Carnation Construction

01-03 - Foundations - Under Slab Plumbing

Properly sloped trenches

Story so far

In the previous step you will have dug the 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 Site mark-out plan and your Excavation cross-section drawing.  Here are my drawings for my evaluation shed...

Site mark-out plan 

Here's the version of the plan for the main house...

House foundation plan 


Excavation cross-section 

Here's the equivalent (and more complex) version for the main house...

House foundation depths 

Also a drainage & sewer ditch

It is further assumed that you have dug (using the digger) a drainage ditch to carry away the water collected by the foundation drainage system and to carry away sewage.  Both the sewer pipe and the drainage pipe need to maintain a downward gradient of 1/4" per foot (1/8" per foot minimum) all the way to their destination (the septic system and the rain water runaway area respectively).  If you cannot maintain the proper sewer gradient then you will need to go to a sewage collection tank and then pump it from there to the septic system (or public sewer).  Details on implementing drainage, sewer, and utility ditches can be found here .

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.

Make sure your plumbing design is correct

Building code specifies lots of plumbing rules

There are lots of building code rules associated with plumbing and that of course applies to the under-slab plumbing too.  Before you pour concrete for the slab you are required to have the under-slab plumbing inspected.  It is very important to get your plumbing right.  The rules specify things such as when it is ok and not ok to use a sanitary T piece to connect two sewer pipes and how often you need to have sewer cleanouts.  Where slab area cleanouts are required, they entail routing a pipe up through the concrete slab so you will want to minimize the number while at the same time meeting the building code.  Generally it is a good idea to over achieve on building codes as they just specify minimum requirements, but in the case of cleanouts they are best kept to the minimum.  Many of the plumbing rules are described here together with my particular design.

Do a plumbing drawing

Before you start cutting and gluing pipes it is worth having all the plumbing drawn up so you have something to guide you.  A drawing will help you avoid mistakes and will allow you to purchase all the necessary connectors ahead of time.


Install drainage pipe

Goes on the bottom

The 4" perforated drain pipe goes at the bottom of the ditch (on the drain rock bed) so needs to be installed first.

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 such as this .  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 

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 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 


Install sewer pipe

4" ABS pipe

The next step is the sewer pipe.  It needs to be 4" black ABS pipe .  There are a full range of branch connectors and angle pieces.  It is glued together using black ABS glue.

Sewer pipe 4 inch ABS 


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 under-slab 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 

Here's a picture that shows the cleanout with all the other pipes and conduit that leave from under the building at the exit point.

Cleanout with other pipes 

Sewer stack

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 slab and then carries up vertically and 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.

Under where a stack will be, a long turn 4" ABS connector piece is used to turn the pipe vertical so it will come up through the slab.

ABS 4 inch long turn 

Sewer pipe turning upwards 

Make the vertical pipe long enough to stick up above the slab by at least 6 inches.  For the moment, glue a cap piece on the end so no no stones can fall down it and no sewer gasses can come up.  Later you will cut the cap off.


Install any other utilities

Take advantage of having an under-slab trench

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 some 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 exit point

Multiple pipes under slab 

Multiple pipes at far side

Multiple pipes farside 

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, you need to get the plumbing inspected by the official inspector from the health department.  The inspector needs to sign the plumbing permit.