Venting new sink

My father-in-law is helping install a new sink in our house tommorow morning. After seeing tonight his plan, I went online and did a little research on plumbing and have a couple of questions I'm hoping maybe some plumbing experts might be able to help with...
The sink location is about 5 to 6 feet from a 2" drain/waste/vent pipe that runs directly into the concrete floor and the other end to the roof. Where my father-in-law would like to tie in is about 6 inches off the ground (the pipe comes out of the ground and at about 6 inches runs horizontal away from the sink for about 10 feet before heading up to the roof). So he would like to run it directly from the sink to the d/w/v 5 to 6 feet away without any vents.

pipe and 3 and a half feet for a 1 1/2" pipe. I've also read that the real kicker is that the distance between a vent and a trap should be calculated by the amount of slope in the line. For example a 2" pipe should not be able to drop more than 2" (with a 1/4" slope per foot) before being vented (so a 2" pipe's max distance between trap and vent at 1/4" slope per foot would be 8 foot).
Our slope will be considerably more than 1/4" per foot since we have about 6 feet between the sink and the d/w/v and will be dropping about 2 feet. So our slope is going to be more like 4" per foot. Does this matter? Do I need to look at a different solution, or will this be fine? I don't know if I am just too paranoid about this. I've also seen the check valve as another option, but don't know that I really want to do that? What do you all think?
Thanks in advance, Matt
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Matt wrote:

<SNIP>
If it runs right to the roof with no fixtures tied in, then it is a vent stack. You cannot dump waste flow into it.
Period.
Jim
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Jim, thanks for the quick reply... when I said it runs to the roof, I should have clarified we have a two story house and the where it goes up from the ground floor you can look outside and see it coming up on the roof. However, the second floor has a bathroom right by where the pipe goes up. Since there is only one pipe coming out on the roof on that side of the house I'm almost positive that those fixtures are vented on this pipe.
More for my knowledge... why would there be a vent stack with no fixtures tied in, what would that be used for?
thanks again for the quick response, Matt
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Matt wrote:

It's very difficult to see your house layour over Usenet <g>
It *could* have been a vent for whatever is buried under the slab.
If 2nd floor fixtures *are* discharging into it (run water/listen), then it *may* be permissible to tie into it. But the only way you'll be able to vent the sink trap is with an air-admittance valve (Studor). You can't tie back into the stack on this floor as a vent and I doubt an "island vent" loop would be acceptable here either.
You'll want the inspector's advice on this one...
Jim
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What would it be venting?

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mm wrote:

It could be venting whatever fixtures are connected *under* the slab.
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There are fixtures under the slab! I'm not going to sleep well anymore at my brother's house.
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This may be predictable and not what you wanna hear, but it sounds like you need a real plumber to at least assess the situation. Perhaps there's one lurking here that'll assist.
In our state at least, there's something you sign before homesale that indicates all improvements were done with permits, to code, etc, and should the next owner have a lot problems with whatever you do/don't do right on that sink, and goes to the municipality to find no permit pulled for the work, you exposure yourself legally for the long term. For the short term, god knows has that's all gonna work out.
-- Todd H. http://www.toddh.net /
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