TEXAS OSSF INSTALLER II STUDY GUIDE (2026/2027)
Based on TCEQ Rules Title 30, Chapter 285, and Current Design Standards
1. What is the minimum vertical separation required between the bottom of a
subsurface trench and a seasonal high-water table in a Type I system?
ANSWER ✓ 12 inches.
2. According to TCEQ rules, what is the minimum horizontal distance required
between a septic tank and a water supply well?
ANSWER ✓ 50 feet.
3. When installing an aerobic treatment unit (ATU), what component must be
installed to permit measurement of the effluent clarity?
ANSWER ✓ A sampling port located after the pump tank or prior to final
treatment/dispersal.
4. What is the required minimum liquid capacity for a septic tank serving a three-
bedroom residence?
ANSWER ✓ 750 gallons.
5. What class of OSSF Installer license is required to supervise the installation of an
ATU with a daily flow design of 5,000 gallons per day?
ANSWER ✓ Installer II (or Installer III).
6. When is a fill system (Type VI) required instead of a conventional trench (Type
I)?
ANSWER ✓ When the naturally occurring soil has a percolation rate faster than 3
minutes per inch or slower than 60 minutes per inch, or when the vertical separation to
a limiting condition is less than 12 inches.
7. What is the purpose of a pump tank in a pressure-dosed system?
ANSWER ✓ To provide storage for effluent, house the pump(s), and allow for controlled,
timed dosing of the dispersal area to improve distribution and resting cycles.
8. What is the minimum horizontal separation required between a spray dispersal
area and a property line?
ANSWER ✓ 10 feet for a residential system; larger distances apply for non-residential or
based on local rules.
, 9. When installing aggregate in a trench, what size gravel is typically specified?
ANSWER ✓ Clean, washed gravel ranging from 0.75 inches to 2.5 inches in diameter.
10. What critical step must be completed and documented before an OSSF
installation can begin?
ANSWER ✓ A site evaluation conducted by a licensed Site Evaluator, resulting in a site
evaluation report and design.
11. What is the required minimum depth of cover over the distribution lines in a
standard trench system?
ANSWER ✓ 6 inches of soil cover over the gravel/distribution media.
12. A four-inch perforated pipe in a trench must be laid with the perforations
facing which direction?
ANSWER ✓ Downward at an angle between 4 and 8 o'clock positions to minimize
sediment entry and promote even distribution.
13. What component is required on the inlet pipe of a septic tank to prevent
disturbance of the scum layer?
ANSWER ✓ A sanitary tee (or baffle).
14. What is the minimum horizontal distance from a building foundation to a
septic tank?
ANSWER ✓ 5 feet.
15. When using chambers instead of gravel in a trench, what is the primary
function of the chamber?
ANSWER ✓ To provide a void space for effluent storage and infiltration into the native
soil while providing structural support.
16. How must a septic tank be leveled during installation?
ANSWER ✓ The tank must be level from end to end and side to side, as per
manufacturer's specifications, typically within 1/4-inch.
17. What is required on the effluent filter outlet of a septic tank?
ANSWER ✓ An accessible, watertight inspection port or riser to allow for maintenance
and cleaning of the filter.
18. When installing a system in expansive clay soils, what special precaution might
the design specify for the piping?
ANSWER ✓ Use of flexible couplings or sleeving to allow for movement and prevent
pipe shear.
Based on TCEQ Rules Title 30, Chapter 285, and Current Design Standards
1. What is the minimum vertical separation required between the bottom of a
subsurface trench and a seasonal high-water table in a Type I system?
ANSWER ✓ 12 inches.
2. According to TCEQ rules, what is the minimum horizontal distance required
between a septic tank and a water supply well?
ANSWER ✓ 50 feet.
3. When installing an aerobic treatment unit (ATU), what component must be
installed to permit measurement of the effluent clarity?
ANSWER ✓ A sampling port located after the pump tank or prior to final
treatment/dispersal.
4. What is the required minimum liquid capacity for a septic tank serving a three-
bedroom residence?
ANSWER ✓ 750 gallons.
5. What class of OSSF Installer license is required to supervise the installation of an
ATU with a daily flow design of 5,000 gallons per day?
ANSWER ✓ Installer II (or Installer III).
6. When is a fill system (Type VI) required instead of a conventional trench (Type
I)?
ANSWER ✓ When the naturally occurring soil has a percolation rate faster than 3
minutes per inch or slower than 60 minutes per inch, or when the vertical separation to
a limiting condition is less than 12 inches.
7. What is the purpose of a pump tank in a pressure-dosed system?
ANSWER ✓ To provide storage for effluent, house the pump(s), and allow for controlled,
timed dosing of the dispersal area to improve distribution and resting cycles.
8. What is the minimum horizontal separation required between a spray dispersal
area and a property line?
ANSWER ✓ 10 feet for a residential system; larger distances apply for non-residential or
based on local rules.
, 9. When installing aggregate in a trench, what size gravel is typically specified?
ANSWER ✓ Clean, washed gravel ranging from 0.75 inches to 2.5 inches in diameter.
10. What critical step must be completed and documented before an OSSF
installation can begin?
ANSWER ✓ A site evaluation conducted by a licensed Site Evaluator, resulting in a site
evaluation report and design.
11. What is the required minimum depth of cover over the distribution lines in a
standard trench system?
ANSWER ✓ 6 inches of soil cover over the gravel/distribution media.
12. A four-inch perforated pipe in a trench must be laid with the perforations
facing which direction?
ANSWER ✓ Downward at an angle between 4 and 8 o'clock positions to minimize
sediment entry and promote even distribution.
13. What component is required on the inlet pipe of a septic tank to prevent
disturbance of the scum layer?
ANSWER ✓ A sanitary tee (or baffle).
14. What is the minimum horizontal distance from a building foundation to a
septic tank?
ANSWER ✓ 5 feet.
15. When using chambers instead of gravel in a trench, what is the primary
function of the chamber?
ANSWER ✓ To provide a void space for effluent storage and infiltration into the native
soil while providing structural support.
16. How must a septic tank be leveled during installation?
ANSWER ✓ The tank must be level from end to end and side to side, as per
manufacturer's specifications, typically within 1/4-inch.
17. What is required on the effluent filter outlet of a septic tank?
ANSWER ✓ An accessible, watertight inspection port or riser to allow for maintenance
and cleaning of the filter.
18. When installing a system in expansive clay soils, what special precaution might
the design specify for the piping?
ANSWER ✓ Use of flexible couplings or sleeving to allow for movement and prevent
pipe shear.
