perimeter drain gradient

[Zeppelin]

I have a twenty-seven foot long perimeter drain along the side of my house. The front corner is 5/8 of an inch higher than the rear corner. Is this enough to allow for drainage from the front of the house to the rear? Or should I increase the gradient? Is there some kind of formulae that can be used [like a quarter of an inch per linear foot, for example]?

Bob Pellow
Parksville, BC

[Reedwalker]

How did you measure so precisely over such a distance?

Did you use a transit level? Line level?

According to the International Plumbing Code Table 704.1 Slope of Horizontal Drainage Pipe


2.5" or less = 1/4" per foot
3-6 = 1/8" per foot
8 and up = 1/16" per foot

[Zeppelin]

Hello Reedwalker,

I have a trim piece circling the house that I checked for level. I then measured down from the trim piece to the top of the perimeter drains that I have excavated to date. Not precise but accurate enough I would guess.

Thus, to use your standard, for anything over eight feet, one should use 1/16 inch per linear foot, which means my drain should drop 27/16 which equals 1 and 11/16, rounded off would equal a drop of 1 3/4 inches. It should drop that amount instead of the 5/8 inches that currently is the case.

Am I calculating that correctly?

Bob

[havasu]

It sounds like your calculations are correct but keep in mind that the code is designed as minimum standards. I've always used 1/4" per foot for any distance, and never had a problem.

[Reedwalker]

No that is incorrect.

The first set of numbers is the pipe size.

So assuming your pipe is 4" ID (inside diameter) it requires 1/8" per linear foot. So a 27' run of 4" would require 3 3/8" of fall.

And I'm sure your method of measurement is adequate.

[havasu]

My bad. I also read the code incorrect. Maybe that is why I still use 1/4" per foot drop on any drain pipes.

[Reedwalker]

That's a pretty good rule of thumb Havasu.