A drain strainer for debris is located in drain line having a side outlet extending to a drain line flow space. The strainer has a body bounded by sidewalls contiguous with an annular forward end wall for abutting with the drain flow space to prevent the passage of debris. An array of apertures in the strainer body, which is hemispherical or planner with upstanding sidewalls, is sufficient in number and size to maintain water flow while extracting and retaining debris occurring in the drain line flow space. A closure assembly is formed by a closure plate that supports a seal acted on by a retainer for forming a fluid tight seal with the side outlet opening of the drain line segment.