A common method for making a curved rim joist for a deck is to cut multiple kerfs in the back of treated 2×10 framing, to make it easier to bend the rim joist to the curve. After cutting the kerfs – or any kind of notch, hole, or end cut, for that matter – you should field-treat the cut or hole to re-establish an envelope of preservative protection, according to both common sense and building codes.
AWPA Standard M4 specifies copper naphthenate and oxine copper (also known as copper 8 quinolinolate) for exterior use. Copper naphthenate is suitable for field treatment of wood in ground-contact or above-ground exterior applications, whereas oxine copper is intended for exterior above-ground use only. Field treatment is also required for preservative-treated wood used in interior locations. For these applications, M4 specifies the use of an inorganic boron treatment that does not normally emit VOCs.