Ensuring proper slope

I am laying down DWV piping under concrete slab. How do I ensure proper constant slope from a fixture to main stack? There are a lot of fittings used.
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Alexander Galkin wrote:

If it's in a straight line, lay a long 2 X 4 along the pipe to ensure the run is reasonably straight. You want approx 1/4" per foot slope. Mark your level at the 1 foot length and tape a 1/4" shim there and you can check individual sections.
Jim
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OR you could just use an angle finder at or about 40 degrees is what most building codes for. Terry Lewis Contracting
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40 degrees, my ass!

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