Begin by determining the exact location and elevation of the top of the existing pipe at the exact point of tie in.

Determine the exact location of the new pipe run and compute the actual length of the pipe.

PIPE DROP= 1/4 per foot for each foot of pipe run. (.25" x pipe length)

fitting allowance= Allow an additional 6" for the fitting allowance of the 3" closet bend on the upstream end of the pipe.

Add the pipe drop and the fitting allowance, then beginning on the top of the existing pipe add pipe drop and the fitting allowance.

That would be the absolute minimum elevation of the finished floor.

Sounds like there's a chance that the sewer line could be significantly above the garage floor level, especially if this is in the rear of the house (do sewers usually run to the street out front?).

Lazypup, is there any way to estimate the sewer line height without breaking concrete?

My main sewer line is in the front of the house, the same side that toilet will be on. It is also below the foundation by about 2 ft. I was wondering of raising the new floor to to the bathroom to 8 or 10 inches will give me enough room to put in the piping for a toilet or if I will have to use an up flush unit. Now keep in mind we have already added a new flooring over the cement of 4 inches. This will give me a step up of 4 to 6 inches into the new bathroom.

As a general rule of thumb the municipal sewer line will run in a easement parallel to the public right of way (road or street) between the curb and sidewalk in the area we often refer to as the tree belt, however, if you live on a corner lot your house sewer may tie into the municipal sewer on either right of way frontage. In addition, in some regions the municipal sewer runs in an alleyway between lots at the rear of the lot. When in doubt call your local water & sewer provider and they can tell you the location and elevation of your tie in point, however, the answer they give you may be a bit confusing also. Some providers will tell you the depth below the center of the roadway or the depth below the top of the curbing. On the other hand, they may give you the depth below the U.S.Coast & Geodetic Survey Benchmark for your lot. The U.S.C.G.S. benchmark may be a couple blocks away and there could be some radical changes in average terrain between your lot and the benchmark. Normally your land title deed will have a site plat or your approved building site plan will show an index that shows the position of the U.S.C.G.S. benchmark and the reference elevation of average surface terrain at one specific survey stake on your lot. It is then the plumbers responsibility to "Shoot the grade" with a builders sight level to determine the depth of the sewer tie in and the minimum elevation of the finished floor where the lowest plumbing fixture will be located.

All structures on slab are required to have a "Main Cleanout" approximately 3' outside the footer wall. You can get a "Best guess" estimate by locating the cleanout then measure the depth below grade of the sewer line at the cleanout, but be real careful here because there is no guarantee that the line won't offset upwards once inside the footer wall.

Now before you decide to install an up-flush toilet you better do your homework. The code requires that we must size the building main drain and house sewer by the actual DFU (drainage fixture unit) load. Typically a house with up to 2.5 bathrooms will have a combined total DFU load of about 30 to 35DFU'd which would then be connected to a 3" line which is rated at up to 42DFU's if run with a 1/4" per foot pitch or 36DFU's if run with a 1/8th inch pitch.

An up-flush toilet is technically classified as a "Sewage Ejection Pump". The codes require that we rate the DFU load of the pump by the output rate of the pump, however regardless of how small the pump is, the codes have established minimums that we must go by.

By example, Under the International Residential Code we must compute for a minimum discharge rate of 14.2gpm and we must allow a minimum of 1.5DFU's per GPM therefore even though your up-flush toilet may have a smaller pump, we are required to allow 14.2gpm x 1.5DFU's for a minimum DFU load of 21.3DFU's. This 21.3DFU value must then be added to the actual existing DFU load of the structure to determine the total DFU load on the house sewer line. As you can see, while the actual DFU load of the structure originally required a 3" house sewer line, with the addition of the up-flush toilet you would then be required to upgrade the house main drain and house sewer line to 4".