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How to deal with the P-trap in Revit MEP

In this article, I want to talk about different ways to deal in Revit with the issue of putting a p-trap to those plumbing fixtures that need one, specially lavatories and sinks, and also floor drains. For other objects such as urinals, bathtubs, washing machines, the p-trap is usually built-in.

What is a p-trap?

The p-trap is a pipe fitting that combines a u-shaped pipe with a 90 degree elbow, as the shaded parts in the following image. For lavatories and sinks, the u-shaped part comes with a drain for maintenance (not shown in this image). The p-trap is necessary to prevent the gases that come from the sewer system, from getting back into the plumbing fixture and, therefore, into the room.

Where is the p-trap family located in the Revit libraries?

If you have Revit MEP or Revit 2013 OneBox, look for a path similar to this:

C:ProgramDataAutodeskRVT 2013LibrariesUS ImperialPipeFittingsPVCSch 40Socket-TypeDWV

There is a p-trap family, named as: Trap P – PVC – Sch 40 – DWV.rfa

What are the issues with the p-trap in Revit?

  • The plumbing fixture families for lavatories or sinks from the out of the box Architectural folders do not have connectors, and of course, they don’t have p-traps.
  • The plumbing fixture families for lavatories or sinks from the out of the box MEP folders do have connectors, but don’t have p-traps.
  • Connecting an MEP lavatory to a sanitary pipe with “Connect into” results in an incorrect connection that uses an elbow to connect the lavatory to the pipes.
  • Connecting the lavatory by manually drawing pipes also results in an incorrect connection that uses an elbow.

  • If we select the elbow, and try to change it to a p-trap, even if the p-trap is loaded, the list on the type selector shows only other elbow families, not the p-trap, because it is by default, classified as something else, not as an elbow.

  • If we delete the elbow, load the p-trap, and try to place the p-trap manually, we will see that the process might be time consuming and frustrating, because the p-trap usually comes rotated in a different orientation, obviously not the one we need, and even though the p-trap family comes with rotating and flipping controls, it is not easy to place it correctly, at least, not in the first try and in a short time.

  • After some time we might get it to be in the correct orientation, but now we have to delete the elbow and adjust the length of the two adjacent pipes. If you had to to do this for every lavatory in a project, that would be really… :banghead:

Alternatives of solution

Solution A
Convert the elbow into a p-trap

I learned this solution from Kristin Woodard, at her lecture in Autodesk University 2012, entitled “Plumbing in Revit MEP: you’re no longer just a subcategory1. She showed that the elbow family has a parameter named as “part type”, that, by default is set to “elbow”. Well, she said, change that parameter to “multi-port”, load the elbow into the project, and now, in the project, select the elbow family, and the p-trap will be listed on the list of “elbow” families. Select the p-trap, and the elbow will be converted perfectly into a p-trap.

Wow, let’s see: Kristin says that if we select the elbow family, then Edit Family > change “part type” to “multi-port” > load the elbow into project again > select the elbow > now the p-trap is on the list of families in the type selector, because now the p-trap is a multi-port kind of part. (The p-trap family must have been loaded into project before, of course). Select the p-trap from the type selector, and voila, this works ! :perfect: It’s one of those things that pay for the price of admission to AU.

I found that if you do this the other way around, meaning, from the p-trap family > edit family > change part type to “elbow” > load into project > select the elbow, now the p-trap is also on the list, because now the p-trap is an “elbow” kind of part.

While this is a wonderful feature, you still need to do this change for all the lavatories in your project, though. Therefore, let’s talk about another solution:

Solution B
Load the p-trap family and assemble it with other pipe segments into the lavatory family

This solution requires some work within the family editor for the content creator of the MEP office. The MEP family is modified to include the required p-trap and other pipe segments, The modification can go as far as you need to. You could include not only sanitary elements but also domestic cold and hot water segments. This solution works well with Copy/Monitor with mapping, to replace or “swap” the lavatories from the architect’s model with your own MEP families with fittings and connectors, simply by specifying the types in the Coordination Settings dialog box of MEP or Revit 2013 One Box.

The drawback of this solution is that you would need to modify several families in the same way, unless you use always a generic lavatory.

Solution C
Use a family that has the p-trap family and other elements, without the plumbing fixture

This is the ultimate solution. Having a family that contains all the necessary fittings and pipe segments, without any actual plumbing fixtures, allows you to avoid the use of Copy/Monitor Mapping, and simply let the architect’s plumbing fixtures stay in your model, where you simply place your families with the fittings and pipes. It requires good knowledge of the family editor, unless you get the family already assembled from other sources. But once you have the family, you don’t need to do more work on the family editor because now you don’t need to insert this into lavatory families as in solution B above.

This image shows three degrees of this idea. The first image on the left contains just the p-trap and two segments of sanitary pipes. The second image2 goes one step further and includes the domestic water pipe segments as well. The image on the right contains everything that you might need: p-trap, tees, cleanout, vertical pipe, domestic water pipes, etc. That image is a kind courtesy of Don Bokmiller, Simon Whitbread, and Joel Londerberg, the authors of “Mastering Autodesk Revit MEP 2012” 3. Please visit www.sybex.com for more information. Look for the zip file in the download that corresponds to Chapter 18, and find the family named as “Sinks and Lavs.rfa”.


From all these options, use the one that best fits your situation. Think of productivity. If a lavatory always needs a p-trap, it makes sense to automate that p-trap somehow, instead of doing that p-trap one by one every time.

Creating the custom families is no easy task, especially considering that it is more difficult now in Revit 2013, due to something that is probably a bug, as shown in the following image:

I had a conversation about this error message with my friend Erik Gray, Senior QA Analyst from Autodesk, in Las Vegas. Erik thinks that this is a bug, due to some new features and error messages introduced in 2013, and he agrees that the error message has no sense in the family editor because the error tells the user to open the routing preferences for the pipe type but we can’t do that because we are in the family editor, not in the project. :confused:

To skip this error, if possible, do the families with Revit 2012.

Well, as my friend Dave Jones says: nothing is ever easy…”

Notes and credits:

See you in our next blog…

2012 Planta1.com, inc. , Alfredo Medina | permission to reproduce this article is granted if the name of the author and the URL of this article receive proper credit.


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