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Laying Drainage Pipes Below Grade

How To Build Your Own Home e-book available


Before You Begin

Tools Of The Trade

Designing Your Home

Building Permit Plans

Excavating Home Site

Prepping For Slab

Pouring The Slab

Installing Sill Plate

cutting Outside Wall Parts

Building Outside Walls

Framing & Raising interior Walls

Exterior Wall Sheathing

Cutting Out Openings

Wrapping The House

Preparing For Roof Trusses

Gettin Roof Trusses Ready

Raising Roof Trusses

Sheathing The Roof

Preping For Shingles

Shingling The Roof

Selecting Doors/Windows

Installing Doors/Windows

Preparing For Siding

Types Of

How To Select Siding

Siding The House

Siding Soffits & Trim

Installing Strapping


Now that you've got a packed level surface for your slab, its time to lay out the drainage system for your home. This must be done prior to pouring the concrete as it will sit below it. If you haven't drawn out the plans for your drainage system yet, it is time to do so. Your design must depict the location and connection of all drains in your home.

Using a copy of the house plans, superimpose a layout of the drainage system over the plans. You will need to design a drainage system below each sink, toilet, tub, and shower. All drains will then run into one central drain, that will empty into the 4" PVC pipe leading from the house to the septic system. Please refer to your local plumbing codes for pipe sizes and drip angles.

You will need a level, shovel, marking paint, drainage pipe of varying sizes, PVC cement and cleaner, string, wooden stakes, and a saw.

  • How to lay out reference points for the walls of each room in your house, on the excavated ground.

  • How to locate and mark exact drain locations in each room of the house.

  • How to locate and mark drain exit from house to exterior drainage going to septic system.

  • How to dig trenches for drainage system.

  • How to lay pipes.

  • Cap off all exposed piping
  • At this point it is a good idea to choose and order your roof trusses in order to receive them in a timely fashion (don't forget the manufacturer needs time to engineer and build them.) For details on choosing roof trusses click here.

    1. Laying out reference points for walls.

      • The first step is to lay out stakes demarking the exact location of the corners of the house, as we did in the excavation process.

      • Connect the 4 stakes with string giving an exact perimeter of the outside walls.

      • Using the same method as above, measure and stake out the perimeter of each room containing a drain (bathrooms, kitchen, and drainage from second floor,) as per your home design plans.

      • Connect each stake with string. This will represent the interior walls of the house.

      • As per your drainage plans, locate and mark each drain with orange marking paint.

      • Using marking paint, mark out piping routes connecting all drains together.

      • When you design your drainage system, avoid using sharp 90 degree turns. Every sharp turn is a potential blockage. It is better to use two 45 degree elbows in place of one 90 degree elbow. This will create a wider curve.

      • You should now have an exact replica of your drainage plans marked out on the ground.

    2. How to ensure a 1/4" drop throughout the drainage system.

      • Using a level, a 1' length of PVC, and a black marker, lay the length of pipe flat on level ground using the level to make sure pipe is perfectly level.

      • Pick up one end of the pipe exactly 1/4" off the ground. Place the level on the surface of the pipe. Using the marker, draw a line on the level locating the end of the air bubble in this position.

      • This will be your reference point for laying the pipe.

      • Drain pipes must lay at an incline of 1/4" per 1' (one foot) length and continue at that incline from the start of the drainage system to the end.

      • If the incline is less than or greater than 1/4" per 1' you will have flow problems. Lay Level On Pipe And Lift One End 1/4 inches Off The Ground Then Mark The level Glass
      • Using the paint markings you have just drawn as guidelines, make trenches throughout your drainage system. Remember the starting point will have less of an elevation than the ending point.

    3. Laying drain

      • Beginning at each drain with a 90* elbow of appropriate size, (depending on type of drain and local building codes) attach a 3' piece of piping to the top of the elbow (do not glue at this point as this is only a dry run.) This will be the piece that will protrude through the slab and will attach to the drain.

      • Attach a predetermined length of pipe to the other end of the elbow, leading away from the drain towards the next drain. Be sure to angle the pipe at 1/4" per 1' piece.

      • Continue connecting fittings and cutting pipe to fit each drain until you have reached the exit point of the house, where it will connect to the exterior drainage.

      • Once you have verified that the entire system is laid properly, it is time to permanently connect the drainage system.

      • From this point on it is important to keep the 1/4" per 1' decline all of the way to the end of the run.

      • Start the first drain fairly close to grade. Each drain will have a length of pipe sticking up through the slab which will be trimmed at a later date. Be sure to clean all joints well with PVC cleaner, and to apply glue all the way around each fitting joint. It is advisable to take your time during this project since once the slab is poured, there will be no going back. Removing a drain or pipe at that point will mean destroying a portion of your floor, subfloor, and slab.

        All Drains Are Connected And Covered Over With Gravel

    4. Connect the central house drain to the exterior drainage pipe going from the house to the septic tank.

    5. Once you have verified that all drains are properly secured and laid at the proper angle, it is time to bury the drainage system in preparation for the slab.

    6. Temporarily cap off all of the exposed drains. This will prevent debris and concrete from entering the drainage the system during construction.

    {Next Step: Preparing For The Slab}


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