The Dept. of Environmental Quality in conjunction with the Dept. of Commerce, provides the following guidance document on setbacks from decks to septic tanks and drain fields.
Information from the State Building Codes indicated that decks over 30 inches in height require a building permit and thus are a "structure". The following is a proposed guidance on setbacks from decks to septic tanks and drain fields:
Decks over 30 inches in height: are required to have a 5 foot separation from the septic tank and a 10 foot separation from the drain fields. The measurements are to be taken from the location of the pier block or other supporting device. In the case of cantilevered decks, the tank and field may be partially under the deck as long as they are 5 feet from deck's supporting device(s).
For existing or new septic tanks that are not or not planned to be the required 5 feet from the deck structure but are accessible for pumping, the local Building Dept. official or a Professional Engineer may certify there are no structural problems associated with the reduced setback for consideration by the Agent.
Decks under 30 inches in height: are not required to maintain a set back from the tank or drain fields to the deck structure, or it's supporting devices (ie pier blocks).
All decks, regardless of height, must: 1. not have any obstruction over, or within 5 feet of the tank that would impair pumping, and 2. be over or within 10 feet of a drain field if they cause the drain field to be inaccessible to a back hoe for repairs.
In the opinion of the Agent if the tank location appears to compromise the decks support structure, the agent should notify the local Building Department.
CDC 02/22/2005, rev. 05/04/2005, rev 06/29.05