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Plumbing Building Code

Star How to navigate your way through the minefield that is plumbing code.


Building code intro

These snippets come from the IRC 2009 rules, but note that building codes vary in different local areas so you will need to do your own checking up on codes in your area.  Also, this is just a few of the building codes that caught my eye and appeared relevant to my case.  It is not a full set, so you should consult a full code book to make sure you meet all the codes.

It is very important to ensure your plumbing is implemented to code right from the start rather than wait for the inspector to tell you it's wrong, because it is really hard to correct plumbing problems once everything is glued together.

It is worth doing a complete CAD drawing of your entire house plumbing scheme showing every ABS fitting and have it checked by the Health Department inspector before you implement it.  There will likely be a fee associated with this service but it is probably worth it.

Building code general notes

Plumbing in walls

Steel shielding plates are required if a pipe is less than 1.5" from the surface.  The distance is measured from the outside of the pipe (not the center).  Feature walls that use 1.5" boarding on 2x4 framing are good for internal walls because you automatically get 1.5" from the surface.

Sewer pipes

4" Sewer pipes are required to have a minimum slope of 1/8" per foot.  Using 1/4" per foot allows them to carry more DFUs (Drain Fixture Units).  For 2" and 1.5" pipes you should use 1/2" per foot.

Note that IRC 2009 does not specify a maximum slope.  Many plumbers however say that a slope of more than 3/8" per foot in a 4" or 3" pipe will cause sewer flow problems.  Most people use a figure of 1/4" per foot for 4" and 3" pipes.

Sometimes it is desirable to only use 1/8" per foot because it allows the waste pipe to also provide the venting function if the rise to the fixture is not more than a pipe diameter.  Note that UPC code gives maximum allowed distances for using the drain as a vent.  The main thing using an 1/8" slope does is avoid wall mounted toilets being too far off the floor when not close to a DWV stack.   

It is not allowed to use a Sanitary-Tee to make a connection in horizontal sewer pipes or to make a vertical to horizontal join.  Use a Wye instead (ideally a long bend Wye).

It is ok, and indeed required if also used for venting, to use a Sanitary-T on a horizontal to vertical connection.

You are not allowed to use Double Sanitary Tees unless the ports are half the size of the main size.

Cleanouts in sewer pipes are required "at the base of each stack".

A working area clearance of 18" is required in front of a cleanout.  Hide the cleanout in a small counter top cupboard and open the cupboard door to access the cleanout.

A cleanout is required as the sewer pipe leaves the building and joins the external sewer pipe.  It needs to be within 30" of the building.  Cleanout needs to be in the direction of flow (or di-directional).  Use a bidirectional Tee so you can cleanout in both directions.

Sewer cleanouts are required every 40 feet and sometimes more if there are bends.

Pipes passing through footings or foundation walls require a pipe sleeve.  In my case the 4" house sewer pipe comes through the wall so I avoid a hole in the slab that could let in water.

Underground pipes must be properly supported by smooth stones or filtered non-organic soil.  There must be no sharp stones and the smooth stones must be distributed to give even support.  You should not use crushed rock up against sewer or water pipes.  Use drain rock or use filtered soil.  You can use crushed rock above drain rock or soil as long as it is a couple of inches away from the pipe.

Sewer piping must be tested using a 10 foot head of water.  You will also need to add extension pipes or caps to any cleanout pipes outside the building perimeter if they haven't yet been fitted with screw sealed caps at their final height.

Never, under any circumstances let rainwater or basement footing drains to discharge into the house sewage system.  Rainwater should go to a separate runaway.

In my house, rain water will be collected to use for watering the garden and potentially toilet flushing.

All household wastewater must go to the septic tank.

I do not plan to make use of grey water from sinks showers and baths.  It will all go to the septic system.  There is plenty of rain in my area and the main issue is having storage tanks.

Potable water

A water service pipe may be permitted in the same trench with a building sewer if the sewer pipe is listed for underground use within a building.  Schedule 40 PVC is approved.  Even though it is approved, it is best to use a different trench.  If they do come close then keep the water pipe more than one foot above the sewer pipe in the trench.

The water service pipe to the building must be at least 6" below the frost line as it enters the building.  It is best to put the water pipe through the rear wall rather than the slab so that the entry point is fixable in case of any problems.  If the frost line is 1 foot (like in my area) then as long as there is about 2 foot or more of back-filled basement wall then things will be fine.  If the frost line is deeper in your area, or your basement is not sufficiently sunk into the ground, then you may have no alternative to coming up through the slab.

The water service pipe to the building must be at least 3/4" pipe and have a pressure between 40 and 80 psi.  A good choice is to use 1.25" pipe that is rated at 200 psi.  Even better is to use a 2" pipe like I do.

Need a whole house water shutoff valve near where water enters the building.  Put this on the inside wall of the basement directly next to where it comes through the wall.

Toilets, sinks, and bidets need a shutoff valve.  Baths and showers do not need a shutoff.

It is good practice to use a shutoff valve on all appliances because it allows the house water to be turned on even before all the fixtures have been installed.

Water service piping installed underground shall have a blue insulated copper tracer wire (minimum 18 AWG) or other approved conductor installed adjacent to the piping.

Drain Waste Vent (DWV)

Never reduce the sewer pipe diameter in the direction of flow.  Eg a toilet is 2" out of the china, which goes to a 3" slightly sloped pipe, which goes to a 4" DWV stack.

Connect into the 4" DWV stacks using sanitary-Tees if the drain is being used as a vent.  If it is not also a vent then use a Wye.

4" and 3" sewer pipes need to slope down-hill at greater than 1/8" per foot.

The total fall of a fixture drain to the DWV stack cannot exceed one pipe diameter if you want it to also count as a vent.  There are UPC code rules that further limit the distance.

For separate vent pipes it is good to always use 2" piping (but it is still good to check it is handling less than the allowed DFU figure for a 2" vent pipe).

The vents go upwards and then horizontally mid way up the wall (more that 6" above the highest water level of the highest fixture on that building level.  The vent then needs to go up to the attic where it can be combined with the other vents from other floors.

UPC code specifies the maximum for how far a vent can be away from a fixture for a given pipe size.  It also cannot be closer than two pipe diameters.

Vent pipes (eg top of stack) should protrude through the roof by 1 foot to avoid any snow buildup.  If the roof is used as a deck then it has to be at least 7 feet above where people can walk.  In my case I will route the vent pipes to the top of the cupola.

The top of a 4" stack combined with all the other separate dry vents can be reduced to 2" prior to going through the roof (3" in heavy snow areas).

Venting for island sinks involves a special arrangement.  See details given later.

ABS pipe must have a pressure rating equal to Schedule 40 or greater.

Use 3" diameter pipe for toilets and any pipes that serve multiple fixtures.  Baths, showers, and sinks can be 2".  Stacks are 4" all the way to the roof.  Vent pipes to the stacks can be 1-1/2" but it is far better to use 2".

A water trap must be installed in the drain line between each fixture and the main sewer pipe. Toilets of course have the trap built in to the fixture.

Laundry sinks require a minimum 2-inch drain (same provision as required for kitchen sinks).  Showers and floor drains also need to be 2" pipe (and trap).


A water closet, urinal, lavatory or bidet shall not be set closer than 15 inches from its center to any side wall, partition, vanity or other obstruction, or closer than 30 inches center-to-center between water closets, urinals or adjacent fixtures. There shall be at least a 21-inch clearance in front of the water closet, urinal or bidet to any wall, fixture or door.

Floor drains

Floor drains are useful and necessary when making an entire room a wet room, eg a bathroom or utility room.

Floor drains need a trap and it needs a mechanism to stop the trap drying out.

Use a trap-primer or a deep-seal design.  But a deep seal design is no good as it just doubles the time to evaporating dry.  It is best to take a feed down a 1/2" pipe from the sink drain to the floor drain.

Could use a physical plug in drains that are not used much.  This would be in addition to a deep-seal trap that building code requires.  If you use a round screw-in fixture drain then the screw-in part can be sealed with eg foam to form a plug.  Tricks like this can be useful if you are month-balling a house for several months.

Floor drains might seem like a lowly thing but they are a plumbing fixture like all other fixtures and are one of the hardest things to get right.

Hot water

Showers and baths need a scolding prevention device.

Bathtubs, whirlpool bathtubs, public lavatories and bidets require temperature limiting by means of a device that conforms to ASSE 1070 or CSA B125.3.  Keep in mind that shower and tub/shower combination fixture fittings must conform to ASSE 1016.  Bidets shall be limited to a maximum of 110°F, while the other fixtures shall be limited to a maximum of 120°F.  In all cases the water heater controls shall not be a suitable means for limiting the water temperature.

More info

The King County Health dept web sites that gives some info on plumbing rules is http://www.kingcounty.gov/healthservices/health/ehs/plumbing.aspx .




The term "drains" refers to the piping that carries away the water and waste (not just the plug hole or floor drain grating).

It is important to get the diameter of drains correct.  Also you must not reduce the diameter as it flows along its path, ie the pipe diameter can only get larger as it goes downstream (to the 4" vertical stack).  Here are the fixture drain sizes (see http://www.kingcounty.gov/healthservices/health/ehs/plumbing/unitvalues.aspx).

Bathroom sink                                1.25"
Bath or bath/shower combination     1.5"
Bidet                                             1.5"
Dishwasher                                    1.5"
Kitchen or laundry sink                    1.5"
Clothes washing machine                 2"
Shower                                           2"
Floor drain                                      2"
Toilet                                             3"

Things requiring 2" drains (showers, floor drains, and washing machine) are a bit of a pain because you cannot use a transparent hair trap because these are only available as 1.5" diameter.  I recommend that you build your own 2" trap (see later).

Code:  Drains, waste and vents (DWV) water tested with a 10' head for 15 minutes (or air tested at 5 psi for 15 minutes). (UPC 712.1, 2 & 3)

Code:  Drain pipes properly sized. (UPC Table 7-3 & 7-5)

Code:  Change in direction from vertical to horizontal or horizontal to horizontal through wye branches or 45° wye branches or fittings of equal sweep. (UPC 706.3 & 706.4)

Code:  Double sanitary tees may ONLY be used when barrel of stack is 2 times larger than inlets, for connecting with a vertical stack. (UPC 706.2).

Code:  No threaded ABS fittings or joints located in a non-accessible location. (UPC IS 5-316.1.59)

Code:  Install 18 gauge nail plates when plastic or copper plumbing is within 1” of face of framing. (UPC 313.9)

Code:  Hangers and straps don't compress, distort, cut or abrade the piping and allow free movement of pipe. Pipes exposed to damage by sharp surfaces are protected. (UPC IS 5-314.1)

Code:  Support plastic lines (eg ABS) at every 4 feet.  Support at each horizontal branch connection. (UPC Table. 3-2)

Code:  Support vertical plastic lines at base and each floor. Provide mid-story guides. (UPC Table. 3-2)

Always use a vent Wye after a toilet so you are safe for future code changes.

Also need to consider the unit values for each fixture type and size drain and vent pipes accordingly.


Code:  Each horizontal drainage pipe provided with a cleanout at its upper terminal and each run of piping which is more than 100 foot in total developed length, provided with a cleanout for each 100 foot, or fraction thereof, in length of such piping. See exceptions. (UPC 707.4)
Cleanouts not required at horizontal runs <5 foot except sinks. (UPC 707.4, ex.1)

Code:  An approved 2-way cleanout fitting, installed inside the building wall near the connection between the building drain and building sewer or installed outside of a building at the lower end of a building drain and extended to grade, may be substituted for an upper terminal cleanout. (UPC 707.4, ex.4)

Code:  Cleanouts required at each aggregate horizontal change of direction exceeding 135 degrees. (UPC 707.5)

Code:  Each cleanout installed so that it opens to allow cleaning in the direction of flow of the soil or waste or at right angles thereto and, except in the case of wye branch and end-of-line cleanouts, installed vertically above the flow line of the pipe. (UPC 707.6)

Code:  Underfloor cleanout not more than 20 feet from access door with an unobstructed 30” wide x 18” high pathway. (UPC 707.10)

Code:  Cleanouts are accessible. 12” clearance required at lines less than or equal to 2”,
18” clearance at lines greater than 2”. (UPC 707.10)

Code:  Cleanouts are required for the kitchen sink.


Code:  Each trap protected by a vent. (UPC 1002.2)

Code:  The developed length of the trap arm not to exceed the following limits. (UPC Table. 10-1):
    Pipe Size         Length of trap arm
    1 ¼”                 2' 6”
    1 ½”                 3' 6”
    2"                     5'
    3"                     6'
    4” & larger        10'

Code:  Trap arms less than 3 feet cannot change direction more than 90 degrees without the use of a cleanout. (UPC 1002.3)

Code:  Trap arms 3 feet and larger cannot change direction more than 135 degrees without the use of a cleanout when installed at the lowest level of the gravity drain. (UPC 707, 1002.3, exception & IAPMO interpretation)

Code:  Vertical distance between fixture outlet and the trap as short as practicable and not over 24” in length except for clothes washer which can have maximum 30” standpipe. (UPC 1001.4)

Code:  trap depth from bottom of output pipe to top of pipe at the bottom of the U must be between 2" and 4".


Every fixture needs a vent after the trap.  Sometimes you can share vents and sometimes you can use a wet vent, but that does not change the fact that every fixture must have venting.  Even toilets (that have the trap built inside) need a vent.  A toilet vent needs to be off the top of the pipe that carries the waste to the sewer stack or use a Wye off the vertical 3" pipe from the toilet flange.

 Toilet vent pipe wye   Toilet vent off vertical

Code:  Toilets must have a vent off the top of the 3" pipe that carries the waste to the sewer stack.  The vent pipe attaches with a 3"-3"-2" Wye to 2" vent pipe (2" vent is the minimum for toilet vents).

It is best to use regular sweep elbows when changing the direction of vent lines.  If you use "vent" fittings, you'll create places where debris can collect and block the vents.

Code:  Horizontal vent above fixture flood rim except island sink, where structurally impossible or when allowed by the local jurisdiction. (UPC 905.3)

Code:  Takeoffs for vents must be above the weir. (UPC 905.5)  

Code:  No flat venting. Vent pipe inverts are taken off above the center line of horizontal drainage pipe. (UPC 905.2)

Code:  A vent shall not be installed within two pipe diameters of the trap weir.

Code:  Vents terminate a minimum 6” above roof line. (10” in high snow load areas) (Only applies if roof does not allow people access) (UPC 906.7)

Code:  If the roof serves any function other than being a roof, such as being a balcony or walkway, the vent must rise 7 feet above it.

Code:  Vent connection of downstairs fixtures must be 6” above the overflow rims of fixtures on the floor above.

Code:  Every dry vent shall rise vertically to a minimum of 6 inches above the flood level rim of the
highest trap or trapped fixture being vented.

Code:  Dry vent pipes must be sloped away from the roof towards the fixture by at least 1/4" per foot.

Code:  Total fall of a fixture drain to the vent cannot exceed the following distances...
    1 ¼”                 2' 6”
    1 ½”                 3' 6”
    2"                     5'
    3"                     6'
    4” & larger        10'


Code:  Vents cannot be less than 1.5" pipe.

Code:  If the vent length is more than 40 feet then increase the vent pipe by one size.

Code:  Air gap - Bath taps on the wall must be 3" above highest water level in a bath.

Code:  Minimum vent for toilets is 2" pipe.

Code:  Not more than 24 DFUs on 2” vent pipe.

Code:  An individual vent is permitted to vent TWO (and only TWO) traps or trapped fixtures as a common vent.  The traps or trapped fixtures being common vented shall be located on the same
floor level. (UPC 908.1)
Where the fixture drains being common vented connect at the same level (to within about an inch), the vent
connection shall be at the interconnection of the fixture drains or downstream of the interconnection. (UPC 908.2)
Where the fixture drains connect at different levels, the vent shall connect as a vertical extension of the vertical drain. The vertical drain pipe connecting the two fixture drains shall be considered the vent for the lower fixture drain, and shall be sized in accordance with Table 908.3.  The upper fixture shall not be a
water closet. (UPC 908.3)

In my view it is best not to use one vent for two fixtures.  Give each one its own 2" vent pipe and only connect more than 6" above the highest fixture water level.

Depending on your code area you may be allowed to use Air admittance Valves, but I don't recommend using them because if they wear out they can let sewer gasses into your home.

Wet Vents

Wet venting can potentially save on piping, but if in doubt it is best to add a dry vent.  Personally I avoid wet venting and I suspect future code changes will make them impractical and/or illegal.

Unless you are connecting each fixture individually to its own vent then you will have some pipe sections acting as a wet vent.  A common arrangement is to have a 4-4-2 Sanitary Tee on the main stack and then three Wyes to pickup the traps from 3 fixtures and then have a 2" vent pipe up from the end.  This is a horizontal wet vent.

Code:  (With a few exceptions) Nothing can use the shit path from a toilet as a vent.  In a sewer stack system the stack cannot be used as the vent for the lower floors because there are toilets in the upper floors.  It needs separate vertical vent pipes.  The sewer stack vent and the vents from the various fixtures and various building levels can connect at the top prior to coming through the roof (because there are no toilets above that height).

Code:  Horizontal wet vent permitted.  Any combination of fixtures within two bathroom groups located on the same floor level is permitted to be vented by a horizontal wet vent.  The wet vent shall be considered the vent for the fixtures and shall extend from the connection of the dry vent along the direction of the flow in the drain pipe to the most downstream fixture drain connection to the horizontal branch drain. Only the fixtures within the bathroom groups shall connect to the wet-vented horizontal branch drain. Any additional fixtures shall discharge downstream of the horizontal wet vent. (UPC 909.1)

Code:  Wet vents cannot be greater than 6 foot developed length. (UPC 908.1)

Code:  Minimum one pipe size larger than the required waste and minimum 2”. (UPC 908.2)

Code:  Wet vents cannot serve as vent to more than four fixtures. (UPC 908.1)

Code:  Cannot use flat venting, ie you must connect the vent tap-off above the half-way liquid level in the pipe.

Code:  Toilet cannot be the upper (upstream) fixture on a wet vent.

Code:  Any combination of fixtures within two bathroom groups located on the same floor level is permitted to be vented by a vertical wet vent.  The vertical wet vent shall be considered the vent for the fixtures and shall extend from the connection of the dry vent down to the lowest fixture drain connection.  Each wet-vented fixture shall connect independently to the vertical wet vent.  Water closet drains shall connect at the same elevation.  Other fixture drains shall connect above or at the same elevation as the water closet fixture drains. The dry-vent connection to the vertical wet vent shall be an individual or common vent serving one or two fixtures. (UPC 909.1.1)

Here is an example of how wet vent configurations are done (but not what I do)...

Remote bathroom

Run the toilet line straight to the 4" vertical sewer stack using 3" pipe where it ties in using a 4-4-3-Wye-45 (or a 4-4-3-3-Double-Wye-45, but NEVER a 4-4-3-3-Double-Sanitary-Tee (Sanitary-Tees are not needed as it is not a vent, and double Sanitary-Tees of this port size ratio are illegal)).  About 2 or 3 feet from the toilet flange (on its path to the sewer stack), you put in an upward facing 3-3-2 wye which is your vent for the toilet.
You run that 2" pipe (toilet vent) sloping upwards to the floor drain and put in a horizontal 2-2-2 Wye for the 2" floor drain trap (this seems like flat venting which is not allowed).  It connects via a 3" length of 2" pipe, a 1.5"-2" adapter, a 90 degree 1.5" bend, and a 1.5" swivel ABS adapter to the output of the hair trap.
Out the back of the Wye you continue the upwards sloping 2" pipe to the bath tub or shower and put in a horizontal Wye for its trap (2-2-1.5 for bath or 2-2-2 for shower) (this seems like flat venting which is not allowed).
Out of the back of that Wye you continue the upwards sloping 2" pipe towards the sink and do a 90 degree long sweep bend up the wall behind where the sink will be.  Half an inch below level with where the arm from the sink's P-trap will be in the vertical 2" pipe you put in a 2-2-1.5 Sanitary Tee.
Out of the top of the tee, you continue on to the attic where you can combine it with other vent lines.
Terminology: At the point ABOVE that last tee at the sink it is a "Dry Vent", below the tee it is a "Wet Vent" which simply means that the sink drains into the vent for the bath/shower & floor drain & toilet, and the bath/shower drains into the vent for the floor drain & toilet, and the floor drain drains into the vent for the toilet.

Kitchen island sinks

Venting a sink on an island in the kitchen present special challenges because there is no way to route a vent upwards from the island.

Code:  Drain loop method requires fittings be of drainage type only (not vent type). (UPC 909)

Code:  Drain serving the island serves no other fixtures upstream from return vent. (UPC 909)

Code:  Island sink cleanout in vertical section of foot vent. (UPC 909)

Plumbing Kitchen Island


Drainage Fixture Units (and general info)

When planning your plumbing piping (both waste and vent pipes) it is necessary to produce a diagram that shows the number of DFUs handled by each pipe leg.

The DFU numbers come from plumbing code and are...
Bath or shower or combo             2 DFU
Floor drain                                   0 DFU
Bathroom sink                             1 DFU
Toilet (1.6L flush)                         3 DFU
Kitchen sink + dishwasher             2 DFU
Laundry washing machine + sink    3 DFU

A 4" pipe for the house drain can handle the following number of DFUs depending on the slope...
1/8" per foot 180 DFUs
1/4" per foot 216 DFUs
1/2" per foot 250 DFUs

My entire house needs to be less than 216 DFUs.

In a horizontal branch application allowed DFUs are less...
4"        160 DFUs.
3"        20 DFUs and a maximum of two toilets.
2"        6 DFUs (no toilets)
1.5"     3 DFUs (no toilets)

If used as a wet vent then even less...
3"       12 FU
2"        4 DFU
1.5"     1 DFU

Vents need to be sized using half the DFU figure, eg a bath would count as 1 DFU for venting rather than the 2 DFUs for drainage.

A 2" vent can have a max length of 120 feet.
A 3" vent can have a max length 212 feet.

Fixture         Drain Size         Vent size              Trap size
Toilet                   3”                     2”                    In Toilet
Kitchen sink          2”                    1-1/2”               1-1/2”
Clothes washer     2”                    1-1/2”                2”
Lavatory sink        1-1/4”               1-1/4”              1-1/4”
Laundry sink          2”                    1-1/2”              1-1/2”
Bathtub                1-1/2”               1-1/2”              1-1/2”
Spa tub                1-1/2”               1-1/2”              1-1/2”
Shower                 2”                    1-1/2”              2”

Sewer pipe from house

Code:  No pipes directly embedded in concrete.  All pipes passing through concrete walls or floors are protected from breakage.  Voids around piping passing through concrete floors on the ground are appropriately sealed.  (UPC 313.1& 313.2)

Code:  Sleeves are used for piping passing through concrete or masonry that are not bored or drilled. (UPC 313.10)

Code:  Pipe trenches parallel to footings offset a minimum 45° from footing bearing line. (UPC 313.3)

Code:  Drains, waste and vents (DWV) water tested with a 10' head for 15 minutes.  Plastic pipe not allowed to be tested with air. (UPC 712.1, 2 & 3)

Water line to house

Code:  Water lines tested to working pressure or 50 psi for 15 minutes. Plastic water piping not allowed to be tested with air. (UPC 609.4)