Pipe gradients for kitchen sink.

[Jon Fairbairn]

I have a kitchen sink (and dishwasher) that is some distance from the gulley. I can find web pages that recommend 1 in 40 to

1 in 120 as the range of gradients that work best on the grounds that solids will be stranded as the liquid flows too fast if the gradient is too steep or settle out if it?s too gentle. I think this is mainly for sewage. Does this really apply to the grey water that one gets out of a kitchen sink?

I?m moving the pipe because it blocks frequently and is hard to get to to unblock. It currently drops through the bottom of the sink cupboard and then runs along and turns a corner under the cabinets before going out through the wall. The main problem seems to be that a biofilm forms on the inside of the pipe, and I would have thought that having the liquid flow past quickly and dry out as much as possible would be the thing to go for.

Any ideas?

[Dave Liquorice]

The building regs have reasonable detail on the required falls and pipe sizes. I think it's Doucment H:

/parth/

URL may well be broken but I'm not makinga tinyURL for 3 characters.

[Andy Burns]

[Jon Fairbairn]

Thanks. Looking through it I can?t find anything specific to the case. This is going from the sink into a gulley, so it?s vented and not a branch. The permitted gradients for branches do go up to 1 in 20, though.

Gnus can unsplit URLs, so it doesn?t bother me.

[Tim Watts]

The long and short is "fall along all of the pipe" is essential.

It's ideal if the fall is "good" - but no fall (or worse a sag) will cause blockages. A continuous fall, even if not ideal is usually manageable.

If good fall cannot be achieved, run the pipe in 50mm and stick some tees with a screw cap end to act as rodding points.

[Jon Fairbairn]

Sure. I have no problem getting it steep enough. What I?m wondering about is the upper limit on how steep it should be.

[Tim Watts]

None really.

[GMM]

That's certainly always been my view but my kitchen sink outlet runs for about 10ft with a shallow slope, through a wall, then vertically for a foot, through a 90 degree turn then along another couple of feet of shallow slope to the gully. Dunno if that's the cause but it makes some pretty 'orrible gurgles when the sink's emptied and I've been meaning to have a fiddle with the geometry for ages to see if it has any effect.

[Tim Watts]

Another way to handle this (and the preferred way on sewer pipes where there are maximum falls quoted in building regs) is to run on a sensible slope, then a 90 degree bend with a gentle bend at the bottom (not a tight elbow) then continue with a run at a reasonable fall again.