Ducts in slab sub base

[Weebles]

Spent the weekend setting out our house.

Hired a theodolite.  Turns out theodolites have gone digital in the 23 years since we last used one ? but a You Tube video helped and we spent a joyous 3 hours setting out all the corners of the house.  Amazingly, we have dug pretty much in the right place.  And we feel more comfortable setting out accurately rather than measuring from a vague boundary hedge.

 

MBC on site Wed / Thursday (in theory).  Our sub-base is going in now (MBC doing the top 150mm MOT type 1).  Just wondering about ducts. for water and BT.  We also need to run the electric armoured cable in.  We know we should have posted this question earlier.  It has been bothering us for weeks.

 

Site plan looks roughly like this.  Its a weird shape (cut off a larger house behind).  Plant room is shaded in yellow.  Intended route of water (blue) and electricity (black, from kiosk) shown.  Both go under the garage slab so we are ducting that tomorrow.  

 

Do we run these ducts at 700mm deep to the plant room and then up into the plant room?  i.e. is the bit I am filling with around 300mm subbase as I type going to have to be dug out to put these ducts in?  Or do they rise slowly and then go in the final layer (which MBC are laying)?

 

BT duct is another question but please help us sort out water and elec first.  Thanks.

 

 

 

 

 

[Jeremy Harris]

I ran all the ducts, and the soil pipe, at around 600 to 700mm down below the slab sub-base top level and just bent them up through 90 deg where they came up through the sub-base.  I used 50mm flexible duct, except for the telephone, which used grey, rigid BT Duct 56, with a hockey stick bend where it came up through the slab.  I had no problem pulling 25mm MDPE or a three core 25mm² SWA cable through 50mm ducting.  In practice a 90 deg bend in flexible ducting is a pretty gentle curve.

 

Not sure it BT insist on Duct 56 being used for the run to the house, but they free-issued us with enough of the stuff, with hockey stick bends, to fit it anyway. 

[Weebles]

Thanks Jeremy.

That answers that one then.  We will dig them in 600-700mm down under the top level and bend them up to go through the upper layers of the sub base.

For BT, if I could get hold of Open Reach I would be delighted.  They keep putting me on hold until they go home!

I ordered some 56 duct online though so will just use that to get into the plant room.  Though don't have a 90 degree bend or hockey stick of that diameter so will see what I can get at short notice.  Will have to worry about where the cable will actually run another day.  Might be routing it round behind the garage after we have finished the build.

Thanks for the quick reply.

[Weebles]

I have seen @JSHarris neat solution for sealing ducts when the house is finished (chicken wire and expanding foam).  What about whilst the slab gets poured etc when there is no pipe in the duct.  Will a plastic bag and a rubber band / electrical tape do the job?

[Onoff]

2 minutes ago, Weebles said:

I have seen @JSHarris neat solution for sealing ducts when the house is finished (chicken wire and expanding foam).  What about whilst the slab gets poured etc when there is no pipe in the duct.  Will a plastic bag and a rubber band / electrical tape do the job?

 

A bit of dpm and duct tape might be stronger?

[PeterW]

Lots of duct tape and either make them higher than the slab by a good way, or just below and encase them in spare polystyrene so you can break them out after you have power floated the slab. 

[Jeremy Harris]

15 minutes ago, Weebles said:

I have seen @JSHarris neat solution for sealing ducts when the house is finished (chicken wire and expanding foam).  What about whilst the slab gets poured etc when there is no pipe in the duct.  Will a plastic bag and a rubber band / electrical tape do the job?

 

 

Just make the ducts way too long and leave them stick out above the pour.  They will stay vertical, held by the holes cut in the insulation.  Here's a photo, showing all the ducts tied together and poking up in the far corner.  When the concrete had cured I just cut the ducts off flush:

 

[Weebles]

Got it.  Great photo, thanks.  What about the other end of those ducts?  Some of ours will be for some for random post build bits - electricity to shed etc.  

Seal the other end in DPM plus duct tape too?.

[Jeremy Harris]

8 minutes ago, Weebles said:

Got it.  Great photo, thanks.  What about the other end of those ducts?  Some of ours will be for some for random post build bits - electricity to shed etc.  

Seal the other end in DPM plus duct tape too?.

 

I just taped plastic bags over both duct ends, with the tape holding the draw cords in place.  I just pulled a plastic bag over the end of the duct, with the draw cord folded back down the outside of the duct, wrapped the draw cord excess around the bag and duct a few times then wrapped tape around to hold it all in place.

[Mr Punter]

Where we are (Sussex) the electric people insist on smooth ducting - 32mm single phase, 50mm 3 phase.

[Weebles]

Finished in the dark an hour ago.  Put in a duct for the electric armoured cable (needs a duct because the location of our plant room / rest of the build / kiosk means we won't be able to leave the armoured cable anywhere convenient to put through a short duct into the plant room.  Put in 63mm twinwall (that's all we had but there is no reason for the electric people to come back on site now so they can't object) in a bit of sand.  Duct taped / heavy duty plastic both ends.

Shifted a tonne of sand tonight (just me and a shovel).  Rolled the last of the recycled crushed concrete ready for the MOT type 1 tomorrow.

Still got to get the 110mm twin wall water duct in.  That is tomorrow's job, along with some BT duct.

Thanks for all the help today.  

[Weebles]

We have a slab! With ducts sticking out of it. All beautifully taped. Thanks everyone. I forgot to allow for electricity to the kitchen island but everything else seems to have gone well.  Ducts aren’t all in perfect positions but we should be able to work around it. Will see for sure when the frame goes up. MBC back in 23rd.