I’m self-building an annex and looking for some advice from those in the know…

 

I had originally intended to exit foul drainage pipes through a section where the foundations are stepped down (and therefore plenty of room with three courses of trench blocks to go through) but hadn’t considered that this section is a long way away from the bathroom.

 

Can I do as the diagram shows and channel through the already-cast concrete footing as well go through the trench block course as shown? I would install a lintel above.

 

Once drains are installed and BC are happy, would I then concrete-in the exit opening?

 

what is best practice for finishing a lintel opening like this? 
 

Any advice appreciated. Ben 

 

what do the building regs for your region say?

My BC officer is off for a week. He had mentioned that two courses of trench blocks were required to exit a 4 inch pipe which I planned for at the other end of the building. Bit to avoid a long run with bends running under the slab, this plan is trying to make the best of a last-minute change to exit the pipe nearer the bathroom. Otherwise I would have just stepped the trenches down in the section to allow for two courses of trench blocks. I’m trying to come up with a solid plan to present to him when he’s back. The pipe will be fully under the concrete slab so I’m hoping he will not have a problem with it. The reason for the post is to get advice on whether it looks like best practice in this situation or if there’s anything I’m missing.

I don't know the answer but I would personally ask SE who designed the foundation - if they say yes (covered by their PI) and provide written confirmation BCO will I believe have to accept. 

Thanks @marmic, I designed the foundations myself through doing research and the BCO is happy with them. So no SE. As far as builds go, it’s a very straightforward one - single story, one bedroom, rectangular footprint, timber frame annex

I suppose it’s down to the BCO whether he is willing to accept the above. I’m on a tight timescale and wanted my next BC visit to be a visual look at the foul drains and block work below DPC. I’m hoping someone can confidently say that doing this would not cause any inherent problems and is good practice in the circumstances (or not as the case may be). A further points to make is that the footings are on very solid ground.

highly unlikley to be going anywhere imo -  but that really doesn't help!  Where does liability sit / calcs?   Hopefully somebody on here can advise/suggest further.  Assume no structural warranty? (they can be picky).

Ours is very similar albeit with an addtional run straight down the middle under a structural spine wall in line with ridge. We used SE - design was almost the same as I had thought but very slightly deeper.  Without SE calcs etc we wouldn't have got past our BCO / structural warranty provider

I don’t think concreting the pipe in is a good move, if there was ever any movement in the foundations it would damage the pipe.

 

We had to get a build over agreement for our extension and where the solid pipe comes through the foundation (with a lintel over) it’s surrounded by shingle to take up any movement.

Simple way is make the finished floor level higher to compensate. Add more insulation and sail over the foundation and add a lintel. But does that add other complications we are not seeing?

@Clark Kent thanks for your input. I’ve just been reading up on this and agree. When using a lintel, something along the lines of 50mm clearance around the pipe and filled with compressible sealant (expanding foam?)

1 hour ago, JohnMo said:

Simple way is make the finished floor level higher to compensate. Add more insulation and sail over the foundation and add a lintel. But does that add other complications we are not seeing?

That would be one way of doing it. My main concern there would be additional material costs and increased labour (I’m going this well, but on a shoestring!). I’m convinced there’s a simple solution. The whole reason for having to make this unexpected opening in the foundation is because I planned for the exit to be on the other side of the property having not considered the underground drainage runs in relation to the bathroom. I’m now going back to the idea of getting the pipes to the original planned exit (where there is plenty of headroom for pipes) with minimal bends and plenty of access, to be fully compliant.

47 minutes ago, BenGillyHills said:

original planned exit

Always best not to change original plans, changes can have big knock on effects.

3 minutes ago, JohnMo said:

Always best not to change original plans, changes can have big knock on effects.

Exactly 👍