Our washing machine leaks from the waste pipe. The waste goes through the inside u-trap then through the wall where it drains around a 90 degree bend and from there the pipe descends under the concrete to where I presume it empties into the sink used by the guttering downpipe. The problem seems to occur when its raining. Is it possible that the exit to the Washing machine waste pipe is underwater when this happens and so refuses to drain any more and backs up into the house? If so how can this be avoided?