What material is typically used for the girder under a wood deck?

Using wood as the material for the girder under a wood deck is standard practice primarily due to its availability, strength, and ease of construction. Wood girders provide adequate support for the structure and distribute the weight of the deck above it effectively. The natural properties of wood, such as its strength-to-weight ratio and workability, make it an ideal choice for residential construction, especially in decks where aesthetic considerations often lead builders to prefer materials that match the decking itself.

Additionally, wood girders can be easily treated with preservatives to resist rot and insect damage, making them suitable for outdoor applications. Other materials like steel and concrete, although strong and durable, can be more expensive and may require additional considerations for corrosion protection or foundation work. Plastic, while lightweight and resistant to moisture, typically does not provide the same level of structural integrity needed for a girder in this setting. Therefore, wood remains the most common and practical choice for this application in residential decking.