Your washing machine's drain water feeds down a drain pipe, like all of the other plumbing fixtures in your house. The drain water normally should never enter the house again, either through the washing machine's drain or another drain. Clogs in drain pipes or vent pipes allow drain water to seep back into the house.
Snake the Drain
A drain snake will come into direct contact with the clog in the washing machine's drain pipe. To snake the washing machine's drain, pull the machine's hose out of the drain tube, then feed down the snake. Spinning the snake's crank clockwise as you feed it into the drain pipe lets the snake bore through clogs, while spinning it counterclockwise when pulling the snake out allows the snake to catch and pull out clogs. You may encounter a difficult clog you cannot push through at first. In this case, pull the snake back about an inch while turning the crack counterclockwise, then spin the crack quickly clockwise as you push it forward with greater force; this should push through the clog.
Check and Clear Vent Pipes
Inspect your house's vent pipes by climbing onto the roof. Shine a flashlight down each of the vent pipes and use a drain auger to dislodge any clogs. For clogs you can't reach using the auger, spray water down the drain pipe with a hose so the clog comes free. If the vent pipes regularly clog with debris from an overhanging tree, place a cover over the vent pipe to prevent more problems.
Check the cleanouts in the front yard. The cleanouts attach to the drain pipe that connects the drains in the house to the sewer system. Cleanouts have covers with a giant knob on them. Remove the cover by clamping a pipe wrench to the knob and turning the cover counterclockwise. With the cover removed, you can see if the cleanout is clear or not. A clear cleanout means the drain pipe does not have any clogs. If you see water or debris in the cleanout, a major clog has formed in the drain pipe. If you have a power auger, feed it into the cleanout to snake the sewer drain line and remove the clog.
Get Professional Help
If you still can't determine the cause of the washing machine's water backing up into the bathtub's drain, call a plumber for professional help. A plumber has the necessary tools and equipment to locate clogs in the drain lines in the house as well as the vent pipes. The plumber also can determine if damage to the pipes is contributing to the problem, and he has the ability to replace the damaged sections of pipes.