The local frost line is 42 inches. The code requires water service pipe to be installed not less than 12 inches deep and not less than 6 inches below the frost line. What is the minimum depth of cover?
- A42 inches of cover
Forty-two inches places the pipe exactly at the frost line with no margin, ignoring the additional 6 inches the requirement adds below it. A pipe at the frost line freezes in a hard winter.
- B12 inches of cover
Twelve inches is the absolute minimum that applies where frost is not a factor. Applying it in a frost region ignores the requirement that actually governs here.
- C54 inches of cover
Fifty-four inches comes from adding the 12-inch absolute minimum to the frost line, treating two independent minimums as if they stack. They do not; the deeper of the two applies.
- 48 inches of cover
Why D is correct
Where two minimum depths apply, the greater controls. The absolute floor is 12 inches of cover, and the frost requirement is the frost line plus 6 inches, which here is 42 plus 6 equals 48 inches. Forty-eight is greater than 12, so the service is buried with not less than 48 inches of cover. In a warm climate with no meaningful frost line, the 12-inch minimum would be the governing number instead.
What this question is testing
The item tests the ability to apply two simultaneous minimums correctly, taking the greater rather than adding them, and to work with a locally published value, the frost line, as given data. It is the same logic used whenever two code minimums bear on one dimension.
On the job
Frost depth is local and it is published by the jurisdiction, not calculated. Call the building department or check the local amendment before you price excavation, because in northern towns the difference between a 4-foot and a 6-foot trench changes the shoring, the spoil handling, and the machine you need. Also keep the service clear of the sewer trench and maintain the required horizontal separation between the two.
Memory technique
Frost line plus six, but never less than twelve; take the deeper number.
Exam tip
When two minimums apply to the same dimension, the deeper or larger one governs; never add them together.
Where to look it up
Chapter 3, protection of piping against freezing, plus the water service installation provisions in Chapter 6. Frost depth is set locally and the UPC states burial requirements differently, so confirm the depth with the authority having jurisdiction and the code your state adopted.