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Community and Q&A

Plumbing too close to edge of slab

User avatar
Hugh Stearns | Posted in Project Management on

A plumber set a vent pipe within a 1/4″ of the edge of the slab on one of our jobs. When we told him to move it, he cried. I was surprised to find that this was not covered in our specification and I could not find it in code documentation. In fact, I could not find it anywhere. We have all seen concrete on the edge of a slab bust out around a vent stack or a piece of rebar that was too close to the edge of the slab.

So what is too close?

If there is a brick ledge this is a moot point as the extra concrete for the brick provides coverage for the pipe. but it you have a 2X6 wall with siding and a 4″ vent stack and the pipe is perfectly centered you only have 3/4″. Is that enough? But wait, you have to have your rebar cage in there too. If you are only using #4’s your are at 1/4″ coverage.

Note: we are building monolithic slab on grade with no basement.

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Replies

  1. Trevor Lambert | | #1

    I would say that requirement would fall under common sense. But unfortunately, you can't rely on people having that. When I hired a plumber to lay my underslab pipes, I gave him a drawing of where every protrusion was to be, within 1/8".

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Hugh,
    In general, you don't want to locate any plumbing pipes (including vent pipes) in exterior walls.

    There are lots of good reasons for this rule -- for example, preventing frozen pipes and making sure that the exterior wall has adequate insulation.

    You've just discovered another good reason for the rule.

  3. User avatar
    Hugh Stearns | | #3

    What was that that was said about common sense? I'm slapping myself. Of course, we should not put vents in outside walls. I'm in a hot climate so freezing is not an issue but insulation is as it typically gets close to 100 most days this time of year. While this is a piece of cake in a bathroom, which this is, in a kitchen, this may be a bit more of an issue where there are frequently no near interior walls. Is there a code restriction on how close the vent has to be to the P-trap?

  4. Trevor Lambert | | #4

    Yes, there is a restriction, and it depends on the size of the pipe and the slope of the pipe. The vent cannot be below the trap weir. So if you have a 1.5" pipe, the furthest the vent can be is 1.5" times the slope of the pipe. If you have exactly 1/4" per foot slope, that works out to 6 feet. I think that's the minimum slope allowed, so if the slope is actually steeper than that, in theory the vent has to be closer. In most cases, no one is going to be measuring the slope. Some codes put a specific number on the distance to account for slight variations in slope, e.g. 5' max for a 1.5" pipe, 8' max for a 2" pipe.

  5. User avatar
    Hugh Stearns | | #5

    Thanks, Trevor. That is good input. This needs to be a design issue. We can size and specify these variables as needed to achieve the design we want. To this point, this has not been on the designer's radar. It will be now.

  6. Malcolm Taylor | | #6

    Hugh,
    Unless you are in Canada, you can vent the sink with an AAV and have no vent stack in the wall at all.

  7. Trevor Lambert | | #7

    Air admittance valves are allowable in many Canadian locations as well (pretty sure Ontario Building Code allows it). However, you still have to meet two criteria which make it just as, if not more challenging than a conventional vent. It has to be 6" above the flood rim level of the fixture. It also has to be accessible. I'm having a hard time picturing how you could have it above the sink top level and yet not buried in a wall somewhere. If you manage to satisfy those conditions and install it, then you get to worry about if or when it might fail.

  8. Malcolm Taylor | | #8

    Trevor,

    My understanding is that south of the border AAVs can be used for any fixture, or even to vent a whole house. In Canada, the use of AAVs is restricted in all provincial plumbing codes to very specific situations. The criteria varies by Province. Here in BC they can only be used on islands, or for renovations where conventional venting is impractical. We don't have the language about flood rim levels, instead they must be 100mm above the fixture drain. For islands I prefer a loop-vent, which doesn't rely on moving parts.

    The larger issue Hugh faces is the layout of plumbing on his plans. I don't show the location of all the drains on my drawings, but I do show the 3" and 4" risers, to ensure there is a path from the fixtures they serve to the exterior. Sounds like Hugh is going to get his designer to do the same.

  9. Trevor Lambert | | #9

    The requirement for being accessible and above the flood rim level is direct from the product installation manual. Even if the code doesn't require it, it only makes sense to follow. Just imagine what's going to happen to an AAV installed below the flood level in a back-up condition. In my limited experience, getting the vent pipe up above that flood level is the most challenging part, and if anything it's even more important for an AAV than a conventional vent. A conventional vent will just drain back out after a back-up, whereas an AAV can potentially leak.

  10. User avatar
    Jon R | | #10

    Evidently they make a "wall box" to house the AAV - making is accessible and at the right height.

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