sewer pipe drop

Hi, Just an intro and a quick question.

I've just started about a 1000 sq. ft addition to an old house I recently bought. It's an old one built in the 20's and the addition is being built on an old school pier/beam foundation. I've contracted the foundation work and it's almost done. The rest will be regular stick built with pre fab trusses which I'm doing myself.

I've done the regular DIY plumbing projects but this will be a pretty big test of my skills and patience. I've purchaced some good books on the subject and am currently boning up...

Now the question. I understand that the min.drop for 3 in. drain is 1/8 per foot. Some of my browsing has lead to drop calcs of 7 feet or so over 60 feet. My drain farthest from the old sewer pipe is around 40 feet so at .125 x 40 is 5 inches, right? 1 inch drop for every eight feet.

Am I missing something or am I on the right track? Digging a 7 foot trench under my existing house would be way out of the realm of my DIY skills
I hope I haven't overlooked a biggie...

Oh yea, I plan on replacing all the old water pipes with a PEX manifold system. Looks a lot simpler than fooling with a bunch of PVC joints.

Thanks in advance.

Now the question. I understand that the min.drop for 3 in. drain is 1/8 per foot. Some of my browsing has lead to drop calcs of 7 feet or so over 60 feet. My drain farthest from the old sewer pipe is around 40 feet so at .125 x 40 is 5 inches, right? 1 inch drop for every eight feet.

Click to expand...

If your run is 60 feet, your total drop or fall with be 60 feet X 1/8" per foot or
60/8ths vertical drop.

60/8ths converts to 7 1/2 inches total drop over 60 feet.

For a 40 foot run, your total drop or fall will be 40 feet X 1/8" per foot or 40/8ths vertical drop.

40/8ths = 5".

Thanks, that's what I thought! My house and the addition is on a 2 foot crawl space so it shouldn't be a problem. I'm reading up on vents and such now...