My friend has just moved house. There is plumbing ready for a washing machine. The waste pipe looks to be running ok, vertical section at about the right height, then slightly downward sloping "horizontal" section. However, the last 20cm of the pipe where it goes into the soil stack goes very slightly "uphill". Is this going to cause a problem? There doesn't look like there is room to lower this last section. The waste pipe is also half buried in the wall (along the length) so changing this bit would also be tricky.