Soil stack into main sewer line - advice please

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Looking for a bit of advice as I'm currently in the process of doing our bathroom and as part we've decided to move the soil stack to make better use of the space. The new stack drops from the side wall just next to the main sewer line which runs the length of the house. This is a 6" clay pipe which I've received permission to branch into, subject to an inspection once the work is complete.

Having already excavated down to the main sewer, my intention is to install a rest bend at the bottom of the stack, starting at ground level. I'll branch into the sewer using a 45 degree clay saddle and then convert this to 110mm PVC using a flexi coupler. Then the part I'm having trouble getting right: joining the coupler to the rest bend.

Because the sewer line is very close to the base of the soil stack, and approximately 1ft below where the rest bend would finish, my original thought had been to use a second 87.5 degree bend immediately after the rest bend, positioned sidewards at an approximate 45 degree angle, where it would connect into the flexible adaptor, using a short length of pipe to achieve the depth. My queries are:

1. Is there a problem having a second bend immediately after the rest bend and,
2. Would the 45 degree angle leading into the clay saddle have a sufficient gradient, if it is only over a very short distance of 1-2ft?
3. Is there a simpler solution, or am I just overthinking this?

I've attached a couple of diagrams to illustrate what I mean.

Any thoughts or comments most appreciated!
 

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Have 'they', (if it's a 6" sewer, than I'd be pretty sure its taking a number of properties, and so will be the Water Co's asset), indicated the method of connection? Saddling onto a clay pipe is not an easy task, usual practice would be to slip a Junction into the main run.

Also, what provision are you making for access in the vent of an issue?
 
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Thanks both for your advice, the water company haven’t shown much interest in suggesting a method of connection. Your point about rodding access did encourage me to reconsider my options. What I’d initially suggested (so far as I can tell from the regs) would require an inspection chamber due to the change in angle.

This has led me to considering making the connection directly into the nearest manhole which is about 2m ‘upstream’ from the new stack, and is on our land. My only issue would be bringing the inlet into the manhole at low level as I understand that internal backdrops are not usually permitted. External backdrop might be difficult as the new inlet would need to be on the same side as the outlet.

The water company do however have intentions to modify this manhole due to a number of recent blockages, so this could be a good opportunity to add an additional inlet if they’ll allow it.
 
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Let me tell you, first thing in the morning is not the best time to be taking photos of a manhole! (several buckets of water later….)

Have taken a few photos and drawn on the direction of flow and a couple to indicate where the foul water joins (yellow- soil, green- surface water, red- new connection)

The topmost surface water pipe was in long before we bought the place and it’s certainly not ideal, but will hopefully be renewing the rest of the path and drainage in a few years time, so no immediate concern.

Low down within the red circle is where I’m hoping to bring the new connection in (as well as a telescopic at the top). Only issue is whether this would be considered low enough to satisfy the water company. I also intend to give them a call today but your thoughts would be most appreciated.

Do excuse the odd pesky turd in the photos.

Edit: there’ll also be a rodding access point just above the rest bend
 

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I'd be very surprised if Building Control will let you connect into that as is. Is the chamber just taking waste from your property, or do others use the run? Given the condition of that chamber, I would seriously take the opportunity to demolish it, (shouldn't take much!), and start again.

If it's shared, it's the Water Co's asset, you'll need permission to connect into it, and I'd be having words about them replacing it, or at least contributing towards the cost of doing so.

Given though you are going against the direct of flow, I'd rethink connecting into the chamber, and go back to Plan A, which was to connect into the line downstream of the chamber. I'd say you will need to use a junction, it is very unlikely, (I'd wager impossible) to saddle onto a clay pipe without breaking it. Just cut the required section out and drop the junction in, use Flexseal Couplings to make the joints. Given the short distance from the main to the stack, fit a rodding eye at the base of the stack just above ground level.
 
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Yeah, I would be too to be honest. It’s used by 6 other houses in the terrace. Ours is in the middle so it branches left and right from here.

Like I say, the water company (South West Water) have intentions to carry out some repairs on the thing so I may see if they’d take me up on a complete rebuild and see what their thoughts are on running the soil waste to there at the same time, seeing as half the path has already been dug up. Otherwise I think I understand now what you mean about the couplers. Would the 45* angle down be enough of a gradient or is not such an issue on short drops?

Really appreciate your help on this, thanks
 
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45° under normal circumstances is way too steep, but for that short drop, and given there plenty of other flow through there to wash things through, I wouldn't worry. (Ridiculously steep falls seem OK on New Build work these days it seems!)
 
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