Step 5 - Slab Prep

Get ready to do a lot of work that is just going to be torn back down. Whew, quite a few man hours go into building the forms for a concrete foundation. If you can afford to hire a helper, now is the time. You'll notice in the videos below that I did some things the hard way. Most professional slab crews will do a slab in this order:

1. Pack the base
2. Cut beam trenches
3. Build the forms
4. Run the plumbing
5. Pressure test plumbing
6. Set the vapor barrier
7. Set the re-bar
8. Pour the slab

If you are a one man show, wait as long as possible to dig the trenches because they will cave in during rain storms. If you do build the forms first, like I did, you will have to rent a special trencher that has the blade all the way over to the side of the machine. Also, don't put the brick-ledge boards on or do ANY plumbing until after you have the beams trenched out.

The videos below outline the steps as I did them ... or did them, un-did them, and re-did them as was the case whenever I screwed up or did something in the wrong order.


The first task is to layout the batter boards with string lines that mark the exact locations of the edges of your foundation. Pick the height of the layout such that you can have a descent of 12" around every edge of your house. To word differently, find the highest point around the edge of your foundation, and make sure the surface of your slab is at least 12" above that level. This will cost you a little extra in base fill material, but it will save you from ever having to sweep water out of the garage.


The next step is to build the forms. I actually ran my string lines to mark the edge of the frame of the house, so I made a little template to space the forms out far enough to allow for the brick ledge.


Don't try to get by without renting a Bobcat again unless you want to waste a lot of time like I did. Do order extra base material to fill the forms or make sure you can get deliveries of more on the day that you have the Bobcat onsite. It takes a lot more base fill than you think, especially if you do a good job of packing it down as you go.  Lastly, leave one section of the forms down so that the dump trucks can back all the way into the foundation area, and to have access to drive the Bobcat in and out to spread the fill and pack it down.


Again, just rent the equipment you need for the job. $200 and the right machine can do two week's worth of hard manual labor in a few hours.


Also, don't run ANY plumbing until after you have trenched out for all your slab beams.  Most of the plumbing can run fairly shallow through the base.  Depending on your situation, use the trencher to set the main sewer line, but a pick and shovel should work for everything else.


   One way to simplify the plumbing for the slab is to put all your bathtubs on the second floor. For your main sewer line, use one straight 4" PVC pipe across the house to connect all the downstairs toilets with as few bends as possible. 'T' into that main line as needed with 2" lines for the drains for, tubs, sinks, and the washing machine.

   Remember to allow for venting on the sewer lines. Technically, each toilet should have a 3" PVC pipe that runs through the wall behind the toilet.  So, you need a 'T' pointing up for the pipe to come up through the slab where the toilet will mount ( center point is 12" from the wall behind the toilet ) as well as a 'T' pointing horizontally to tie the vent pipe into. Place the horizontal 'T' upstream of the 'T' that is for mounting the toilet. You also need a couple of short pieces of PVC pipe and a 90 deg. long sweep elbow to route the vent pipe up through the slab where the back wall behind the toilet will be. I like to use 4" pipe for the sewer lines, but I reduce down to 3" for the vent pipes. Of course, pay attention to how the sweeps in those 'T's are running. The path should always be down and out. Remember to also have a vertical 'T' to receive any sewer from the upper floors.

   That main horizontal sewer line should also gradually descend as it exits the house. Plan to have your septic tank located such that it is on a straight shot, aligned with your main sewer line as it exits under your slab.


   For pressure lines, use soft copper for all under slab work. There should be no joints of any kind under the slab, and you just bend the tubing as needed to come up or go down. Starting at your water service line outside the slab area, attach a shutoff valve (I recommend a simple PVC ball valve if your incoming line is PVC.) Sweat a brass threaded fitting onto the end of the soft copper tubing, and then, after it cools, screw it into the shutoff valve. Run that line under or through a hole in the forms and then bend it up. Create your first manifold here. You will need to 'T' off for hot water and for every direction that you need to run water.

   If multiple locations needing water are off in the same general direction, create another manifold at each intermediate location. At each manifold, you simply bend the soft copper up so it will come above the surface of the slab. Then you sweat on the needed 'T's and run the outgoing lines back down into the base fill and across to the next location needing water. I like to protect that soft copper with plastic sleeves or insulation sections as I do each run. Do practice ahead of time before you try to "sweat" soft copper tubing ... it's a pain.

   Finally, pressure test all your lines before you pour the concrete.   Create a temporary bridge from the cold water side to your hot water side so that you can pressure up the hot water lines as well.  Leave the pressure on the lines for at least 24 hours, and check for any signs of moisture at the joints or along any of the runs.   If you have the equipment, you could pressure the lines with air and have a pressure gauge attached.  Then just check for any drop in pressure to indicate a leak.  That's a good final test.


Watch dumpsters at construction sites for 'almost' used up chop saw blades. These can fit in a regular skil saw and be used for cutting re-bar.


6" mesh makes great supports for re-bar in your slab beams.


After you get your vapor barrier laid down, it is time to tie in that re-bar.


A 6" wire mesh will really strengthen up your slab. Make sure it stays as close to the upper surface of the slab as possible without being above it anywhere. Also, before the concrete trucks come, put a coating of tar around all exiting pipes. This will help prevent insect intrusions later.

   You will definitely need help on slab pour day. I found that it was less expensive to hire a crew than it was to rent all the equipment and owe a bunch of favors to all my friends and family that I would need out there.  Also, concrete is often less expensive for weekday delivery.

   Remember to put all the anchor bolts down.  You could drill a hole in the middle of a 2X4 block, and use that as a little template to position all the bolts the same distance from the brick ledge.  Try to keep the slab misted with water after it sets up, especially if you pour during the hot months of summer.  It is best to also mist the slab the next day.  Finally, let the concrete cure for a few days before you start pulling the forms off.

Bonus:  Add a storm shelter - Shown below are the forms of an 'under-slab' shelter.   It only goes down to the depth of the natural ground at the back side of the house, and so anyone would have to lay down to get inside.   The forms allow for pouring the walls and ceiling of the shelter and the floor is poured later when the cement trucks come back to do the driveway.

Note the anchor bolts in the forms.  These will be used to attach boards for the lid to sit on.


Remember to install a P-Trap and drain line in the bottom so no moisture will collect in there.

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