Water should only ever flow in one direction into your home and never in the other direction. When excess water causes a pressure shift in your pipe, water can start to move backwards and flow into your main water supply. That poses the health risk of your drinking water being contaminated.
John the Plumber Hamilton has years of experience in draining and de-clogging drains and pipes of all kinds. Pouring corrosive drain cleaners down your pipes to loosen a clog is not an option to us, and sometimes a block is so bad that a snake or hand auger will only make a small hole in it, or push the blockage further down.