soil pipe height calculations

[walkern]

Hello,
I'm attempting to lay the soil pipe and inspection chamber on part of my extension myself (a new toilet) so did a few calculations and below is my toilet, two inspection chambers and the main sewer outlet that I believe I need to accommodate the change in directions.

From what I've been reading you should aim for 1:80 and 1:110 degree drop and inspection chambers ideally 30cm below surface.

The existing sewer outlet (the pipe I have to join) is 40cm below surface. Doing some calculations on this 11.2m run if I have the first shallow inspection chamber at 30cm it gives a 1:112 drop (i.e. 0.1m across 11.2m). If I change it to 20cm or 25cm it goes to 1:60 ot 1:75 I believe.

Is it ok to make the first inspection chamber this close to the surface (presuming you can cut them as most seem to be 30cm depth) or would 1:112 be ok?

Thanks.

 

[noseall]

Pipes (officially) should have 12" of cover.

We have laid plastic on the smallest of falls but you have to be uber precise when laying and gravelling around the pipe. You work with what you've got.

 

[Ian H]

You really want 1 in 40 fall on a 4" drain.

 

[John D v2.0]

It seems from the approved documents like if you have loo(s) connected your falls can be pretty flat. I guess that's because there won't be much deposited.
But I think although it might be fine, the final word is down to the building control person, so it's worth checking with them if it'll cause a lot of rework.
Regarding the cover, another option if the traffic will be heavy on top is to bridge over the pipes with concrete.

 

[walkern]

Thanks. I read the slope didn't need to be as steep if there's a toilet attached. But i completely forgot to take into account the slope of the ground. It's pretty flat but I'll do another measurement..

 

[Nige F]

Is it possible to exit the wall from the toilet above ground then run along the wall to a vent pipe and drop there underground? Pictures would be useful.

 

[mfarrow]

Our house has 1:200 slope rains and although a pain to set (renewed after extension) they work OK and passed Severn Trent inspection.

I concreted mine in at intervals to keep it lined up then pea shingled afterwards. I would normally go straight to pea shingle but didn't want any flat spots.

 

[Ian H]

How are you measuring these tiny falls?

 

[John D v2.0]

Personally I'd use a water level and sit posts of a known heigh on the pipes, but apparently you can get laser measuring devices that tell you relative heights to ridiculous accuracy.

 

[Ian H]

If I lay one with tiny falls (which would be a replacement from A to B) I pipe up best I can with a level, run water down the pipe then bed the pipe with the camera inside to try and have it not holding water.

I don't think you could lay plastic pipe to such a small tolerance with a normal level.

 

[mfarrow]

Maybe it was nearer 1:150. Anyway, 1800 level with a wedge taped under it at one end, and a close eye on the bubble.

 

[walkern]

Thanks, I've just ordered a 15m water level, should be arriving tomorrow

 

[^woody^]

With drainage, just make sure that one end is lower than the other.

Use the 3m pipe as a levelling board and then lower the one end 75mm. No need for fancy levelling devices. If it looks right it is right. The bco won't check it for level, as it's more to do with what will be doing down the section of pipe and how often.

 

[walkern]

Thanks everyone.

I got the tubing, it was only a few quid, filled it with anti-freeze (I read it was less liable to bubbles and was coloured so made it easier). I've worked out there's a drop of 14cm so my 40cm drain is actually 53cm and so need to start the pipe 40cm down to give a nice drop of 1:80.

But yes, I'll do the above Woody. It was more a case of working out prior how far to dig down, and whether it was feasible.

Plus I enjoyed making the level and trying it out, and I can use it for my main extension

 

[John D v2.0]

Glad you worked out out but don't use anti freeze it's wasteful and better not to use more chemicals than you need to! Just use water. At least you didn't try brake fluid