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Question DetailsAsked on 5/30/2017

What is the main reason for not running a 2" sub pump line and guter together through a 4" PVC pipe 30' out

This is a follow up question submitted on 05/25/2017 by (Hartwell). using a Y to attach the 2" sub pump pipe outside the house and gutter to a 4" PVC pipe under ground 10" running out to the curve 30'

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1 Answer


The risk is freezing up or tree debris getting into the drain line and blocking it, causing backup of the sump pump discharge - or potentially backing up into the sump pump pit. To avoid that, I recommend having the sump pump discharge into the underground drain line with an open entry - free air for say 1/2" or more all around the 2" line and the 2" line not going down into the connection more than needed to hold it in place, so if the underground line plugs up the sump pump water can overflow the connection to the ground.

I would connect the sump pump into the 4" - not the downspout. My normal mode is to run the underground drain almost to the foundation and at that end have a wye and two 45's - the twinned 45's leading up to the sump pump pipe (which would have a 90 on the end of it pointing down into the 4" riser from the 4" 45's) and the downspout going into the wye. The reason for two 45's rather than a 90 - 90's tend to build up leaves and dirt in the bottom "corner" of the bend, whereas twinned 45's provide a positive flushing action.

This is also a good idea for the downspout - so it can continue to drain even if the underground drain is clogged, rather than back up to the roof and overflow the gutters.

And of course, provide surface slope and compacted soil, or an impervious surface, so any overflow runs away from the foundation - maybe through a gentle swale over the underground drain pipe ?

Connections - I would have the sump pump come into the drain above the rtoof connection - so maybe a downspout connection typically using a downspout to underground drain fitting like following (though that price is high by about 4 fold), with the sump pump coming in above at a wye and an open wye (wye facing up) pn the downspout just above the undergrond drain as an outlet for the roof drainage in case of underground drain blockage. Personally, I would NOT use the sump pump entry wye as a roof drainage emergency overflow - too much chance of leaf blockage and such, and icing issues if in freezing area.

One thing - in almost all areas running the roof runoff and sump pump discharge to the curb is illegal - most require you to discharge it on the lawn or in a natural drainage not at least 10 feet from the sidewalk or curb so it soaks in, so check your local building restrictions.

Also of course - with a 30' run be sure you have a place to run a hose in to flush it out to clear roof debris annually, and in an area with any ground freezing conditions 10" depth is not going to cut it - has to be well below frost depth to have any chance of working right. The underground line IS going to need flushing out by running a hose with a jet nozzle on it through its length from top to bottom every few years (or even annually if you get a lot of tree debris off your roof) to keep it clear and to remove the significant amount of dust/silt that washes off most roofs.

Answered 3 years ago by LCD

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