Drainpipe fall gradient

[Handyman]

What's the minimum fall that's acceptable for 100mm plastic waste pipe? Is it 2deg or 4deg?

In my case, it's a 4m length which I'm hoping to hide the pipe (including the fall) within the depth of a 6" floor void. At least I'll be able to work out how much will be visible at the lower end, and how much I've got to disguise.

 

[Nige F]

Historicaly it was1 in 40 but plastic is so smooth that I would just get it into the available space

 

[Handyman]

1 in 40

What's this in degrees? My 'O' level trig was so long ago, I've lost my log tables...

Are you saying that because the plastic is smooth (or might not be, once my family have had a few acidic curries....) that I can get away with even 1 deg fall??

 

[Nige F]

I haven`t a clue..my maths is so bad that`s why I `m a plumber not an architect......believe me I would have been ..UNI was free in those days and drains were laid in 2 foot lengths of salt-glazed clay Yes I`m saying you can lay plastic to a very shallow fall

 

[doitall]

9 Degs or 2.5%

I think

 

[croydoncorgi]

1 in 40 is 1.43 degrees - I think!
9 degrees would leave the lumps behind!

 

[ChrisR]

Rule of thumb for smallish angles - one degree is about one in 60 (think equilateral triangle).

So at 1 in 40 you need a 4 inch drop in 4 metres

 

[Handyman]

So at 1 in 40 you need a 4 inch drop in 4 metres

So (and forgive the mixed units), that means, on a 4" pipe, the end of a 4m run will protrude approx. 2.5" below a 6" joist (allowing for the flanges on the connectors). I can deal with that, no problem.

That means I can install a bog upstairs, bring the pipe down through my airing cupboard, and run it to outside under my kitchen floor, bringing out through the wall at a point where I can connect to the existing soil pipe without hacking up the concrete floor of my utility room, and without losing headroom in my cellar.

Excellent. Thank you, guys.