A below-grade concrete floor slab rests on documented well-drained sand-gravel soil classified as Group I in 2021 IRC Table R405.1. Under R506.2.2, what is the finding on omission of the usual 4-inch base course?
- AThe base is mandatory with no exception
- BThe base may be omitted only for vehicle-use slabs
- The stated soil condition qualifies for the base-course exception
- DA vapor retarder substitutes for the base course in every soil
Where to look it up
https://core-docs.s3.amazonaws.com/documents/asset/uploaded_file/3836/Lennox/3042182/2021_International_Residential_Code.pdf
Why C is correct
R506.2.2 generally specifies a 4-inch base course on prepared subgrade where the slab is below grade. It excepts the stated well-drained or Group I sand-gravel soil condition. That exception does not automatically prove every site’s bearing capacity or waive a separately applicable vapor retarder, slab thickness or structural requirement. The base course and vapor retarder are separate provisions; one is not a universal substitute for the other. See printed p.5-25. Source: https://core-docs.s3.amazonaws.com/documents/asset/uploaded_file/3836/Lennox/3042182/2021_International_Residential_Code.pdf