Waste pipes under raised house?

[Crofter]

My wee house sits up on piers, varying from about 150-900mm off the ground.

I need to connect the kitchen sink to the SVP, which is at the other end of the house, about 8m or so away. This is all at the 'shallow' side of the house, where the piers are less than 300mm tall.

My original plan was to run the waste pipe through the insulated 300mm deep space within the floor (I'm using JJI joists). However I'm wondering if there is actually anything at all wrong with just running it underneath the house- this would be much easier. Will there be any sort of frost risk? In some ways it isn't really different to running pipes under a normal suspended floor, after all.

[ProDave]

Make sure there is a GOOD fall and no sags in the pipe for stagnant water to collect and freeze. then I think you will be okay.

Not a great deal different to me having the same under the static caravan. I don't think the token bit of wooden screening around the bottom will do much to keeop the temperature under it above ambient.

[Nickfromwales]

For the few extra few bucks, run all the exposed pipe work in 110mm, reducing only after the vertical bend. 

[Crofter]

Just now, Nickfromwales said:

For the few extra few bucks, run all the exposed pipe work in 110mm, reducing only after the vertical bend. 

What's the advantage to that?

[ProDave]

Less chance of sag I would say?

[Crofter]

I was going to support it on loops of wire stapled to the underside of the joists, so 600 centres.

I've got about a 1:40 fall available, it works out about the same whether I run the pipe within or beneath the floor.

[Oz07]

Less likely to block, easier to maintain, more stable pipe, no flow restriction/future proof and probably performs better at 1:40 as opposed to a smaller pipe

 

+1 to 110mm

[Nickfromwales]

Less issues with sag, which is VERY problematic with the smaller bore pipes, but the main advantage is a near zero risk of freezing, or issues relating to partial freezing.

Another pro is the the inevitable accumulation of 'sludge / slime' in the bottom of the pipe is nowhere near as problematic in the larger pipe as it would be in 1"&1/2 and 1"&1/4 wastes. At the end of the run, instead of a bend going up use a branch and plug the open horizontal end with a rodding eye. That way you can clean / clear it in no time at all.

[Crofter]

All good advice, thanks.

The pipe run will be: sink+dishwasher, then about half way along the washing machine will join, and then it will meet the main stack under the toilet (where the shower and basin feed in as well). So yeah I'm up for increasing the pipe diameter.

[Nickfromwales]

Most pukka static caravan pipework is done that way as it's fit and forget.

[ProDave]

Mine came "pre plumbed" off site in the small bore stuff all apart from the loo.