Detached garage plumbing

jonzAugust 16, 2008

I have a few questions about plumbing a new detached garage.

First off, I'm dealing with existing construction...slab already in place and walls completely framed and complete. This is a new home im purchasing...otherwise I would have planned this all advance. I will be installing a slop sink, house bib and possibly a small water heater to start. Might also add a half bath in the future as well.

My questions deal with domestic water supply to the garage from the house:

1. In the trench from the house to the garage, can I run my cold water piping in this same trench with my plastic eletric conduit feed to the garage sub panel?

2. What type of piping should I use? Copper, Pex, PVC?

3. What diameter should the pipe be? (100' run from the house)

4. How should I stub the pipe up into the garage? Should I dig under the foundation and chip the concrete inside the garage and bring it up that way to protect from freezing?

5. How deep should I trench? (New York...cold winters)



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1. yes you can

2. copper if you cna afford it. if not use the PVC, though i am not sure if it has to be pvc or CPVC. PVC itself is not allowed IN a home, but can supply a home. Lazypup can clear this up.

3. depends on what kind of flow you need. of course, if your house is fed by a 3/4" supply, then going any larger gets you nothing. i would use the same size as what feeds the house.

4. yes, you dig it up. but no you don't necessarily chip it away. you cut the concrete. your plumbing cannot come in direct contact with teh concrete.

5. at least 18" below the frost line. i would imagine for you that is going to be 4-5 ft down.

lastly, check with your local permit office. by doign what you are talking about you WILL require a permit. and the bad thing is they may even demand you install a fully funtional bathroom with a shower and iinstall a kitchen. the garage then becomes an apartment, evne though you park a car in it.

    Bookmark   August 16, 2008 at 11:51PM
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Thanks for the reply.

Yes, I figured i'd cut the concrete (lack of wording on my part) So the pipe cannot come in direct contact with the do I go about patching the floor back up with concrete once the pipe is in place? Pipe insulation?

Copper it shall be but I would still entertain PVC if its allowed.

So, the permit deal. Why do you say I would require a kitchen? Does adding a half bath make this necessary or does it deal with having water supply to the structure?

    Bookmark   August 17, 2008 at 12:51AM
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While you're trenching, you may want to go ahead and check how your septic for the 1/2 bath would be hooked up in the future. If you can lay some lines now, it may save $$ in the future.

Also, check with your utility district for the depth requirements. In my area water and electrical can't be in the same trench unless each is kept at their required depth. Makes repair later much easier.

    Bookmark   August 17, 2008 at 11:05AM
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you don't patch teh concrete back to teh pipes. you cna sleeve the pipes and pour concrete patch to that, but the pipe itself cannot be in teh concrete. this is why when you dirve by a building under construction you see PVC sticking up out fo the foundation, pipe protection. if you go look you will see that some areas have a 12x12 open section around the pipe. this is where the tubs/showers will be!

the reason i said that is that due to teh 1/2 bath, SOME locations will assume that the garage will be used as a rental apartment and therefore it must have a kitchen. always best to check first before you go making firm plans!

i agree with the above, if you do this go ahead and put stuff int eh trench now to support future upgrades. no sense having to dig it twice.

    Bookmark   August 17, 2008 at 11:42PM
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This discussion has already gotten so far out of hand that I hardly know where to begin.

Perhaps we should start by clarifying a couple code issues here.
1. Code requires that all buried plumbing lines must be buried a minimum of 6" below the average frost depth.
2. The supply and sewer lines may be laid in a common trench if both the sewer pipe and the supply pipe are a type of pipe that is listed as approved for use inside a structure.
3. If either pipe is a type that is not listed as approved for inside a structure the pipes must be laid in separate trenches and there must be 5ft of undisturbed soil separating the trenches.

Before you even think about solving the problems associated with running water to an unheated structure in a cold climate you must first determine if you can establish an approved drainage system.

This is a separate structure therefore the drains may not in any manner connect to the DWV (Drain, Waste & Vent) system in the house. It will require laying a separate sewer line and connecting it to the house sewer line before the house sewer line connects to the municipal sewer main or the septic tank.

The first step is to determine if you have a municipal sewer or septic tank then determine where the sewer line enters your house. You then determine the shortest distance you could run a line from the proposed garage site to the house sewer line.

As an example, let us assume for the moment that you have a municipal sewer and the house sewer enters the structure near a front corner of the house. The post states that your garage is 100 from the house, and I am assuming the garage is behind the house and you mean 100 from the rear of the house where you would be starting the run of the water supply line. If this is the case you would have a 100 sewer line from the garage to the rear corner of the house, then for illustration let us allow 35 to continue the line parallel to the end of the house to the front corner and another 25 parallel to the front of the house to meet the existing house sewer line. The proposed sewer line to the garage would then be 100Â+35Â+25Â+5 of building drain under the garage to the proposed fixture location for a total of 165Â. Code requires a minimum of a 3" sewer line and a 3" line is required to have a minimum of ¼" per foot pitch so the total drop in the line would be 165 x .25"= 41.25"

Code requires that at the point where the sewer line exits the garage the line must be 6" below the average frost depth. In my region the frost depth is 36" although in upstate NY is may be much more than that, but for illustration I will use 36".

With a frost depth of 36" the sewer line must exit the structure at 36"+6"=42" below average grade. Previously we computed the required drop for the sewer line to be 41.25" so at the point where we intend to connect the garage sewer to the house sewer the existing house sewer line would need to be 42"+41.25"= 83.25" below grade. (Assuming that the entire lot is level of course).

    Bookmark   August 19, 2008 at 8:24AM
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Your assumptions are correct LP, frost in NY is 36", requiring pipes and building footers at 42"

    Bookmark   August 19, 2008 at 10:19AM
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lazypup -- what is the rationale for requiring a separate sewer tie-in for outbuildings? Just curious about that requirement.

    Bookmark   August 19, 2008 at 10:37AM
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A much simpler suggestion - install a garden hydrant (hose bibb that sticks up out of the ground and self-drains) *near* the garage. That way you have water service to the garage without having service *in* the garage that starts requiring drains and so forth, but you still have a nearby hookup for a garden hose for car/hand/etc washing.

For the half-bath, I take it this is just a shop bathroom so you don't get the house muddy/greasy when you're working in the garage? Just put a composting or holding tank (no water hookup) toilet and a bottle of Gojo out there.. much simpler for occasional use.

For hot water, you can get portable water heaters that plug in to a regular electric outlet and have garden hose inputs and outputs.

    Bookmark   August 19, 2008 at 8:01PM
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I'm still shaking my head
Ad a cart, a couple hose fittings to 4G heater and double the price. Isnt internet marketing great?

I think the idea was a slop sink and future bath, pretty hard with a hydrant outside

Perel, I dont think you have ever priced or experienced a composting toilet ever.

    Bookmark   August 19, 2008 at 10:42PM
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Thanks for all the replies.

I never would have thought all that would be involved in just adding a sink to my garage for washing my hands after changing the oil on my cars :)

perel, very correct with the assumption of the half bath. I never intend to make an apartment in there. Just a convenience thing I thought of so the wife wouldn't be upset with me messing up her baths after I was working in the yard.

lazypup, thanks for the informative post. I was looking through my NYS plumbing code book today. If im correct, using copper for the cold water supply and PVC for the waste, I would satisfy code requirement that they can be in the same trench..correct?

So, the drainage cannot run back into the house from the garage...I'm assuming that means I would need to locate the drainage feed outside to my cesspools from the house and tie into that under the ground..correct?..all while maintaining proper pitch. What happens if the pitch from the garage requires the pipe to be lower then the house feed to the pool?

I appreciate everyones suggestions and help.

    Bookmark   August 19, 2008 at 10:47PM
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Obviously PEREL is not a plumber.

It is true that you could run a water line out to the garage and install a "lawn hydrant" for an exterior hose connection, however under most codes when we install a lawn hydrant all exposed piping and the control valve must be painted bright yellow and there must be a conspicuous sign posted stating that is "Non-Potable Water". In turn you may not under any circumstances use that water to supply any fixtures that are intended for human use.

In regards to composting toilets- Gone are the days of the "outhouse"

"composting Toilets" may not be installed at any location where a septic tank or municipal sewer main is accessible.

    Bookmark   August 20, 2008 at 12:10AM
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I've used a composting toilet before. Wouldn't be my choice for a primary toilet, but then again I'm not trying to live off the grid. (I found the lack of air conditioning a bigger deal in that house than the choice of toilet, FWIW) I hadn't realized how expensive they are, though - for the low and mostly liquid amount of use this toilet would be getting, a basic holding tank portable toilet would probably be sufficient. For a hundred bucks or so you can get one that flushes from and to small holding tanks.. not as good as a real plumbed toilet, but probably close enough for a shop toilet.

And yeah, I agree the price on those portable hot water heaters is high.. but if you need a Listed appliance for that use, guess you have to pay extra for that.

    Bookmark   August 20, 2008 at 12:19AM
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It was mentioned perhaps, but if you cant maintain the pitch of the waste line, to the tie-in, a sewage ejector may be in order.

    Bookmark   August 20, 2008 at 11:28AM
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I agree that this discussion has no doubt gotten far more complex than what they originally intended but you cannot blame me. If all they originally wanted was a hose bid near the garage I would have began by suggesting a lawn hydrant, however the original post was a bit more complex than that:

Quote from the original post:- "I will be installing a slop sink, house bib and possibly a small water heater to start. Might also add a half bath in the future"

I am merely pointing out that if they intend to install a sink or perhaps a half bath in future the code requires an approved drainage system as a pre-requisite to install any fixtures inside the structure. Taking this to the next level, we have no yet even begun to discuss the problems of installing fixtures I an unheated structure in a cold climate.

If you were asking my personal opinion I would say the whole idea will soon prove to be cost prohibitive.

NOW FOR THE NEXT QUESTION: "What is the rationale for requiring a separate sewer tie-in for outbuildings?

ANSWER: All of the DWV (Drain, Waste & Vent) lines inside a structure eventually combine into one line before passing through the footer wall to exit the structure. That single line is properly defined as the house or building "Main Drain". Code requires that we must install a "Main cleanout" on that line within 3 of the footer wall. For structures built over a basement the main cleanout is generally installed immediately inside the footer wall in the basement. For structures on slab it is generally located outside the structure within 3 of the footer wall. For structures on a raised foundation with a crawlspace the cleanout may be inside or outside the structure.

The main cleanout officially designates the end of the "Main Drain" and the beginning of the "house or building Sewer".

On a residential property the "House Sewer" is generally consider the primary sewer line for the property and it would discharge directly into a "Municipal Sewer" or a "Septic Tank.

Code only allows one line to connect to the municipal sewer or the septic tank, therefore when we have additional outbuildings that are equipped with plumbing fixtures the "building sewer" from the outbuildings must tie into the "House Sewer" before tying into the municipal sewer or septic tank.

It must also be mentioned here that all DWV and Sewer Lines are sized by the DFU (Drainage Fixture Unit) load calculation method. The code gives us a list of all the fixtures that may be connected to a drainage system in both residential and commercial plumbing systems. The list then gives us a minimum size of drain opening or pipe and the DFU load value for each fixture. (IRC table 3004.1 & table 3201.7)
Lavatory 1-1/4" pipe & 1DFU, shower/tub 1-1/2 pipe and 2DFU, Shower stall 2" pipe and 2DFU, watercloset with 1.6gpf or less flush min. 3" pipe and 3DFU. A watercloset with greater than 1.6gpf = 3" line and 4DFU, kitchen sink 1-1/2" pipe and 2DFU, A Baptismal pool in your church is a 2"line and 3DFU while an office water cooler is 1-1/4" line and 1DFU and an Autopsy table at your local coroners office is a 2" line and 3DFUs.

The code then gives us another table that lists the maximum number of DFUs that may be conveyed by each size of pipe. (IRC table 3005.4.1)
For the DWV lines within a structure the table lists the following
1-1/4" =1DFU

When designing a DWV system we must begin at each fixture and attach the code required size of drain line then at every junction where two or more pipes combine we must compute the total DFU load at that point and select the appropriate size of pipe from that point onward.

When we have more than one structure equipped with plumbing on a building lot we must do basically the same thing. We begin by calculating the total DFU load for each structure. At every point where two or more building sewers combine we must add the DFU load for each structure to determine the total combined load on the sewer line at the junction then we consult another table that defines the maximum number of DFUs permitted on each size of pipe.
Example from IRC table3005.4.2
3" with ¼"/ft pitch 42DFU
4" with ¼"/ft pitch 216DFU
3" with 1/8"/ft pitch 36DFU
4" with 1/8"/ft pitch 180DFU
(The IRC permits a 3" line to run with a 1/8"/ft pitch only with the expressed written consent of the local authority having jurisdiction)

    Bookmark   August 20, 2008 at 3:41PM
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I realize the original post is dated, but I have a similar situation as Jon did. I do not recall that his garage was an unheated space. My garage is heated year round and I have a lift station downhill in the basement of my house that ejects my usage to the street muni sewer 220 feet away with 15' of head. I believe the station is plenty large for both buildings, three foot diameter and four feet deep. Only a single ejection pump, not sure if code requires redundant pumps. I want to put a full bath and kitchen in the garage loft and run the waste down to the basement lift station. From reading lazypups response I am confused. One says, "This is a separate structure therefore the drains may not in any manner connect to the DWV (Drain, Waste & Vent) system in the house" and in another response, "Code only allows one line to connect to the municipal sewer or the septic tank, therefore when we have additional outbuildings that are equipped with plumbing fixtures the "building sewer" from the outbuildings must tie into the "House Sewer" before tying into the municipal sewer or septic tank.". Should the first comment read system "from" the house or is there a lift station exception or something? Not knocking anyone, just confused. I would assume that running a waste line from my garage to my home's lift station would be no different than running another interior waste line into the station from a bath add on or something. My concern is that I have about a 20 foot drop from my loft to the station in the house. That's some fast moving poop!

    Bookmark   July 16, 2011 at 1:26AM
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