Your contractor didn't treat the cut ends of pressure-treated sills. What code and AWPA M4 say

A homeowner raised the question on Reddit's r/Carpentry: a contractor replaced rotted sills, studs and a rim joist with pressure-treated lumber and left the cut ends bare because the wood was already treated. IRC Section R317.1.1 settles it: cut ends, notches and holes drilled on site in treated wood have to get a field treatment that follows AWPA M4. The same section requires treated or naturally durable wood for framing that rests on a concrete or masonry foundation wall less than 8 inches above exposed ground.

AWPA explains the reason on its site. Sapwood absorbs preservative in varying amounts, while heartwood, juvenile wood and the wood around knots take up only a little, so a cut can open a weak spot. For interior uses the association names borates such as Bora-Care or a saturated borax solution. For exterior wood above ground it names oxine copper, and for ground contact copper naphthenate, sold as Copper Green, CopperCoat or QNAP. AWPA classifies sill plates as a UC2, interior damp, application.

Viance, which makes preservatives for treated lumber, says M4 calls for field treatment of every cut, hole or abrasion that may go past the treated zone, and that a deck stain or sealer is no substitute. The company also says field treatment is a condition of warranty coverage for most preservative makers. The copper naphthenate used for this contains 2% copper and goes on bright green before weathering to light brown.

The same contractor skipped priming cut ends on new cedar siding and caulked joints instead of flashing them. Western Red Cedar Lumber Association guidance says siding works best finished on all six sides before it goes up, and that end cuts should always get primer or stain. The same guidance rejects caulk as a substitute for flashing and warns against relying on caulk alone to keep water out.