Internal downpipe from flat roof

I've got a flat roof that is completely enclosed on all sides, by mine and next doors house on the sides, and pitched roof's front and back. The only way for rain water to get off the roof is down through the roof internally into the utility room below. I need to connect the 65mm down pipe to the waste i have for the washing machine which is 40mm, any idea's? I understand that connecting to the 40mm pipe may mean I don't have enough capacity in very heavy rain but as i see it the worst that will happen is I’ll have water sitting on the roof for a short period. Recommendations about the best way to connect the 2 different size pipes and any other advice welcome.

strap on boss.. View media item 10662

Hi. What you are suggesting will end in tears, the washing machine waste is plumbed into the soil system. In most areas soil and rainwater have separate systems, if you connect to soil/waste system you will in affect be venting the soil system via the outlet, creating odours and compromising the soil system with rain water. In the event of the down pipe blocking (and it will)or backing up due to volume of water the pressure at the proposed jointing point may be sufficient to push joint apart (kitchen full of water). As for standing water on enclosed roof, if a blockage causes water on roof to build up unnoticed the weight can cause problems (1 cubic foot of water weighs 62.5 lbs). Re visit the situation and engineer a way around your proposal. Good Luck

Thanks for the advice, back to head scratching!

Out of interest how have you ended up with a flat roof with nowhere to drain from? It sounds like you really need to install a new drain run from the outlet to where it enters the underground run. In the past when I've drained rainwater internally I've had to use cast iron pipes with mechanical seals for the same reasons justlead1 mentioned. Furthermore if you don't have an overflow and it blocks up whilst you're away it will end in tears that way too.