How much slope is needed for drainage?
All of your drain pipe (with one exception) must slope slightly downhill. Usually 1/8" per foot is plenty for drainage, this is the same as the commonly recommended 1% slope. Note that I said the drain pipe must be sloped rather than the French drain.
My experience is that many waste line installers simply install a straight sewer line from house to septic tank or house to sewer main, regardless of the building slope, provided that we have at least 1/8" per foot, preferably 1/4" per foot, or more. Lower slope or nearly-flat drain lines are more likely to clog.
- The leach lines themselves, however, should be set level. In a conventional gravity system, the pipe from the house to the septic tank, and the outlet pipe from the tank to the distribution box or leach field, should both slope downward with a minimum slope of 1/4 inch per foot.
- * Your septic system site plan is typically drawn right on top of your property survey showing the septic tank 'setbacks' with tank 5-10 feet from the house, the leach field at least 20 feet from the house, at least 100 feet away from wells and streams, 25 feet away from dry gulches, and 10 feet away from the property
- The cost to build a septic tank system varies widely, depending on your location. According to SepticTankGuide.com, a standard or conventional gravity system for a 3-bedroom home on a level site with good soil would likely cost from $1,500 to upward of $4,000.
Generally, minimum slope for water to run off is 1% (1/8" per 1'). However, minimum slope for a flat roof by building code here is 2%. (1/4" per 1'). You have more than enough slope for your roof, however you will likely need some sort of membrane roofing since it is very low slope.
- A 110mm foul drain taking the discharge of less than 1 l/s should be laid at a 1:40 (25mm per metre) fall. A foul drain taking the discharge from a minimum of one WC can be laid at 1:80 (12.5mm per metre). Gullies incorporating in foul water or combined drainage systems must have a 50mm minimum water seal.
- Once trench is dug, add a 3" layer of gravel along bottom (Image 2).
- Digging a Drainage Ditch. Using a shovel dig a trench that is six inches wide to make a drainage ditch.
- Drainage Pipe in Fabric Lining. Lay the drainage pipe in a fabric lining in the dug ditch.
- Fill Trench with Sand.
- The minimum pitch for shingles is 1:6, which is equivalent to 4 inches rise to 24 inches run. The minimum pitch for a standing seam metal roof is 1:4, or 3 inches rise to 12 inches run. The minimum pitch for roll roofing is 2:12, or 2 inches rise to 12 inches run.
Standard Horizontal Drainage Pipe Slope
|PIPE DIAMETER||MINIMUM SLOPE|
|2 1/2" or smaller||1/4" per foot|
|3" to 6"||1/8" per foot|
|8" or larger||1/16" per foot|
- In civil engineering, the invert level is the base interior level of a pipe, trench or tunnel; it can be considered the "floor" level. Conversely, the obvert level is the highest interior level, and can be considered the "ceiling" level.
- Slope percentage is calculated in much the same way as the gradient. Convert the rise and run to the same units and then divide the rise by the run. Multiply this number by 100 and you have the percentage slope. For instance, 3" rise divided by 36" run = .083 x 100 = an 8.3% slope.
- According to plumbing code, drain pipe has to be sloped a minimum of 1/4-inch per foot and a maximum of three inches per foot or vertical. A slope of less than 1/4-inch per foot will cause constant drain clogs and a slope of more than three inches will allow the water to drain without the solids.
Updated: 17th October 2019