Which of the following is not a valid material for cleanout fittings?

The correct answer indicates that wood is not a valid material for cleanout fittings. Cleanout fittings play a critical role in a plumbing system by providing access for cleaning and maintenance of drains and sewer lines. These fittings are subject to environmental conditions, including moisture and potential corrosion, which is why materials must be durable and resistant to degradation over time.

Plastic, brass, and lead are all materials commonly accepted for use in cleanout fittings. Plastic is favored for its corrosion resistance and lightweight properties, making it suitable for many plumbing applications. Brass is known for its strength and resistance to corrosion, making it a reliable choice for fittings. Lead has historically been used in plumbing due to its malleability and resistance to corrosion, albeit with declining usage due to health concerns.

Wood, however, is not considered a suitable material for cleanout fittings. It is not resistant to moisture and can easily decay, warp, or break down when exposed to the conditions typically found in plumbing systems. Thus, the selection of materials like plastic, brass, or lead is ensured for functionality and longevity in plumbing applications, while wood is unsuitable and poses a risk to the integrity of the system.