Soil Pipe & Floor Joists

[pyramids.of.mars]

Hi

I need to reposition the soil stack for a new upstairs layout. The new stack will need to run for ~6m under the kitchen floor before connecting to the existing drain pipe.

I've measured 17cm between the underside of the floor joists and the concrete slab. Subtracting 10cm for the pipe, I have approx. 7cm spare, i.e. not enough for a 15cm fall (1:40 on a 6m run by my maths). The joists are perpendicular to the pipe.

As an alternative to excavating a run in the slab, would removing 4" sections from the first ~6 joists and supporting their newly-formed ends on bricks cemented into the slab satisfy the building regs ?

Thanks GD.

[gasman pete]

My maths is not that good i know it should be 2 1/2 degrees.....hence the tees are 92 1/2 ofests pete

[Roger Mills]

2 1/2 degrees is about 1 in 25 - so a 6 metre length at that gradient would require a fall of 24 cms - so allowing 11cms for the pipe, an overall clearance of 35 cms would be needed.

Not sure whether that's necessary - probably not.

[Roger Mills]

You'll still need something to support the floorboards which run over the pipe - so you can't remove *all* the joist at that place or, if you do, you'll need a metal bridge or something.

How deep are the joists. Sticking with your 1:40 slope (but see also the other reply suggesting you need 1:25) you need a clearance of 15 + 11 = 26 cms, and you've only got 17, so you need another 9. If the joists are only

100mm high, you ain't got a lot to play with! If they're more, you should be able to notch the bottoms to take the pipe - starting with 9cms on the first joist, and progressively less on the others. If you support the joists immediately either side of the notches, I don't see a problem with building regs.

[pyramids.of.mars]

Joists are 10cm

The pipe will run diagonally(ish) across the floor, so removal of joists (or notching) would be staggered. I'll use strategically positioned T&G chipboard to avoid floorboard coverage issues.

copy of the regs i've just dug out says that 1:80 is OK for 100mm pipe with flow rate up to 6.3 ltr/s. Anyone care to comment ? Only have two WC's would be attached, a revised calculation of 7.5cm for the fall +

11cm for pipe = 18.5, reducing the problem cosndirerably - I reckon only the first ~3 joists would need to be touched.

All this will be in a full plans submission, but this upfront research is invaluable as ever !

cheers GD.

[Jonathan Pearson]

where has the 1:40 come from, IIRC the fall per metre needs to be between 18 and 90mm, so over 6m would give 18 *6 = 108mm, however again IIRC the BCO may let you go down to 9mm per metre (if you have 2 toilets?? on pipe), giving a fall of 54mm over the 6m

Jon

[The Natural Philosopher]

I think 1:60 is allowable for a soil pipe...but you need a rodding eye where the vertical meets it.

I can see no reasons why, if you have a decent slab underneath, you cannot cut the joists and support them, provided there is a damp proof later between the bricks and the wood.

DO provide at least one means of rodding the underfloor section though. You OUGHT to have a manhole cover at the downstream end of the section...i.e. your underfloor section should be dead straight and exit to the outside into a pot with a cover. I recommend you don't try to get away without this.

[pyramids.of.mars]

Hi

the 6m run will join the existing vertical 'outlet' pipe. The outlet is embedded in the slab at the front of the house and leads directly to the septic tank. This existing stack joins vertically onto the outlet.

Is there a minimum angle that the 6m run needs to connect to the outlet ? As it stands, it would be the angle of the fall discussed above, i.e., very shallow !

I'd put a rodding eye at both ends of of the 6m run.

cheers.

[charlieB]

Check out Marley plumbing design guidance

".....larger diameter 82, 110 and 160mm branch runs may be laid flatter at 9mm/metre fall where the discharge flow rate exceeds

2.5litres/second."

That's basically 1 in 100 gradient.

One other thing to watch out for is the bend radius at the bottom of the vertical pipe - this is probably more important than the gradient as is everything falls down the pipe and smoothly round the bend it should carry on easily.

As others have said - make sure you have/maintain rodding access to the new pipe

Charlie

[pyramids.of.mars]

this is really useful....thanks Charlie.

[Roger Mills]

You might still have a problem!

It seems to me that, in order to get a fall, the bottom of the 'horizontal' pipe needs to sit on the slab at the point where it joins into the stack. This might be difficult - if not impossible - to achieve.

If the stack joins into a socket whose top is flush with the surface of the slab, you'll have to fit a tee into the socket - and the branch of the tee will be a fair bit higher than the slab.