Ideal drainage pipe gradient advice needed.

[SteeVeeDee]

Hi,

I'm doing a main sewer connection for foul and surface drainage in the next few weeks/month. We're in a deep clay area and the sewers are all combined systems around here. The house just now is four bedrooms with family bathroom, w/c, and kitchen. It will later likely be (drawings still in process) 6 bedrooms, 3 en-suite shower rooms, main bathroom, kitchen, w/c, and utility. There may be a toilet in the detached garage later as well.

There is an existing manhole on the edge of the garden that is well built and looks to be 80s/90s. They left a stub so that i can join on. 

I'm wondering what the ideal gradient is? I've got a connection to the main at around 1.5m and don't have that far to travel so have plenty space to make it the ideal pitch. I keep seeing maximum or minimum gradient but I cant find the ideal. Is it halfway between the max and min? I'm mindful that clay can heave, although we have moderately mobile clay here. 

I think I'm going to use clay supersleeve from the existing manhole to my own manhole but open to advice. I'll build my manhole by laying engineering bricks or concrete liners, and run out a section of clay so that I can then convert to plastic. I'm going to use plastic up to the existing house for cost and convenience but I will change this to clay when I do the extension and renovation later.

Steve

[Russell griffiths]

All in plastic, why use clay unless it’s been specifically asked for. 
ideal fall is 1::40. 

[bmj1]

Use plastic for your drainage. The modern plastic pipes are very sturdy. Much better.

[saveasteading]

More information needed. You have told us that you have 1.5m of distance between the existing fixed points, but not the height difference.

 

The best slope depends on the gradient and the water quantity....you say it is combined, so we also need to know if you have a huge catchment for rain.

 

Plastic is very much easier to work with than clay. I wouldn't dream of using clay unless I was building a motorway or the liquid is nasty.

 

[Nickfromwales]

Just lay the normal plastic pipes as said, these are very robust. Just leave a good 200mm below, around and above and backfill with plenty of pea shingle to allow a bit of wiggle room. 

[SteeVeeDee]

23 hours ago, saveasteading said:

More information needed. You have told us that you have 1.5m of distance between the existing fixed points, but not the height difference.

 

The best slope depends on the gradient and the water quantity....you say it is combined, so we also need to know if you have a huge catchment for rain.

 

Plastic is very much easier to work with than clay. I wouldn't dream of using clay unless I was building a motorway or the liquid is nasty.

 

My bad writing skills strike again!

 

The drop to connection on the existing manhole from ground level is 1.5m. I've not done the length yet but it has a 90* turn that I will 45* each side of the new manhole, probably 10m and 5m ish. I can measure it later.

Water is very hard. We'll get a filter for drinking water later but will be impractical for a whole house filter given the likely water demand, although I'd prefer it.

Roof will be pretty big but mot a mansion. Fairly typical 6 bedroom detached. 

Plastic is easy to use but it goes brittle over time and the seals perish. The seals also often get nicked during installation although thats from being careless, which I rarely am.

I'm 45 now and don't want any major works when I get old. I've regularly seen clay pipework in practically new condition, despite being over 100 years old. Plastic pipe literally wears out and breaks up when jetted or rodded. 

I don't mind plastic when it is easily inspected or replaced but don't want it anywhere underground. The gutters and rwp will be cast iron. My wife would prefer copper gutters and rwp but the mobile mechanics will have that away in no time. 

[Nickfromwales]

1 hour ago, SteeVeeDee said:

Plastic is easy to use but it goes brittle over time and the seals perish. The seals also often get nicked during installation although thats from being careless, which I rarely am.

Nope, that’s completely wrong information, sorry. UG stuff is near idiot proof and extremely robust, and as it’s not got UV getting to it has the resultant longevity.

 

Plastic soil stacks rising externally have been fine for 30+ years in direct sunlight. 
 

Not sure where you’re getting your information from, but it’s duff. 🫤

[Russell griffiths]

I thought clay pipe now uses plastic and rubber connectors, no sockets and hemp packing any more. 
the clay is straight and the connector looks just like a plastic slip coupler. 

[saveasteading]

54 minutes ago, Russell griffiths said:

clay pipe now uses plastic and rubber connectors, 

That's right. More flexible and a better seal. Clay is for veey heavy loads and for nasty chemicals from factories (though I've seen rotted ones).

 

You've come to the right place if you're picking up such strange and erroneous information.

 

Next.... two 45 ° turns isn't right. You would generally  get away with it if only rainwater but it needs to be roddable for foul.  So a rodding point and 90° turn is needed. Or 2 at 45°. Plastic is normal for these too. 

 

When you have measured, post a sketch.

 

Water hardness is not relevant.

 

Next tconsider  aluminium gutters.  In this situation plastic is not best, but cast iron is expensive and rusts, plus is low capacity.

There is copper effect available in aluminium. 

 

 

 

[SteeVeeDee]

On 09/07/2025 at 15:02, Nickfromwales said:

Nope, that’s completely wrong information, sorry. UG stuff is near idiot proof and extremely robust, and as it’s not got UV getting to it has the resultant longevity.

 

Plastic soil stacks rising externally have been fine for 30+ years in direct sunlight. 
 

Not sure where you’re getting your information from, but it’s duff. 🫤

 Contracted to a plumbing company for four years straddling covid and had to attend a lot of dig-ups. Particularly when family all came back to lockdown together and the drains were wroking harder than usual. Also been building extensions and doing renovations for circa 25 years. My information comes from personal experience. 

[Nickfromwales]

5 minutes ago, SteeVeeDee said:

 Contracted to a plumbing company for four years straddling covid and had to attend a lot of dig-ups. Particularly when family all came back to lockdown together and the drains were wroking harder than usual. Also been building extensions and doing renovations for circa 25 years. My information comes from personal experience. 

Understood, just is the opposite of mine, with ~35 years on the coal face. I can still taste the damn coal  

[SteeVeeDee]

2 minutes ago, Nickfromwales said:

Understood, just is the opposite of mine, with ~35 years on the coal face. I can still taste the damn coal  

I think plastic is good in general, and always what I've used. I've never actually fitted a clay system entirely, only small repair sections to connect the fernco to. Its not infallible though and I don't want to cut corners for my own home. I've seen some real horror shows when working with drainage engineers, and dealing with human waste isn't glamorous. I'm fine to use it where its accessible but I don't want to have to be digging up my own driveway in 20 years, with my wife is giving me the side eye. 

[Oz07]

Get a good brand of pipe. I've picked some cheap ones up which feel half the weight of proper brands. Also make sure they come straight I've sent some bananas back once. Mustve been stores wrong for ages in sun or something. 

[Nickfromwales]

2 hours ago, SteeVeeDee said:

I think plastic is good in general, and always what I've used. I've never actually fitted a clay system entirely, only small repair sections to connect the fernco to. Its not infallible though and I don't want to cut corners for my own home. I've seen some real horror shows when working with drainage engineers, and dealing with human waste isn't glamorous. I'm fine to use it where its accessible but I don't want to have to be digging up my own driveway in 20 years, with my wife is giving me the side eye. 

If you fit it properly it'll probably outlast you. UG pipework is never accessible....