In the installation of a building drain, what size stone is used for the initial backfill under the bottom of the trench when the bottom can be kept stable and free of water?

• A. Stone 1 inch or larger
• B. Stone less than 1 inch
• C. Concrete
• D. Rock larger than 2 inches

Answer: (B)

The key idea is using a bedding material that provides a stable, well-drained base for the pipe. Stone smaller than 1 inch is ideal for the initial backfill under the trench bottom because it compacts well around the pipe, fills voids, and creates a smooth, stable surface without creating hard blocks. This tiny-gravel bed also lets water drain away, helping keep the pipe area dry and reducing the chances of movement or settling as you continue backfilling. Larger stones would leave gaps and uneven support, which can shift or pinch the pipe; concrete would be rigid and non-draining, and rock larger than 2 inches is too coarse to bed the pipe evenly. So, using stone less than 1 inch provides the right balance of support, uniform contact, and drainage.