Ask Dave    |    About us    |    Members
Home    |    Articles    |    Plans    |    Custom Plans    |    Dictionary    |    Our eBooks    |    Newsletter    |    Member Photos

Plumbing and Ventilation 3: Questions on Plumbing and Ventilation

Adapting Plastic To Cast Iron Pipe
Mechanical Vents
Pex Pipe
Pouring A Shower Pan
Adding Ducts To Central Heating Or AC
Installing A Shower Valve
What Is A Wet Vent
Installing A Range Hood Vent
Toilet Space

Question about adapting plastic to cast iron pipe:

We are renovating an old house that has a septic system. The pipe to the septic is cast iron. How can we change this pipe to plastic pipe?


Actually, this is not a big problem. The cast iron pipe hub should be either cut off with a reciprocating saw with steel blade or by renting a large cutter for cast iron, if you have the room. I have done this a few times using the recip saw. Just a warning - cast iron pipe is very hard, but also very brittle. It can be broken with a hammer. Once the pipe is cut off a "mission" fitting, a rubber fitting that slips over the pipe and fastened to the pipe with stainless steel clamps, is used between the cast and the ABS or PVC. Here is a pic of one:

Photo of a mission fitting.

These fittings are the heavy duty rubber with s/s clamps not to be confused with the mj - mechanical joint couplings, which are much lighter and not approved for burial. They may need rubber bushings to match different sizes of pipe, such as 4" cast to ABS, or 4"cast to PVC.

These fittings are generically called Mission, but Fernco is also a brand in my area. Your retailer will know which one you need and the size of bushing if needed, too.

Question about mechanical vents:

How do I vent a washing machine without hooking up to a vent stack or through the roof?


You must be asking about a mechanical vent. You can buy these in most hardware stores. They are not acceptable by the plumbing code, but in a pinch they are good. Don't hide them in a wall, in case you need to replace it, keep it accessible. Install it above the highest water level of the washer.

Here is a picture of the mechanical vent you need. Install it after the p-trap coming off a tee. Use a 1 1/2" female pipe adapter to screw it into. Use Teflon tape or dope on the threads. They cost under $5. Here is more info about the vent at Amazon.

Photo of an auto trap vent.

Question about pex pipe:

I see a lot of people using pex for domestic water in new houses. In your opinion what are the advantages and disadvantages of using it.


Pex is an excellent product for hot and cold water. Use the fittings that insert and are crimped on instead of the bulky outside fittings. You can rent the tool to crimp each fitting. The only restriction that I know of is Pex buried under concrete, check out your supplier if going this way.

Question about pouring a shower pan:

Do you know about the type of cement to be used for the shower pan?


There are actually 5 types of portland cement that we use in mixing concrete with variations of the first 3 with air entrainment. The two most common are Type I (Type 10 in Canada) called Normal Portland Cement. This is an all purpose blend which is the most popular. Type III ( Type 30 in Canada) is a high early strength mix.

For a shower pan a Type I should be used. For small amounts of concrete pre-mixed bags are handy. The cement and aggregate are pre-mixed, all that is needed is water. For a bit larger job, one can buy an aggregate mix of sand and gravel called navvy jack which is mixed with cement in the proportion of 6 shovels of aggregate to 1 of cement to give about a 3000 psi mix.

Question about adding ducts to central heating or ac:

I am refinishing the basement and need two additional air ducts in the bedroom and bathroom from the existing central air conditioning plenum. I need suggestions and/or diagrams on the proper assembly. I want to use rigid ducting and am looking at possibly 4" or 6" diameter with the total length of both locations of 8 linear feet.


Use a 5" or 6" diameter duct. You need to buy an adapter from the 5" duct (for instance) that will fit a standard 4x10 or 3x12 floor register. The adapter or boot is secured to framing in the wall or a square hole cut in the floor. The other end which goes into the plenum is fitted through a proper sized hole in the plenum, notched about 1" x 1" to form tabs. Each alternate tab is bent into the inside of the plenum and its alternate is bent onto the outside. Space out 3 screws around the duct on the outside and screw these tabs into the plenum. Wrap proper foil adhesive tape (not duct tape) around the outside of the 5" duct and the plenum to seal off any air leaks. Also tape the joints with this tape as well.

Question about installing a shower valve:

Can you tell me how to install a shower valve in my new bathroom. The two 1/2" water pipes are stubbed out between two 2X4's in the wall. I just don't know how to mount the valve between them. Is there some type of special bracket that is made to do this?


Sometimes the valve has brackets to screw to backing between the studs, Most times the valve doesn't have anything for this at all. Regardless, backing, a 2x4 or 2x6, is fastened on the flat in between studs and centered with the shower drain. Usually the instructions will give the distance the valve should be back from the finished shower wall, depending on the finished trim you have selected.

The valve is then fastened to the backing with plumbers metal strapping. The hot and cold water supply should connect directly to the valve inside the wall, they should not be stubbed out. The plumber probably stubbed them out as a temporary service so he could turn the water on. So before removing the supply pipes from these stub outs turn the water off.

Depending on the type of valve you got it will take either 1/2" female iron pipe adapters or 1/2" copper pipe sweated fittings. If you sweat the copper pipe fittings to the valves remove any rubber washers, etc before heating the valves up, so you don't damage them.

The shower head should be on the same center line vertically. For a shower only, the valve is centered horizontally at 54" above the finished floor with the head at 78" above the finished floor.

For a tub/shower combination, the valve is centered with the drain and 15 1/2" above the tub rim, the spout is centered 4" above the tub rim and the head is 78" from the finished floor. If you have the original instructions with the unit follow their specs.

Question about what is a wet vent:

What is a wet vent? How do you vent tubs and showers?


A wet vent is a vent that is also used as a drain. Maximum trap arm length is 10' for a toilet to the vent and a sink and tub or shower is 5' from the vent.

Diagram of drainage and venting system showing roof, attic, ceiling, vent, sink, tub or shower, drain stack, cleanout, basement or crawlspace and pipe to sewer or septic tank.

The wet vent as shown on the drawing drains the sink as well as acts as a vent for the tub or shower.

The vent off the sink, tub or shower may also return to the main vent as an option to have only one pipe protruding through the roof.

Usually the drain stack is below the toilet and close to it, with a cleanout on the bottom.

Toilet drains are 3", sinks and tubs are 1 1/2" and showers are 2". A tee is allowed for a vent connection but only Y's are allowed for drains.

Question about installing a range hood vent:

Where can I find info on putting in a rectangular stove hood vent?


The standard size vent duct for a range hood is 3 1/4" x 10" or 7" diameter.

You can purchase the duct and transition pieces, angles, etc. at most building supply stores.

You'll find that the 3 1/4x10 duct has a wall and roof cap that are compatible to it. The duct should be insulated in the attic to prevent condensation. Try to keep the duct as short as possible with as least number of 90 degree turns as possible. Each fan has a rating which allows so many feet of duct to the outside, 90 degree turns use up a certain amount of this allowable footage.

The duct comes in a flat piece which is cut to length before assembling. Screw the joints to prevent separation with 1/2" x #6 pan head sheet metal screws and tape the joints with silver duct tape not the regular duct tape. The wall cap is used on an exterior wall where the duct discharges. The roof cap is used on the roof. You use one or the other.

Run the duct through the wall and down the cabinet. The duct should extend down through the cabinet by about 1/2", then cut the corners and fold the sides and ends back against the bottom of the cabinet.

The range hood should have a lip which is screwed in place which extends up about 5/8 to 3/4 into the duct under the cabinet. The hood is then screwed in place to hold it secure.

The 14/2 wire with ground comes through the wall, no electrical box, into the hood knock out hole. Use a box connector to protect the wire. Make a hole in the drywall large enough to make room for the connector. Connect the wires from inside the unit accessible from under the hood.

Diagram of standard range hood measurements.

All you should worry about for now is getting the duct in and coming through the drywall where the cabinet will go. Then you can put the drywall in and after the cabinet is in you can carry on with the 90 degree elbow and cut the cabinet bottom for the duct to pass through. The wire is no problem, just estimate where the hood will be, aim high rather than low. Leave the wire sticking out of the drywall at this point. You can always cut the drywall later and move the wire, if you are close.

The wire usually is 3/4" down on the right side of the hood and about 7 1/2" offset from center, as viewed facing the installed hood. The top of the cabinets in an 8' ceiling are at 7' leaving 1' for airspace. The cabinet over the range is set 24" above the range to the bottom of the hood. The hood is 6" high, leaving the cabinet 18" high.

Question about toilet space:

How much space for a toilet, is normal (the smallest) to have between a vanity and a bath tub?


Our code requires at least 30", usual it's 36".


< previous article
Your source for building tips, woodworking & furniture plans, house plans and building advice directly from Dave...

Hi, I'm Dave Osborne. With over 50 years experience as a journeyman carpenter, foreman and contractor in heavy construction I enjoyed working with apprentices and sharing the tricks of the trade that others shared with me. Now I get emails from Members all over the world and we include many of my answers in our Free Monthly Newsletters. Some of my answers include drawings and instructions specific to a project, but may also answer your questions. I use correct construction terminology, so you can confidently inform your building supply dealers or contractors exactly what you need.


The Benefits of Membership

Membership gives you full access to our hundreds of how-to articles, woodworking plans, converters, calculators and tables. Our Stair Calculator is one of the most popular on the internet. We have projects you can build for (and with) your kids, furniture for your wife, and sheds and gazebos. If you run into a problem or need advice your Membership includes unlimited email questions to me through our Ask Dave quick response button.

Join us!

Get FULL ACCESS to our site!
Subscribe Now !
Click here

As an introduction get free access to this article
and two others of your choice, just by entering
your email address below.

Receive our FREE Monthly newsletter which contains a
free set of woodworking plans each and every month.



Already a Member? Login here:



Forgot your username or password? Click here.
Info on our Memberships.