Civil-Environmental Engineering

Frequently Asked Questions*

1) How do you size the leachfield for a house?


  • Find the design flow rate in gallons per day (gpd) from the county requirements (see the list on the Installers Page).  This is usually dependent on the number of bedrooms in the house.  As an example, a 3 bdrm house in Butte County would require 360 gpd.
  • Check the soils report to see what application rate the county inspector will allow.  We will assume a value of 0.7 gpd/sf for our example.  This gives a trench bottom area of 360 gpd ÷ 0.7 gpd/sf=514 sf required.
  • Decide on the width of trench to use.  This usually is set by the width of the backhoe's bucket, but must be within the range the county allows.  In this example we'll use 24".  This gives a total trench length of 514sf ÷ 2 ft=257 lf.
  • Divide this distance into equal length trenches, which must be shorter than the county's maximum trench length.  In this case, since Butte County's max. length is 100 lf, 3 trenches will be used:  257 lf ÷ 3 trenches=86 lf each.
  • Add on the space that will be required for the central manifold and distribution boxes (it can be down the middle [preferred], or on the end).  We will use 3 ft, so the field must be 89 ft long, end to end.
  • To determine the width, add the minimum spacing the county requires to the trench width.  For Butte County this is 6 ft.  This means that, in effect, each trench takes up 2 ft + 6 ft=8 ft of property (except the last trench, which doesn't have another trench to stay away from).  In this case the entire primary field will need to be 2trenches x 8 ft + 2 ft=18 ft wide.
  • So the primary field, in this case, must be at least 89 ft x 18 ft .  The secondary or replacement field must be the same size.
  • If a more square shape is required to fit the two fields on the property within the allowed setbacks, you can recalculate using more trenches.  In this example, going to four trenches would result in (2) 68 ft x 26 ft fields.
  • Or, you can re-figure it with a wider trench.  Using the widest trench Butte County allows, 36", and two trenches would result in (2) 89 ft x 12 ft fields.  Using three 36" trenches would give you (2) 61 ft x 21 ft fields.
  • A bed system is typically the most compact, though most counties require it to be oversized since there are no sidewalls to accept flow.  Butte County requires a 50% increase, so for our example 514 sf x 1.5 =771 sf, or 28'x28'.  With the 3' added for the distribution boxes in the center this gives us (2) 28'x31' fields.

With our example of a three bedroom house in Butte County, then we have the following options:

    89'x18'     68'x26'   89'x12'    61'x21'   28'x31'


Sometimes the easiest way to decide is to make a scale sketch of the property (showing the setbacks), then using graph paper, cut out an example of each field and move them around on the drawing.  Just remember that you need two fields, but you usually would be allowed to mix and match the configurations, if you need, to make them fit the property (e.g. you might plan on an 89'x18' field-style primary because it fits well close to the septic tank, and a 28'x31' bed-style replacement area that is further away and would require installation of a pump at that time).



*The following page is a summary for quick reference only and may not represent the latest standards.  For complete and up-to-date information, see each counties official website.