I am adding a laundry room the drain right now goes directly to the main sewer line . Would I need to install a p trap on the washer drain? Thanks.

i am adding a laundry room the drain right now goes directly to the main sewer line . would i need to install a p trap on the washer drain? thanks.

Not only will you need to install a "P" trap,(see image) but you will have to run a vent up out the roof or revent back in the attic to a existing roof vent. Good luck, Tom