Plumbing Fixtures, Fixture Fittings, and Appliances
Clearances and fixture unit values
Water closet compartments are the classic exam item: not less than 15 in from the centerline of the fixture to any side wall, 30 in of clear width, and 21 in of clear space in front. Lavatories follow similar logic, and accessible fixtures follow the accessibility standard adopted locally (grab bars, knee clearance, operable parts).
Know which fixtures require a trap, which require an air gap or air break, and which may discharge indirectly. Food-handling areas require hand sinks; drinking fountains and bottle fillers have their own rough-in heights; floor drains need trap primers where the trap can evaporate.
Fixture fittings
- Slip joints are permitted only where accessible.
- Escutcheons close the gap where pipe passes through a wall or floor.
- Fixture stops on the supply; shutoff valves ahead of appliances.
- Backflow protection at the fixture, not just at the service.
Common traps
- Mixing up 15 in (centerline to wall) with 12 in or 18 in.
- Assuming a floor drain trap never needs priming - it does.
- Forgetting that a fixture's drain size is governed by its fixture unit rating, not by the visible outlet size.
- Placing a cleanout or trap in a concealed, inaccessible location.
Rule of thumb
30 in wide, 15 in to the centerline, 21 in in front - and if a question mentions an inaccessible slip joint, the answer is usually "not permitted."
Worked example: Two lavatories in a 60 in alcove: each needs 30 in of width, so the arrangement just fits with the fixtures set against the side walls and centers approximately 15 in from each wall.