Question

When planning a new kitchen, space gets tight fast.

Many ask if the sink’s trap arm can block a dishwasher.

The trap arm is the horizontal pipe from the P-trap to the wall.

It must slope to the vented drain and still leave room to slide the unit.

Answer

No, the trap arm should not block the dishwasher when it stays inside the sink base cabinet. Keep the P-trap and trap arm in the sink cabinet. Route the arm tight to the back wall and into a wall stub-out at the correct height. Bring the dishwasher hose through a high side hole to an air gap or disposal/tailpiece. Leave the dishwasher bay clear of rigid drain piping.

Best-practice layout

• Keep the entire P-trap and trap arm in the sink base.
• Route the arm along the back wall to the stub-out.
• Hold about 1/4 inch per foot of slope for drainage.
• Typical drain stub-out center is 16–18 inches above the floor.
• Use wyes or combos on horizontals, not a sanitary tee on its back.
• Drill a high hole in the cabinet panel for the dishwasher hose.
• Use a deck air gap where required; otherwise, use a high loop.
• Connect the hose to the disposal inlet or a branch tailpiece.

Why this works

The dishwasher bay remains open for the appliance and insulation.

The trap stays vented and accessible for service.

A proper slope prevents standing water and odors.

Helpful resources

• International Plumbing Code basics
• ASPE guidance on kitchen drainage
• Dishwasher installation manuals (example)

Always check local code for air-gap requirements and stub-out heights.