How can I find out where my drains flow to

longwalks1

The alleyway down the side of our house has 3 visible drains/inspection holes. 1 rectangular original one (1930’s semi), a round manhole cover beside our rear extension and a small one right beside the extension that the kitchen sink and dishwasher waste go into.  I’m assuming they all flow out front to the street?

am planning digging down about 8” to create a sub base for a patio out the back and wondering how can I be sure there aren’t any pipes under where I’m going to dig/level? 

Le_Kirk

Check your deeds if you own the house.  If you don't have your deeds, a trip to the land registry (on-line) gets your deeds (which should come with a plan) and often the drain lines are shown on there - mine are.  If you don't own the house, ask the freeholder (you'll have to anyway if you are planning any building work - even a patio).

Martin_the_Unjust

Open the manholes and have a look at how far down the pipes are and which way they run,I would be very surprised if they were not a lot more than 8"

vw100

Contact the water company of your area if all else fails. However even their plans may not be accurate but give a rough guide only.

DevilDamo

As @Martin_the_Unjust mentioned, your best bet would be to lift up the covers and identify for yourself as to how and where they flow. Once the covers have been lifted and you've established the invert levels, you could hazard a guess as to how deep they are at certain locations. Even if there were any existing record drawings, they are not to be relied upon as they're in most cases "as designed" layouts as opposed to "as built" layouts. Your water authority would not normally have details of those drains, especially if they were or are private.