Sewer - Adequate Fall?

[mike2016]

My Finished Floor Height is 55.16 and my invert level is 54.15 so I have a meter fall to play with. The Foundation buildup is 550mm to the hardcore where the pipes will be laid so a 150mm sewer pipe would have an invert level 300mm above the drain it will ultimately connect to. The longest run would be 5m + 10m + 5m (Front of house, side of house, to foul drain) so 20m. With a 1:60 fall that should work ok shouldn't it? I can decrease the foundation depth to get more drop but wanted to sanity check if this looks ok here before I commit to the foundation design in a couple of weeks time. Thank you! 

[Russell griffiths]

Why are you using 150mm pipe. 110mm is standard, it might connect to a larger pipe but the one from the house will be 110, that might give you a bit more wiggle room. 

[mike2016]

ok, my bad, let's call it 4" ?!! But overall my sh*t won't congeal to a halt with a fall of 350mm over 20 m? 

Edited March 23, 2023 by mike2016

[saveasteading]

1:60 is fine. Just make sure it goes in to that all the way and doesn't have flatter areas.

As the table shows, flatter is ok for heavier flow which you will probably have occasionally.

110mm will flow better than 150mm because the flow is more concentrated/deeper.

[Conor]

1 in 60 is fine on paper but if you are slightly off when laying the pipes then you've hardly any margin for error. I'd be decreasing the cover on your pipes where they leave the house to get you the little bit more fall. You can have the pipes come though the wall at any depth really, just need sufficient build over the pipe.

[saveasteading]

1 minute ago, Conor said:

you are slightly off when laying the pipes

Why would you be? Actuaally i will answer this myself. We put in 90% of the drains ourselves, at the designed 1:60. This after sacking the groundworker who ignored the spec and did it 'just off the bubble' as they put it.  At their slope the 70m runs would have been very deep indeed. Ie 3m instead of 2m.

So 1:60 is fine as long as it really is. 

The first qualification of a groundworker is hard work in wet trenches. Sums and quality are down the list so they need management, usually.

[ProDave]

9 minutes ago, saveasteading said:

Why would you be? Actuaally i will answer this myself. We put in 90% of the drains ourselves, at the designed 1:60. This after sacking the groundworker who ignored the spec and did it 'just off the bubble' as they put it.  At their slope the 70m runs would have been very deep indeed. Ie 3m instead of 2m.

So 1:60 is fine as long as it really is. 

The first qualification of a groundworker is hard work in wet trenches. Sums and quality are down the list so they need management, usually.

It really is simple.  Work out how much your required fall falls in 1 metre.  Make a spacer of that amount and tape it to one end of a 1 metre level.  Then you set all the pipes level with the bubble.  What can be so difficult?

[saveasteading]

10 minutes ago, ProDave said:

Make a spacer of that amount and tape it to one end of a 1 metre level

That's what we did once it was a clear run past connections. On top of a straight 3m timber for an overall check.

 

But you won't get most site or project managers going down a trench to check. So do you trust the groundworker? Good  that's OK then.

 

I had a long drain run once, to a soakaway chamber 1.5m wide. The gw ignored 1:100 on his drawing and as instructed , and used 1:60. He didn't have to buy the extra pc ring and didn't understand the fuss:   "all drains go in at 1:60".

Only later did I wonder how much gravel he would have poured in for his ease.

Gws were hard to get at the time.