The soils information for the lot classifies the ground as a well-drained sand and gravel mixture in Group I, and the crew intends to place the concrete floor slab directly on the prepared subgrade with no 4-inch course of clean graded stone beneath it. What should the inspector conclude?
- AA base course is required, because every slab on ground has to be placed over 4 inches of clean graded material with no exception available.
The 4-inch base course is the general rule and it carries an exception. Reading the requirement as absolute makes the inspector call for fill on a site where the code already accepts the natural soil as a capillary break.
- BA base course is required here, and the code allows it to be omitted only where the finished slab will carry vehicle traffic.
Vehicle traffic is not a variable anywhere in this section; the exception turns on the soil beneath the slab. Bringing in the loading is a garage-slab instinct that has nothing to do with why the base course is there.
- No base course is required, because the code waives it where the slab sits on well-drained ground or on Group I sand and gravel soils.
- DNo base course is required, because the vapor retarder placed under the slab serves exactly the same purpose as the gravel.
The base course and the vapor retarder do different jobs. The gravel breaks capillary rise and gives the retarder a level bed, while the sheet stops vapor diffusion, and neither one substitutes for the other.
Why C is correct
The base course requirement carries an exception where the slab is placed on well-drained ground or on sand and gravel mixture soils classified as Group I. With that classification documented for the lot, placing the slab on the prepared subgrade is compliant and the inspector records the basis.
What this question is testing
Whether the candidate can find and apply an exception tied to a named soil classification, and keeps the base course and the vapor retarder as separate requirements rather than interchangeable ones.
On the job
Sandy regions have entire subdivisions built without a stone base, and inspectors there field this question constantly. The professional habit is the same as with the drainage exemption: ask for the document that classifies the soil. When the classification exists the exception is legitimate and well established; when nobody can produce it, four inches of stone is far cheaper than the argument.
Memory technique
If the ground is already gravel, you do not need to buy gravel.
Exam tip
Group I soils turn up in two exceptions, foundation drains and slab base course; recognize the phrase.
Where to look it up
IRC Chapter 5, the concrete slab-on-ground provisions at R506.2, including the base course paragraph and its exception.