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1554 posts

Uber Geek

Topic # 225600 25-Nov-2017 21:54

How do drain layers get minimal slope on pvc pipe?

I have a slight downslope away from my house. I want to run 90 mm pvc storm water pipe from a catch basin/access pit in a straight line down the slight slope to 8 m away to discharge to daylight near my garden.

The access pit has a female socket for the pipe at (I presume) 90 degrees.
The top of the access pit is supposed to sit flush with the top of the ground.

Note that this drawing is not to scale and also exaggerates the slope of my lawn. It shows what I would like to do with the pipe.

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## DarthKermit

4647 posts

Uber Geek

Trusted

Measure the distance from A to B, determine the minimum amount of slope per metre you require and calculate how much lower the outlet needs to be from the inlet.

## DarthKermit

4647 posts

Uber Geek

Trusted

I think that 1 in 50 is the minimum slope for drainage. So for every 1000 mm you'd drop 20 mm. 8 * 20 = 160 mm of drop over 8 metres.

1554 posts

Uber Geek

Thanks for the reply. 1 in 100 is I think the absolute minimum. It's an 8 metres I need the pipe to go.

But how do I get the pipe to achieve the desired angle? Corrugated pipe just follows the hole you stick it in...

## dgashby

65 posts

Master Geek

So the question then becomes how do you dig a trench with the right grade?

A low tech approach would be to use a string-line, level, tape measure, and a plain old garden stake.

But this being Geekzone, you could use a laser instead of the string line :-)

1554 posts

Uber Geek

No, I can dig the trench ok.
I don't know how to get the 90 degree angle the pipe comes out of the access pit into the actual angle I need it to be! It is so frustrating.

## dgashby

65 posts

Master Geek

Place the stake in the ground at the outlet end of the pipe. use the a string line between the top of the sump and the stake.  Adjust the height of the string on the stake to make it level and then mark that position on the stake,  That's where you measure the drop height from - also add in the extra height difference between the top of the sump and the lower edge of the pipe at the sump end.  Put a mark lower down on the stake where drop height is.  Start digging your trench.  When you start getting close to the right heights along the trench use the string line between the lower pipe edge in the sump and the lower mark on the stake.  Keep digging where the string touches the ground.

Another high teach approach would be to use a excavator fitted with Trimble Earthworks but that might be a little over the top :-)'

 Just saw your updated picture, if you're using corrugated pipe as you mention, won't it just bend to the correct angle as it attaches to the sump.  Even if you're using solid pipe, depending on the diameter, won't it also bend to some degree over the first meter or so?

## DarthKermit

4647 posts

Uber Geek

Trusted

Because that type of pipe is flexible, it's impossible to lay it on a perfect continuous slope down.

What you could do is dig a trench and run a length of builder's string in the trench, pull it tight and fix it on the slope you want to achieve.

Then run the flexible pipe in the trench to match the downward grade of the string as close as possible.

## PhantomNVD

2503 posts

Uber Geek

2 people support this post

It’s called a ‘running offset’

1554 posts

Uber Geek

I'm not using flexible pipe, I'm using 90 mm thin wall, solid pvc. Like the 60 and 80 mm kind on your downspouts.
I don't know how to hook the pipe up to my access pit without it being stuck there with zero slope.

1554 posts

Uber Geek

PhantomNVD: It’s called a ‘running offset’

Oh crap, that looks expensive as hell. Those fittings cost a fortune.

## dgashby

65 posts

Master Geek

Phantom's solution gets my vote, it looks like the way a professional would do it.

## DarthKermit

4647 posts

Uber Geek

Trusted

Sorry, must have mis-read the above.

What sort of access pit are you using? If you use a hole saw to cut a hole thru its side (assuming it's a plastic one), there should be enough give to allow you to angle the pipe down a little bit. It's not like solvent bonding a pipe into a spigot where it's a snug fit and there's no way to insert it on a bit of an angle.

1554 posts

Uber Geek

DarthKermit:

Sorry, must have mis-read the above.

What sort of access pit are you using? If you use a hole saw to cut a hole thru its side (assuming it's a plastic one), there should be enough give to allow you to angle the pipe down a little bit. It's not like solvent bonding a pipe into a spigot where it's a snug fit and there's no way to insert it on a bit of an angle.

It's an Everhard with a pitboss adapter siliconed into it, so yeah it has a female socket that the 90 mm slides snugly into and it theoretically cemented into with solvent cement.

I just assumed there was some way of using a connector- like an elbow or combination of them to get a slight angle. Like I could use a 22 turned down and a 15 turned up to get 7 degrees but that's far too steep.

## DarthKermit

4647 posts

Uber Geek

Trusted

One person supports this post

If you can't angle the pipe from your pit directly at the outlet, perhaps use one 15 degree bend and rotate it so it only angles down enough to get the slope you need.

The outlet pipe doesn't need to run directly at 90 degrees away from the pit, does it?

1554 posts

Uber Geek

DarthKermit:

If you can't angle the pipe from your pit directly at the outlet, perhaps use one 15 degree bend and rotate it so it only angles down enough to get the slope you need.

The outlet pipe doesn't need to run directly at 90 degrees away from the pit, does it?

Yeah, that wouldn't be a bad idea!
Have the pipe coming off the side not off the front of the pit then put a couple of 45 turns in to line it back up to where I want the outlet.

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