Construction - Concrete Work - Plumbing and Electrical Rough-in Under Slab Floor

This is the fifth post on the concrete phase of construction and I apologize for its length but countless surprises challenged my DIY-ness and I would be remiss if I did not pass them along for whatever they may be worth.

With regard to the concrete per se, all that remained was to "sling" in the gravel sub-base and pour the slab floor. However, both tasks had to wait until the plumbing that resides below the concrete had been roughed in. The rough-in comprised not only the typical waste/sewer system but, in our case, the complete water supply system as well. And a considerable amount of electric could be roughed in before the pour. (Reminder:  Click on any photo to enlarge it.)

Waste Plumbing Rough-in
In conjunction with pouring the footing under the front wall of the house, a short section of 4" PVC pipe was inserted below the footing forms with a foot or so extending

Waste plumbing rough-in; notice the that it connects with
and exits through the section previously installed
under the footing

beyond the forms on both the outside and inside to facilitate running the sewer line. The same was done with 2" PVC for the water line. The outbound sewer line connected directly to the 4" but the water line merely used the 2" to hold space until a 1" water supply pipe between the house and the water main at the street could be installed.

Installation of the waste lines was a matter of trenching into the excavation just deep enough to have the correct fall for proper drainage.  The downstream pipes were well below the grade and the upstream most pipes were almost at grade. The only thing that complicated the installation was trenching through soil that was more like stone than dirt, thanks to a layer of what old-timers called "hardpan" that presented even more of a challenge by not having seen a rain for almost a month.  The good news was that it will make a great base on which to rest a house.


The installation of the waste system was less complicated than our iteration of the water supply system but it does give meaning to the old saying, "Trifles make perfection but perfection is no trifle".  It requires a lot of know-how and attention to detail.  Fortunately, between two friends and I, we had enough composite knowledge to put together a system that passed inspection the first time.

PEX Rough-in for Water System
Our water supply system comprises PEX (cross-linked polyethylene) tubing as much as possible.  PEX was used extensively in Europe for potable water several decades before finding its way to the US in the mid-80s (it was used in the US before then for radiant floor heating but not for potable water).  Even though it is not used yet by many tradesmen, it meets code in most places and has many advantages over copper and plastic pipes. (For a thorough discussion, go to design guide for PEX plumbing.) Its major advantages for our project are simplicity, significantly lower cost and DIY-friendliness.

The myriad design options for PEX are well covered in the above link.  Suffice it to say that, for our project, only two major decisions had to be made. First, do we run separate "home run" lines to each appliance and fixture or do we run trunk lines to areas like the kitchen and bathrooms then branch off to the individual appliance or fixture?  Second, do we run the lines beneath the concrete floor or run them more conventionally through the framework of the house?  We opted for home runs and below the slab.  Actually, the two are linked in that only continuous tubes of PEX, free of any fittings, can be buried.  By marrying home running with burying, the rough in for the supply system was essentially complete before pouring the floor, much less waiting for the framing to be done, and at a fraction of the cost of doing it later.

Underground Installation
If PEX is buried, it needs to be protected from injury while the rock sub-base is slung in place and the slab itself is poured.  Moreover, it must be protected from chafing where it enters and exits the concrete.  These issues are usually handled by trenching into the dirt, covering the PEX with dirt, sand or pea gravel and providing short sections of conduit for the tubing where it leaves the dirt, sand or gravel and passes through the concrete. My lack of experience made me cautious to the extent that I decided to use conduits for the entire length of each home run -- from a couple of feet above the floor level at the upstream end near the water main and water heater to a couple of feet above the floor at the downstream end in wet walls near the appliances and fixtures.  The loosely-fitting conduits gave me the security of being able to retrieve the PEX if, for some reason, it wasn't working out as intended.  Also the conduits obviated the need for trenching in that they were robust enough to lie on top of the grade and withstand the slinging of the rock sub-base and the weight of the concrete.  Most were buried in the gravel, a few showed after the gravel was slung but barely.  Using so  much PVC did increase cost but not significantly.

When the decision was made to enclose PEX in conduits, I experimented in the aisle of my favorite home center using a length of 1/2" PEX, various diameters of Schedule 40 PVC pipe and 45 degree fittings.  As a result, I decided that 1 1/2" PVC pipe would be the smallest possible diameter for ease of insertion of the PEX for the 20 - 30' distances required and for the bends necessary to enter and exit the concrete. And, supposedly, the tubing could just as easily be removed from the conduits in the future if necessary.

Wrong!  As a trial, I glued together the PVC for one of the longest runs, tried to push the 1/2" PEX tubing through it and found that it hung up and couldn't be budged past the rough edge of the second 45 it encountered.  I tried to run an electrical fish tape in from the other end so as to pull the PEX and it too hung up immediately. Consequently, it was necessary for all 20 home runs to push the PEX past each fitting before gluing the fitting.  Fortunately, the PEX was not effected by the PVC primer or the adhesive so there was no danger of gluing the PEX to the conduits.  If a PEX line needs to be retrieved and replaced in the future, the thought comes to mind that a plumber's snake would negotiate the rough fittings and could be used as a fish line for pulling the tubing through the conduit or, better yet, the old tubing could be used to pull the new through the conduit.  

As it turns out, 1 1/2" PVC was the right choice.  PEX is a stiff material with a strong memory that wants to stay coiled up. Over distances of 35", 1 1/2" barely allowed the PEX to be pushed through.  Even the 10' sections of pipe between fittings offered sufficient resistance that I think a smaller diameter conduit could not handle easily. In one instance, I tried pushing two PEX tubes through in tandem and quickly decided that the effort was not worth the savings on conduit.  

Controlling the Entry and Exit Points for the Conduits
The biggest challenge to running the PEX system was to have it enter and exit the concrete at a perpendicular angle and be properly located inside the wet walls adjacent to fixtures and appliances.  To have any part of the conduit exit outside the wall would be unacceptable. 

The supply system rough-in completed:  Bathrooms to the
left, kitchen island sink and laundry to the right; future
manifold area in the background; second kitchen sink out
view to the right

Consequently, to control the ends of the conduits,  I shop-made salvaged 2 x 4 supports.  One set had 2 1/4" holes for the conduits and an identical set had 7/8" holes for the PEX.  Five foot sections of 4" PVC pipe were used as posts to hold the 2 x 4s in place.  Since the buried portion of the posts would remain in the concrete, I chose plastic posts over metal to eliminate the possibility of the metal rusting, expanding and cracking the concrete.  Wood posts were out of the question for termite reasons. Eventually, the PVC posts will be sheared off flush with the floor and filled with concrete. In many cases, the stub-outs of the waste plumbing rough-in could be used in lieu of or in tandem with temporary posts.  I chose 4" posts only because 2-hole metal straps were available for fastening the 2 x 4s to them and the pipes were left-overs.

Manifolds with cut-offs for each home run

Looking ahead, the beginning ends of the PEX lines will be joined to copper manifolds with individual cut-offs for each line, much like a circuit breaker for each electrical circuit in a service panel. Eventually, copper pipes will run from the tankless water heater to the hot water manifold(s) while PEX will suffice between the water main shut-off and the manifold(s) for the cold water supply. 

The PEX at the fixture/appliance end of the lines will be supported by brackets attached to the framing and stubbed out of the wall for uninterrupted connection to faucets and appliances.  Or perhaps, I will use a copper stub out. In either case, only three or four fittings per line will be necessary -- two at the manifold end and one or two at the fixture/appliance end. And, of course, there will be no need for cut-offs adjacent to the fixtures and appliances because cut-offs are built into the manifolds.

Bracket-supported PEX for direct
connection to faucets

PEX with copper connection for faucet

Materials Cost
The rough-in cost for the waste and supply materials was only $300. However, when the cost of PVC conduits was added, the total cost was $841. I consider the $541 for the conduits to be cheap insurance against damaged PEX lines and not having the option of retrievability in the future.

Other Advantages to Home-running
Compared to trunk and branch systems, the home run system requires more PEX tubing but far fewer fittings. Since the cost of tubing is much less than the cost fittings, home running is cheaper. And, a traditional copper pipe system would cost even thousands more due to the higher cost of copper and, for underground installation, someone who is capable of soldering with silver solder. Another advantage to an underground home run system is quicker hot water. Compared to a home run system run through the framing, the quickness derives from the shorter distance between the water heater and the fixtures.  A trunk in a trunk and branch system requires a larger pipe (usually 3/4") than the branches (1/2").  This means that, compared to a home run system with all 1/2" pipes, there is more cold water in the system to be pushed out by the hot water.

Home Run Electrical Conduits

Electrical conduits from future breaker panel area to areas
that would be hard to reach by running wire through the
 framework of the house, e.g., the island sink in the kitchen
to the right in the picture or the front wall of the house to the
left

After the water supply rough-in was complete, one last job remained before pouring the floor -- running electrical conduit.  Being able to make long runs sub-concrete simplified getting service to areas remote from the future breaker box, especially where the open floor plan (living room, dining room and kitchen) and cathedral ceilings would significantly complicate running wires through partitions and ceilings. I ran 3/4" conduit for two reasons -- being unfamiliar with pulling wires through conduits, I figured bigger was better and 3/4" gave the option of  pulling two or three sets of wires when only one or two might really be necessary.  If one or two sets remain unused in the future, there would be no downside

Stub-in for future floor receptacle in the form of a space-
holding tin can; the black pipe in the background, the water
 supply line coming in from the street, is wrapped with pipe
 insulation to keep the water cool as it passes through the
thermal mass under the floor when it is warmed by the AGS
 system eventually

since I fell heir to several spools of wire when helping to ready our former church for a remodel. 

The need for a floor receptacle was met by using a tin can to hold space during the pour.  A hole saw in the drill press, provided access for the conduit  through the bottom.  The rotating laser located the top of the can 2" above the pour, a stone supported it and a length of rebar piercing the bottom the can and driven into the soil at an angle stabilized it.

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October 2019 Update
The final rough-in for the plumbing was completed four years later.  Here is the link to the later post on the plumbing rough-in.  Of special interest is its description of the manifold, tankless water heater and the automatic overflow prevention, all part of the PEX home-run system.