The North Carolina Division Of Environmental Health controls the state rules/regulations regarding on site septic systems (OSS). A copy of the state regulations concerning OSS' can be found at:
Septic system are based on a "Gallons Per Day" flow rate/processing capability. NC requires systems to be capable of handling a minimum of 120 gallons per day for each bedroom, or designated sleeping area, with a minimum capability of 240 gallons per day for any OSS. That is based on a typical maximum of two persons in each bedroom. If there are to be more than two persons per bedroom the rates are calculated at 60 GPD per person (i.e. 3 in a bedroom is 180 GPD for that bedroom. It basically works out to 60 GPD per person. In your example a three bedroom home, with 2 in each room, would require a minimum capacity of 360 GPD. Many systems are sized at 500 GPD and larger, although there have been systems smaller than 500 GPD.
The first item of information you would need to know is what the current system is capable of handling/processing. Somewhere near the septic system is generally a tag describing the system and its capabilities. Many times it is gone from aging, etc. The local department of health, either county or city, should have a permit record for the current system and can tell you what its basic capabilities are. Once you know what is there you can then move on to decide if the system is currently of sufficient size to handle your new 4 bedroom requirement.
If your system is not capable of handling the increase in residency the Division Of Environmental Health can point you to lists of licensed individuals and companies that can make the recommended changes. Keep in mind that you should never allow your usage to approach the listed maximum processing capability of the system. For example if you have a system capable of processing 500 GPD and you are moving up to an 8 person occupancy that would require a system processing at least 480 GPD. As you approach the systems maximum capability you risk the very real chance of overloading the system or finding its weaknesses. There are acceptable buffer zones that should be maintained.
Good luck on your purchase.
Emmanuel J. Scanlan
PS Inspection & Property Services LLC
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