Where treatment is required
IBC 2304.12.1.2 covers wood framing members, including sheathing, that are in contact with exterior foundation walls and are less than 8 in (203 mm) from exposed earth. Those members must be naturally durable or preservative-treated. The 8 in figure is the same in IBC 2018, 2021 and 2024. There is no 4 in cutoff.
Sheathing is included
The trap is forgetting the words "including sheathing". Work every member against the 8 in line:
- Sill plate on the foundation wall, underside 7 in above exposed earth: under 8 in, so it must be naturally durable or treated.
- Wall sheathing lapping down the foundation to 5 in above exposed earth: also under 8 in, so it must be naturally durable or treated too.
Both are covered. Protecting only the lowest member ignores that the plate at 7 in is also within 8 in, and protecting only the plate ignores the sheathing.
Two equal options
The section says the member shall be "of naturally durable or preservative-treated wood". Naturally durable wood is an equal alternative, so a framer who says pressure-treated lumber is the only choice is wrong. Forgetting the naturally durable option is a listed common error.
Things that do not satisfy the section:
- Fire-retardant-treated wood, which addresses flame spread, not decay.
- Wood dried to 19% moisture content or less. Under 2303.1.9.2, 19% is the limit before treated wood is enclosed; it does not replace treatment.
- Untreated wood with a sill gasket under it. A gasket is not one of the two materials the section allows.
Treatment standards
IBC 2303.1.9 says lumber, timber, plywood, piles and poles supporting permanent structures that Section 2304.12 requires to be preservative-treated shall conform to AWPA U1 and M4. Do not mix it with the other standards in this area:
- ASTM E84 and UL 723: flame spread tests for fire-retardant-treated wood.
- ASTM E2768: an alternative FRTW test.
- ASTM A153 and ASTM B695: zinc coatings on fasteners.
- ASTM A653 G185: exterior connectors.