must be strong enough to support the weight of the roof deck, the insulation, and the roofing material. The purlins are secondary structural elements, and they are only required to support the weight of the roof deck. Therefore, the cost of the truss should be greater than the cost of the purlins.
Option B is incorrect because the ratio of the cost of truss to the cost of purlins should be less than 4. This is because the truss is only one of the many components of a roof. Other components include the roof deck, the insulation, the roofing material, the flashing, and the gutters. The cost of all of these components must be considered when determining the total cost of the roof.
Option D is incorrect because the ratio of the cost of truss to the cost of purlins should be less than 4. This is because the truss is only one of the many components of a roof. Other components include the roof deck, the insulation, the roofing material, the flashing, and the gutters. The cost of all of these components must be considered when determining the total cost of the roof.