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Drain in trench...your thoughts?


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Shouldn't the trench be back filled or at least filled with gravel or something? Poly will never support and prevent wet concrete from filling the trench and you can't possibly want the length of that pipe poured within the slab? Nor would you want the pipes coming up through the slab without a larger pipe to serve as a collar so slab movement won't stress the plumbing.

I'm speaking somewhat out of turn because I've not seen the drawings, but what I've described would be standard construction practices.

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Originally posted by Donald Lawson

Found this on a pre-pour steel reinforced slab inspection today.

Pipe runs about 9 feet down the middle of the trench. Builders said he knew it wasn't right, but thinks putting some mastic on it and wrapping it with poly will be fine.

I disagree.

Your thoughts?

No effin' way. Tell 'em to call off the concrete delivery till they fix that mess.

They can't embed plumbing pipes in concrete.

Mastic & poly my ass. . .

- Jim Katen, Oregon

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Embedding pipes directly in concrete does not allow for slab movement or expansion/contraction. Tell your builder to put sand rather than gravel in the trench (under and over the pipe), as it is even more forgiving. At penetrations, especially through walls, use collars (larger pipes).

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I should have been more precise in my description. This is a monolithic slab on grade, no gravel/sand goes in the trenches....trenches = perimeter and interior beams. The area for the slab is escavated forms are put up and then Select Fill is brought in. After the Select Fill has been leveled out, the perimeter and interior beams are dug. Typically these trenches will be from 12 to 18 inches wide and from 24 to 36 inches deep, depending on what the foundation plans call for.

Normally, we'll see a PVC drain running across a trench (beam) at the bottom. It will be covered with mastic. However, I've never seen one ran along the trench like this one.

I had 2 major gripes about it, 1) You'll have to jack hammer the beam should you have a future plumbing problem, 2) Any movement (settlement) by the beam will likely damage this section of drain

My other gripe was that someone should have checked the plans and measurements (designer/engineer, plumbing contractor and foundation contractor) and made sure this did not happen. (I found two other drains that were off the mark)

Thanks for the replies, I'm searching my IRC now for some back up reference.

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It seems to me that the pipes should run throught the fill, not through the concrete.

Putting a pipe through the middle of the beam changes the cross-sectional shape of the beam. When you look at the plans, do they show these beams with 4" holes in the middle of them?

Putting the pipes at the bottom of the beams seems like it would have the same problem, the beam would end up smaller than it was designed to be.

All of this stuff should be in the plans. Where do they show the pipes to be?

- Jim Katen, Oregon

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