DEAR TIM: My city allows homeowners to install their own plumbing. But to get the permit I need to supply them with a rough-in plumbing diagram. What is a plumbing diagram? Can I just do a bathroom plumbing diagram since all I am doing is adding a new bathroom to my home? Is it a good idea to include a bathroom plumbing vent diagram when I submit my paperwork to get my permit? Dave B., Cincinnati, OH

DEAR DAVE: You should count your blessings that you live in a community that allows you to do your own work. Some areas of the nation do not allow owners of single-family homes to do their own work. I happen to be a master plumber, among other things, and it is my opinion that certain homeowners can do better work than some of the plumbers I have seen in my years in the field. If you can install the piping correctly and it meets or exceeds the plumbing code, why shouldn't you be allowed to do the work?

A rough-in plumbing diagram is a simple isometric drawing that illustrates what your drainage and vent lines would look like if they were installed, but all of the other building materials in your house were magically removed. You would see the pipes in three dimensions, and be able to see all of the connections, the pipe sizes, fittings and vent piping.

This rough-in plumbing diagram shows exactly what the drain and vent pipes will look like once installed. IMAGE CREDIT: Tim Carter

A plumbing diagram is a really useful tool that can save you lots of money and headaches. The drawing communicates to the plumbing inspector that your system will meet the minimum standards of the code. When you apply for your permit, an inspector studies your rough-in plumbing diagram making sure the pipe sizes are correct and that you are using the correct fittings at the places where pipes connect to one another.

For example, let's imagine you think it is okay to install a fitting connecting two drain pipes together at a 90-degree angle, and this pipe is in the ground under a concrete slab. In almost every jurisdiction I know of, this is an illegal connection. You draw this and the plumbing inspector corrects this mistake before you install the pipe and have to rip it out to make the correct connection using two 45-degree bends to make the 90-degree turn.

You should just have to do a bathroom plumbing diagram for your job. It would be unreasonable for you to have to make a drawing of your entire plumbing system, especially since it is hidden behind the walls. You will be expected to show how and where your new drains will connect to your existing plumbing system. This is very important as the new branch line you install must connect to an existing plumbing drain in the right location.

The diagram will absolutely have to include a bathroom plumbing vent diagram. Vent pipes on plumbing diagrams are drawn as dashed lines while pipes that carry water are drawn as solid lines. You will need to show how and where your new vent pipe connects to an existing vent pipe or exits the roof as a new separate vent. If you are going to go through the roof with this new vent pipe, be sure it is sized correctly to prevent getting choked with ice. This can happen in your climate in periods of severe cold weather if the pipe exiting the roof is too small a diameter.

Be sure you call out the size of each pipe shown on your rough-in plumbing diagram. Show all changes of direction correctly. If you intend to use a fitting that turns a hard 90-degree angle, show that on your drawing. If you intend to use fittings that turn just 45 degrees, draw that acute angle on the paper. It is not that hard to make a plumbing diagram so long as you can visualize the pipes in your head before you install them.

If you struggle to make your rough-in plumbing diagram, the first thing to do is ask your local plumbing inspector or the plumbing department if they have any sample plumbing diagrams you can study. Once you see how they are drawn, it will make complete sense to you. If they do not offer a free sample, consider purchasing a copy of your local plumbing code. Often the code books have great examples of the isometric rough-in plumbing diagrams.

When you see a sample rough-in diagram, you will notice that certain simple symbols are used to indicate the fixtures you will eventually connect to the pipes. Be sure to use these symbols properly so the inspector reviewing your drawing doesn't think you will be connecting your toilet to your bathroom vanity pipe!

To make the drawing, you will discover that a drafting 30-60-90 plastic triangle will be invaluable. The 30-degree angle side of the triangle is perfect to illustrate pipes that run horizontally with the code-required slope. You can buy a triangle like this at stores that sell drafting supplies. Some computer software also allows you to draw lines at specific angles.