Modified P-Trap - still in code?

hookoodookuJanuary 28, 2007

After replacing the garbage disposal unit under the sink, I had to rework the P-Trap drainage and wanted to see if I've created any code violations I need to correct later.

I've got a double sink. The original installation was pretty strait forward: Drain #1 goes down to a P-Trap, the P-Trap connects directly to the stub comming out of the wall. Drain #2 (where the disposal is located) comes out of the disposal almost horizonally (slight down slope) and connects to the down pipe of Drain #1 just before the P-Trap with a special Tee.

Well after replacing the disposal, the drain comming out of the disposal is now lower than the previous disposal. The stub comming out of the wall, is mounted so high that the original configuration is now impossible to reproduce (at least not without getting an extra tall P-Trap, but at 10:00pm, that wasn't going to happen).

The new configuration has the tail of the P-Trap for Drain #1 replaced with a Tee coupling (i.e. I had to cut the tail of the P-Trap short and put a regular Tee fitting there). The Tee is being fed by the tail of the 2nd P-Trap. The 2nd P-Trap is mounted as high as possible (i.e. used the 90 degree turn down pipe leaving the disposal, and had to cut is short to fit the P-Trap as close to the disposal exit port as possible), but is within 1/2" of the height of the 1st P-Trap. All connections are compression fittings with no sort of sealant used.

Anything I'm going to need to go back and rework later?

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Your current configuration has a number of code violations.

You have a regular Tee connecting the two drains but code requires a baffled tee:

Ref IRC-2707.1 & UPC-405.4: Directional fittings are required for continuous wastes from a disposal or Dishwasher (I.E. Wyes, ComboÂs or TeeÂs with baffles).

Your Tee is connected by means of slip joints:

Ref. IRC-3201.1 & UPC-1002.3: Maximum of one (1) slip joint on the outlet side of a P-trap

NOTE: slip joint type P-traps and waste fittings are made to Tube Standard and the single slip joint permitted on the discharge side of the P-trap is the "Trap Adapter" that transitions the slip joint tubing to the Pipe size to match the drain piping in the wall.

You can rule out looking for a higher P-trap because P-traps are made to ASTM specification and they are all the same. Increasing the height of the P-trap would increase the water level in the trap above the code maximum of 4".

1. Install the exact same make and model of disposal and waste kit as was originally installed.
2. Remove the factory supplied ¼ bend on the disposal output and use a Disposal Type" end waste kit. (An ordinary end waste kit has a molded ¼ bend on the cross tube that is intended to be connected to a sink drain tailpiece. A "Disposal Type" end waste kit has a straight cross tube with a flange connection on the input end. The factory supplied ¼ bend in the disposal kit is discarded and the cross tube is connected directly to the disposal port. This keeps the cross tube about 3" to 4" higher. The disposal type end waste kit comes with the code required baffled Tee.)
3. Open the wall and lower the elevation of the waste arm.
4. If you are under the IRC and if the waste arm is a 2" pipe you could cut the trap adapter off the waste arm then install a Wye with a trap adapter on the end and the side port, then install separate traps from each sink. (Note: It must be a 2" waste arm because if you install separate traps the load must be computed as separate sinks with a 3DFU value for each trap. Under the IRC the maximum permissible load on an 1-1/2" line is 3DFU however if it is a 2" line the maximum load is 6DFU and separate traps would be permitted. (Note-The UPC prohibits connecting two traps to a common waste arm)

    Bookmark   January 28, 2007 at 5:03PM
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If I'm reading it correctly, your #2 suggestion is how the original disposal was connected. But once I completed installing the new disposal, the disposal port was no longer high enough to connect to the other sink drain's baffle Tee leading to the P-Trap.

However, I just dug the old baffled tee and p-trap from the trash. If I shorten the tail of the tee enough, I might be able to reinstall the drains the way they were. But I'm talking the side input of the tee has to sit on top of the p-trap. We're talk that I had to tighten down on the nut of the joint til just before it starts to bite, and then finish pushing the tail down in the p-trap. We are talking that the side port of the tee is so close to the p-trap, I'm going to have to cut the two extra long tightening tabs off the nut because it will interfere with tightening the nut on the side port of the tee should the nut happen to finish tightening with one of these tabs under the side port of the tee.

Any code issues on the minimum size of the tail of the baffle tee as it enters the p-trap?

    Bookmark   January 29, 2007 at 12:43AM
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The maximum allowed vertical on the input to a trap is 24"(except for a laundry standpipe) but there is no minimum. If they are tight together that is fine.

Cutting the ears off the nut is no problem either. They are only there as a convenience and in fact you can buy nuts without them.

    Bookmark   January 29, 2007 at 1:06AM
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Well, it will only cost me the price of a baffled tee, so I guess I should try it.

Even before I cam here, I already knew I'd have to make some changes. Code or not, I came to the realization that I've got two spots where the water is flowing "backwards" through slip joints, and that forward edge of the slip joint is a great place for 'stuff' to start building back up.

However, just for my general education, I've got a couple of questions regarding the code.

1. You referenced the fact that I'm not using a baffled tee. But why does that mater since the tee is after the p-traps? I mean I understand the reason for the baffle when the tee is just under the 2nd sink (drainage from sink 1 could splash back up into sink 2).

2. What's the deal with only one slip joint... and if most codes only allow one, then why do they even make a coupling since water would HAVE to flow against at least one of the slip joints (for example are those really only meant for venting, in which case we don't care about air going "against" the slip joint).

3. Why in the world would two traps have to count as two loads when the two traps are fed from the same water source? Asked another way, why does it make a difference in the required pipe size if the two sink bowls are connected before or after the p-trap.

4. Before I went ahead and did this, I did some quick web searching and found references to utilizing double traps. But when I did some more researching tonight, I found a bunch of references indicating it shouldn't be done. Since you didn't say so, I'm guessing there's nothing wrong with double traps (provided you size the pipe accordingly). But why would so many say NOT to double trap?

Thanks for all your input.

    Bookmark   January 29, 2007 at 1:50AM
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You don't even want to get me started on making a list of the items we can commonly find in a hardware or home supply center that are not code compliant. Example:Offset closet flanges, 4x3 reducing closet flanges, comprssion couplings for sink drains, comprssion couplings for sink drains, Flexible tailpieces, Compression couplings for PEX (These are approved for HVAC but not plumbing) and the list goes on and on.

Code allows up to three sinks to be connected to a common trap providing all three sinks are the same type, in the same room and there is not more that 30" horizontal separation between the drain openings.

Even though your may only have one water supply, once the sinks are filled they can be simultaneously discharged. When multiple sinks are connected to a common trap the volume and velocity of flow is limited to the volume of the one trap therefore the load is determined by the trap size. When multiple sinks have separate traps they can each discharge simultaneously so the drain must be capable of receiving all at once, therefore we must compute each sink as a separate loak.

Using a separate trap for each sink is not double trapping. In fact, code requires every fixture to have a separate trap and the use of a single trap for multiple sinks is an exception to that rule.

The code prohibition against double trapping is when two traps are placed in series where the water must flow through one trap before entering the second one. An example of this would be if you were to install a trap from your disposal to the Tee as a cross tube, then install a second trap on the output of the Tee to the waste arm. In this instance the discharge from the disposal would be double trapped.

    Bookmark   January 29, 2007 at 3:03AM
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Very good explination for question #3 and #4. I can definitely see the logic for the code in those situations.

But what about the limitation on slip joints after the p-trap? And what about what is wrong with connecting two p-traps with an non-baffled tee (for discussion sake, let's assume that after the p-traps, I've upgraded the pipe size to 2" to account for the two loads)?

And of course your answer has generated another question... what is wrong with 'true' double trapping (putting traps in serial)?

BTW, thanks for taking the time to answer these questions... I appreciate the education you're giving me.

    Bookmark   January 29, 2007 at 11:04AM
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If you connecting two P-traps to a common waste arm the TEE must also be rated for the combined load.

The prohibition against double trapping is to prevent sewer gasses from being trapped in the section of line between the traps because sewer gasses in high concentration are extremely flamable, potentially explosive and may contain biotoxins.

    Bookmark   January 29, 2007 at 11:17AM
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