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Under Slab Plumbing


Properly sloped trenches

Story so far

In the previous step you will have dug the drainage trenches to the right depths and very importantly will have made them the right 1/4" per foot slope.  The trenches will have been dug as per your foundation drawings and your Excavation cross-section drawing.  Here are my drawings for the main house...

Foundation plan 

Excavation heights - East West


Also a drainage ditch

It is further assumed that you have dug from the house (using the digger) a drainage ditch to carry away the water collected by the foundation drainage system.  The drainage pipe needs to maintain a downward gradient of 1/4" per foot (1/8" per foot minimum) all the way to its destination (the rain water runaway area).

Line bottom of under-slab trench with drain rock

To keep the 4" perforated drain pipe off out of the dirt, put about an inch of 1.5" drain rock in the bottom of the trenches.  Make sure you maintain the correct 1/4" per foot slope and adjust the thickness of drain rock layer to get this exactly right.  Don't leave any gap in the drain rock coverage as you want to have the pipe properly supported all the way along its length.  You need the drain rock in all the under slab pipe ditches.  You can if you want add the drain rock after the drainage pipes have been glued.


Install drainage pipe

Use proper type of drainage pipe

Always use the rigid pipe rather than the flexible type with crinkles because dirt gets trapped in the crinkles.  Some building inspectors will not allow the flexible crinkle type to be used.  The type to use is this .  Connectors come in various branch types and angles.  You can get a version without perforations for use in ditches that need to carry water rather than collect it or weep it out, but under the slab you need to use the perforated version.

Perforated drain pipe   Drain pipe bend

Drainage 4" pipes will not have filter cloth sock

Many people insist that 4" perforated drain pipe should be covered with a sock of filter cloth to keep soil from getting in and blocking it.  I prefer to leave off the sock and instead have a good arrangement of cleanouts, down which a long cleanout brush can be inserted and/or a high pressure hose pipe.  I worry that over the long life of the house that any filter cloth would itself become blocked and there would be no way to clean it.

How water will get from Form-a-drain to drainage pipe

There will be a hole drilled in the bottom of the Form-a-drain as it passes over the drainage pipe.  To catch the water, the drainage pipe will have upward facing T pieces.  To guide the water from the Form-a-drain to the T pieces, short pieces of solid wall 4" drainage pipe cut with a 1.5" slot will be used.  A cap piece cut in half with a 1.5" gap completes the assembly.

Drainage pipe under Form-a-drain 

Drainage T piece 

At the far end of the pipe a slightly different arrangement is used to allow the 4" pipe to be slightly higher...

Form-A-Drain and 4in Pipe Far End 

The design is relevant at this stage because the T pieces need to be installed directly under where the Form-a-drain will be.  The outer edges of the inner and outer Form-a-drain are known because that is what the batter board strings indicate.  Temporarily attach plumb bobs down from the batter board strings above the drainage pipe position, and then the center of the T piece upward facing hole needs to be 3/4" from the position indicated by the plumb bob (half the 1.5" width of the form-a-drain ).

Plumb bob 

Glue the drainage pipe

The 4" perforated pipe and the various connectors and branch off pieces are all glued together using PL Premium construction adhesive.

PL Premium construction adhesive 

Here's how things look after the drainage pipes that will be under the footing have been glued.  The two connectors at the top of the picture are for connecting the drainage pipes round the outside of the footings.  The lower two connectors are the T pieces positioned under where the Form-a-drain will be.

Drainage pipes under footing 

You need to route the pipe and implement the T pieces for all the points where the drainage pipe goes under the Form-a-drain.

While installing the pipe and arranging the drain rock underneath to support it, use a spirit level to ensure the proper slope is maintained.

On the opposite side(s) of the house from the exit point, you put upward facing right angle connectors and route the pipes up to the surface outside of the walls and footings.

Drainage pipe vertical 


Sewer pipe

Not under the slab

Typically sewer pipe is also installed under the slab, but that is not my preferred approach.  I have opted to route the 4" sewer pipe through the wall just above the slab rather than through the slab.  This reduces the chance of water leakage.  The down-side is that a raised floor (or wall bulge) is needed to hide the sewer pipe as it routes through the basement.  The raised floor is also needed for any bathrooms in the basement because the plumbing fixtures, particularly toilets, need to be higher.



The sewer will get from the building down a trench that maintains the 1/4" per foot downward gradient to the septic system (or public sewer).  Where the house sewer pipe joins the trench sewer pipe it is a good idea (and a building code requirement) to put in a cleanout.  The best type to use is a two-way cleanout so you can clear both upstream and downstream blockages.  You should put the cleanout just outside the footing within about 30" from the outer edge of the footing.  A piece of sewer pipe connects upwards from the cleanout to about a foot above grade and a screw cap goes on the top.

Two way cleanout 


Sewer stack

In my case, none of this is relevant at this stage because I chose to route the sewer pipe through the wall rather than through the slab.

For detailed information go to Plumbing Building Code and Plumbing DWV (Drain, Waste, Vent) .

Sewer plumbing is referred to as DWV (Drain, Waste, Vent) plumbing to stress the point that you need a vent as well as a drain so that there is "air is behind water".  This ensures that water drains properly.  You need to provide a pipe up to the roof near to the various plumbing fixtures.  The best way to do this is to provide a DWV stack (or stacks) which is a vertical pipe that comes up through the building and then out through the roof.  Plumbing fixtures then connect into the stack.  The stack should be 4" ABS pipe all the way up except for the last part through the roof that is only for air and so can be 2" ABS.

At the bottom, a long turn 4" ABS connector piece is used to turn the pipe vertical from the 1/4" per foot sloped 4" ABS pipe that goes to the sewer collection tank in the yard.

ABS 4 inch long turn 


Install any other utilities

Take advantage of having an under-slab trench

This not relevant to my case as I route utilities round the outside of the house rather than under it.

If you are taking advantage of having a ditch under the building to get utilities such as electricity, data, and random other stuff  to the other side of the building, then now is the time to install this pipe and conduit in the trench.  It needs to be low enough in the trench to pass under where the Form-a-drain will be.  It is best to avoid making unnecessary holes up through the slab, so the best plan with these utilities is to route them under the building and out the other side, and then route them vertically up the wall on the far side.  You can feed them through the wall below grade if you want, but it will still be easier to waterproof an entry point in a wall rather than in the slab.  One way of allowing for future random stuff is to install 4" non-perforated drainage pipe so you can retrofit other stuff later.

It is best not to use the same trench for both a sewer pipe and a potable water pipe so you should use a separate trench (eg round the outside of the building) for the potable water pipe.  In many areas it is against building code to use the same ditch.

With lots of pipe and conduit it does take a wider trench and it can be a bit challenging getting them to all fit below the height that the Form-a-drain will be at.

Multiple pipes at far side

For the moment just leave the various utility pipe and conduit on the far side sticking up above grade with plenty of length to spare.

Vertical pipes 


Get plumbing inspected

This an absolute requirement

Before it is covered in rock if there is a sewer pipe under the slab you need to get the plumbing inspected by the official inspector from the health department.  The inspector needs to sign the plumbing permit.  Even if you don't have any plumbing under the slab it is still a good idea to talk (eg via email) to the health dept inspector to tell him that there is no under-slab plumbing and get his agreement that it is ok to pour the slab without having a plumbing inspection.