Insufficient fall on sewer
Discussion
Hi chaps
About to start on a big double storey side and rear extension.
We have a manhole close to the rear of the property.
We have a sewer that runs perpendicular from a neighbouring house before joining ours and running to front (currently under kitchen)
We're having to redirect this sewer down the sideway and reconnect it at the front manhole by the street. Builder wants to eliminte/redirect them out of the way.
Anyway, he lifted all the manholes today (he's not actually starting for 8 weeks) and was very surprised by the lack of fall between the rear and front of the property. It's currently only a 10cm fall - over a distance of what must be 60 odd feet.
We used to get problems with blockages, but not for about 10 years or so.
Anyway, question.
If the fall on the drain is too shallow, isn’t it Thames Waters job to sort?
It’s technically a public sewer and it feels like i might have to pay to put right a poor design. With or without our new extension.
Existing manholes have been down for well over 15 years, if not original.
House is circa 1928 - Semi detached.
About to start on a big double storey side and rear extension.
We have a manhole close to the rear of the property.
We have a sewer that runs perpendicular from a neighbouring house before joining ours and running to front (currently under kitchen)
We're having to redirect this sewer down the sideway and reconnect it at the front manhole by the street. Builder wants to eliminte/redirect them out of the way.
Anyway, he lifted all the manholes today (he's not actually starting for 8 weeks) and was very surprised by the lack of fall between the rear and front of the property. It's currently only a 10cm fall - over a distance of what must be 60 odd feet.
We used to get problems with blockages, but not for about 10 years or so.
Anyway, question.
If the fall on the drain is too shallow, isn’t it Thames Waters job to sort?
It’s technically a public sewer and it feels like i might have to pay to put right a poor design. With or without our new extension.
Existing manholes have been down for well over 15 years, if not original.
House is circa 1928 - Semi detached.
Had to run some sewer pipe myself a few years ago. The fall is related to the size of pipe - but as long as there is a fall everything should be OK.
I managed a 1 in 100 fall for a 4 inch pipe over about 50 metres - your figures suggest a 1 in 180 fall - which will probably be fine for reasonable size pipe. (No, I did not dig it by hand, used a chain trencher as one end was getting on for a metre deep).
It's drains that go uphill that tend not to work well.
I managed a 1 in 100 fall for a 4 inch pipe over about 50 metres - your figures suggest a 1 in 180 fall - which will probably be fine for reasonable size pipe. (No, I did not dig it by hand, used a chain trencher as one end was getting on for a metre deep).
It's drains that go uphill that tend not to work well.
Yu don't need much of a fall at all, some of the sewer long sections I have seen are surprising in terms of how slight the fall can be. You also get problems if the fall is too great, especially on comers.
If it is a public sewer you need to start talking to Thames Water. You need their permission to make new connections and/or increase the discharge. I would have expected issues with the fall to be Thames Water's problem, but I cant see them changing it to suit you as long as it works ok at present....
If it is a public sewer you need to start talking to Thames Water. You need their permission to make new connections and/or increase the discharge. I would have expected issues with the fall to be Thames Water's problem, but I cant see them changing it to suit you as long as it works ok at present....
Fastpedeller said:
My understanding is that the ideal fall needs to be 1:40 - apparently the Romans found this to be correct ..... who am I to disagree with them! Much shallower and flow won't work well, much steeper and fluid will flow too quickly and leave solid behind.
This was my undertsanding as well.Also, that in some circumastances the local authority will allow for a 1:80.
But what if the drain was built with a 1:180, like mine?
Shouldn't be my problem to put right, surely.
Thanks for the swift replies guys.
Roughly speaking a rain/storm sewer sewer can fall the same as the diameter
Ie 100 diameter 1 in 100, 150 diameter 1 in 150 etc.
Foul water is more concerned with trying to achieve a self cleansing velocity than capacity. Roughly 1 in 40 for a 100. If you connect 1 toilet to the run it can be 1 in 80 as the flush helps move things along. If you connect 5 toilets you can go 1 in 150 which may be the case here if the neighbours are on the same run.
Ie 100 diameter 1 in 100, 150 diameter 1 in 150 etc.
Foul water is more concerned with trying to achieve a self cleansing velocity than capacity. Roughly 1 in 40 for a 100. If you connect 1 toilet to the run it can be 1 in 80 as the flush helps move things along. If you connect 5 toilets you can go 1 in 150 which may be the case here if the neighbours are on the same run.
Labbetts said:
This was my undertsanding as well.
Also, that in some circumastances the local authority will allow for a 1:80.
But what if the drain was built with a 1:180, like mine?
Shouldn't be my problem to put right, surely.
Thanks for the swift replies guys.
Your understanding is incorrect. I've recently approved some 600mm pipes at 1:450 for a new build school so flatter gradients can be quite acceptable. Also, that in some circumastances the local authority will allow for a 1:80.
But what if the drain was built with a 1:180, like mine?
Shouldn't be my problem to put right, surely.
Thanks for the swift replies guys.
Your fall sounds quite flat for a small pipe, but probably ok.
I presume you are having to move the sewer as part of the conditions implied by Thames?
What did they say when you made the request to move their asset? Did they mention having to correct fall?
Normally, they will ask for a survey to be completed prior to you building. Off the back of this, they will then outline what needs to happen. If you haven't got their OK, your builder should not be adjusting or touching it - it's their asset and if there are issues with fall, if you adjust the system, they could try and pin on you at a later date.
Of course, if you do involve them, they may try to restrict your plans so I guess that may influence whether you go to them or not.
If the fall is poor and cannot be adjusted, there are ways around it. You may be able to get away with it, but changing the route will obviously lengthen the pipework and reduce the fall further. A pumped chamber could be a solution, or you may be able to chase the pipework back upstream to get a better run.
What did they say when you made the request to move their asset? Did they mention having to correct fall?
Normally, they will ask for a survey to be completed prior to you building. Off the back of this, they will then outline what needs to happen. If you haven't got their OK, your builder should not be adjusting or touching it - it's their asset and if there are issues with fall, if you adjust the system, they could try and pin on you at a later date.
Of course, if you do involve them, they may try to restrict your plans so I guess that may influence whether you go to them or not.
If the fall is poor and cannot be adjusted, there are ways around it. You may be able to get away with it, but changing the route will obviously lengthen the pipework and reduce the fall further. A pumped chamber could be a solution, or you may be able to chase the pipework back upstream to get a better run.
guitarcarfanatic said:
I presume you are having to move the sewer as part of the conditions implied by Thames?
What did they say when you made the request to move their asset? Did they mention having to correct fall?
Normally, they will ask for a survey to be completed prior to you building. Off the back of this, they will then outline what needs to happen. If you haven't got their OK, your builder should not be adjusting or touching it - it's their asset and if there are issues with fall, if you adjust the system, they could try and pin on you at a later date.
Of course, if you do involve them, they may try to restrict your plans so I guess that may influence whether you go to them or not.
If the fall is poor and cannot be adjusted, there are ways around it. You may be able to get away with it, but changing the route will obviously lengthen the pipework and reduce the fall further. A pumped chamber could be a solution, or you may be able to chase the pipework back upstream to get a better run.
Thanks for all the great replies chaps.What did they say when you made the request to move their asset? Did they mention having to correct fall?
Normally, they will ask for a survey to be completed prior to you building. Off the back of this, they will then outline what needs to happen. If you haven't got their OK, your builder should not be adjusting or touching it - it's their asset and if there are issues with fall, if you adjust the system, they could try and pin on you at a later date.
Of course, if you do involve them, they may try to restrict your plans so I guess that may influence whether you go to them or not.
If the fall is poor and cannot be adjusted, there are ways around it. You may be able to get away with it, but changing the route will obviously lengthen the pipework and reduce the fall further. A pumped chamber could be a solution, or you may be able to chase the pipework back upstream to get a better run.
I haven't engaged said water company yet. So nothing has been mandated yet. The builder wants to re-route the soil pipes down the side access, to simply eliminate them from any future headaches (no doubt a preferable option). There will be a few build overs and around 5m, or thereabouts of additional pipework. It works well at present, despite the shallow fall, so hoping the water company will be ok.
I suggest you carry out an accurate survey of the sewer and plot the runs onto the proposed layout.
Then you can have a look at what will work.
Thames Water will want to see the existing layout and the proposal, they won't allow foundations to be within 100 mm of the side walls of the sewer if less than 1100mm deep, that increases to 600mm away if the sewer is deeper than 1100 but not more than 2m
No manholes allowed within the building and rightly so.
If your alterations are likely to make the drainage fall outside of what they would normally allow, it has been known for the builder to carry out the changes before the works start , whilst you will still need to apply for the build over agreement , you won't be asking for changes.
They will charge you £299 for an online application.
Do not approach them until you know exactly what you are doing.
Then you can have a look at what will work.
Thames Water will want to see the existing layout and the proposal, they won't allow foundations to be within 100 mm of the side walls of the sewer if less than 1100mm deep, that increases to 600mm away if the sewer is deeper than 1100 but not more than 2m
No manholes allowed within the building and rightly so.
If your alterations are likely to make the drainage fall outside of what they would normally allow, it has been known for the builder to carry out the changes before the works start , whilst you will still need to apply for the build over agreement , you won't be asking for changes.
They will charge you £299 for an online application.
Do not approach them until you know exactly what you are doing.
Edited by Busa mav on Thursday 12th January 13:52
I would revise that last sentence to 'until you know what you are proposing.'
What ever you propose you are going to need to engage with Thames Water. Shared drains are now, by recent statute, the responsibility of the loacal Water Board. Doesn't mean they have always been. Indeed, the Water Board may have had no involvement at all in the construction and routing of the drain.
If you would like to start in 8-weeks then you need to be apply to Thames Water ASAP.
What ever you propose you are going to need to engage with Thames Water. Shared drains are now, by recent statute, the responsibility of the loacal Water Board. Doesn't mean they have always been. Indeed, the Water Board may have had no involvement at all in the construction and routing of the drain.
If you would like to start in 8-weeks then you need to be apply to Thames Water ASAP.
mk1fan said:
I would revise that last sentence to 'until you know what you are proposing.'
What ever you propose you are going to need to engage with Thames Water. Shared drains are now, by recent statute, the responsibility of the loacal Water Board. Doesn't mean they have always been. Indeed, the Water Board may have had no involvement at all in the construction and routing of the drain.
If you would like to start in 8-weeks then you need to be apply to Thames Water ASAP.
Many thanks for your thorough summation.What ever you propose you are going to need to engage with Thames Water. Shared drains are now, by recent statute, the responsibility of the loacal Water Board. Doesn't mean they have always been. Indeed, the Water Board may have had no involvement at all in the construction and routing of the drain.
If you would like to start in 8-weeks then you need to be apply to Thames Water ASAP.
NeilC said:
Roughly speaking a rain/storm sewer sewer can fall the same as the diameter
Ie 100 diameter 1 in 100, 150 diameter 1 in 150 etc.
Foul water is more concerned with trying to achieve a self cleansing velocity than capacity. Roughly 1 in 40 for a 100. If you connect 1 toilet to the run it can be 1 in 80 as the flush helps move things along. If you connect 5 toilets you can go 1 in 150 which may be the case here if the neighbours are on the same run.
That's near enough corrrect from a Building Regs point of view, but you can only go to 1/150 if the pipe is 150mm dia and not 100mm.Ie 100 diameter 1 in 100, 150 diameter 1 in 150 etc.
Foul water is more concerned with trying to achieve a self cleansing velocity than capacity. Roughly 1 in 40 for a 100. If you connect 1 toilet to the run it can be 1 in 80 as the flush helps move things along. If you connect 5 toilets you can go 1 in 150 which may be the case here if the neighbours are on the same run.
Equally, this is a building regs allowance, and regardless of number of WCs connected some of the more belligerent water authorities would still not accept it unless 10 properties are connected.
AlmostUseful said:
That's near enough corrrect from a Building Regs point of view, but you can only go to 1/150 if the pipe is 150mm dia and not 100mm.
Equally, this is a building regs allowance, and regardless of number of WCs connected some of the more belligerent water authorities would still not accept it unless 10 properties are connected.
I actually made an error.Equally, this is a building regs allowance, and regardless of number of WCs connected some of the more belligerent water authorities would still not accept it unless 10 properties are connected.
Fall is 200mm and arhitect is proposing that we upgrade the pipe from 100mm to 150mm PVC - says it gives very little additional resistance.
I don't know why, but I had it in my mind from the first time I read the thread that this was your own drain, as it's not it's a slightly different approach.
A 1/90 fall isn't awful, it's certainly not ideal however and unless your replacing/diverting on a like for like basis you'all struggle to get it approved. Upgrading to 150mm may seem an ideal option, but you need to ensure you have a 150mm connection at the lower end to tie in to, you won't get approval for a 150mm pipe to go into a 100mm pipe.
Your 200mm drop is also reduced to 150mm because you drop 50mm at the U/S invert to ensure level soffits which reduces your gradient further to 1/120.
I've done very little with Thames so wouldn't like to comment with too much certainty, but in my patch I submit these things to Suth West Water, Southern Water, Wessex and Severn Trent - Wessex wouldn't think twice about accepting such a proposal, SWW would make me submit far more calcs than are truly required but will eventually accept that their guys know less than me and its just easier if they give up, Southern would take months to respond to my letter because they're too busy so I'd forget about it and give it to one of my grads to sort out and STW would accept it with some careful caveating.
The key here is asset protection, if what you're doing is in anyway damaging to the fabric or performance of their sewer they'll say no. Personally I'd be happy with two or three houses going into a six inch pipe at one in one twenty but I'm not the one approving it!
Also my keyboard is doing strange things on the iPad so apologies for the sudden turn into typing full numbers rather than using digits...
A 1/90 fall isn't awful, it's certainly not ideal however and unless your replacing/diverting on a like for like basis you'all struggle to get it approved. Upgrading to 150mm may seem an ideal option, but you need to ensure you have a 150mm connection at the lower end to tie in to, you won't get approval for a 150mm pipe to go into a 100mm pipe.
Your 200mm drop is also reduced to 150mm because you drop 50mm at the U/S invert to ensure level soffits which reduces your gradient further to 1/120.
I've done very little with Thames so wouldn't like to comment with too much certainty, but in my patch I submit these things to Suth West Water, Southern Water, Wessex and Severn Trent - Wessex wouldn't think twice about accepting such a proposal, SWW would make me submit far more calcs than are truly required but will eventually accept that their guys know less than me and its just easier if they give up, Southern would take months to respond to my letter because they're too busy so I'd forget about it and give it to one of my grads to sort out and STW would accept it with some careful caveating.
The key here is asset protection, if what you're doing is in anyway damaging to the fabric or performance of their sewer they'll say no. Personally I'd be happy with two or three houses going into a six inch pipe at one in one twenty but I'm not the one approving it!
Also my keyboard is doing strange things on the iPad so apologies for the sudden turn into typing full numbers rather than using digits...
AlmostUseful said:
I don't know why, but I had it in my mind from the first time I read the thread that this was your own drain, as it's not it's a slightly different approach.
A 1/90 fall isn't awful, it's certainly not ideal however and unless your replacing/diverting on a like for like basis you'all struggle to get it approved. Upgrading to 150mm may seem an ideal option, but you need to ensure you have a 150mm connection at the lower end to tie in to, you won't get approval for a 150mm pipe to go into a 100mm pipe.
Your 200mm drop is also reduced to 150mm because you drop 50mm at the U/S invert to ensure level soffits which reduces your gradient further to 1/120.
I've done very little with Thames so wouldn't like to comment with too much certainty, but in my patch I submit these things to Suth West Water, Southern Water, Wessex and Severn Trent - Wessex wouldn't think twice about accepting such a proposal, SWW would make me submit far more calcs than are truly required but will eventually accept that their guys know less than me and its just easier if they give up, Southern would take months to respond to my letter because they're too busy so I'd forget about it and give it to one of my grads to sort out and STW would accept it with some careful caveating.
The key here is asset protection, if what you're doing is in anyway damaging to the fabric or performance of their sewer they'll say no. Personally I'd be happy with two or three houses going into a six inch pipe at one in one twenty but I'm not the one approving it!
Also my keyboard is doing strange things on the iPad so apologies for the sudden turn into typing full numbers rather than using digits...
Thanks mate. Spot on response.A 1/90 fall isn't awful, it's certainly not ideal however and unless your replacing/diverting on a like for like basis you'all struggle to get it approved. Upgrading to 150mm may seem an ideal option, but you need to ensure you have a 150mm connection at the lower end to tie in to, you won't get approval for a 150mm pipe to go into a 100mm pipe.
Your 200mm drop is also reduced to 150mm because you drop 50mm at the U/S invert to ensure level soffits which reduces your gradient further to 1/120.
I've done very little with Thames so wouldn't like to comment with too much certainty, but in my patch I submit these things to Suth West Water, Southern Water, Wessex and Severn Trent - Wessex wouldn't think twice about accepting such a proposal, SWW would make me submit far more calcs than are truly required but will eventually accept that their guys know less than me and its just easier if they give up, Southern would take months to respond to my letter because they're too busy so I'd forget about it and give it to one of my grads to sort out and STW would accept it with some careful caveating.
The key here is asset protection, if what you're doing is in anyway damaging to the fabric or performance of their sewer they'll say no. Personally I'd be happy with two or three houses going into a six inch pipe at one in one twenty but I'm not the one approving it!
Also my keyboard is doing strange things on the iPad so apologies for the sudden turn into typing full numbers rather than using digits...
In your view, would I be better sticking with a 100m pipe then?
The diversion isn't radical.
It will run parallel to the exitisting pipe (front to back), albeit about 1.5m nearer to the boundary.
Does it only serve the two properties?
Touch and go really, bit too slack at 100mm really, but not enough flow to keep a 150 pipe flowing sufficiently.
Building Reg's would allow the 150mm with about 3 WCs in so I'd be happy with it if private.
But SFA6th and 7th are quite specific in what they allo.
If it's only 2 houses Could you guesstimate the amount of toilets, wash hand basins, kitchen sinks, baths, dishwashers and showers in and I'll do the maths on Monday and get back to you.
If it's more houses than 2 just go with the 150mm.
Touch and go really, bit too slack at 100mm really, but not enough flow to keep a 150 pipe flowing sufficiently.
Building Reg's would allow the 150mm with about 3 WCs in so I'd be happy with it if private.
But SFA6th and 7th are quite specific in what they allo.
If it's only 2 houses Could you guesstimate the amount of toilets, wash hand basins, kitchen sinks, baths, dishwashers and showers in and I'll do the maths on Monday and get back to you.
If it's more houses than 2 just go with the 150mm.
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