Which parts of a deck have to be treated or naturally decay resistant?

The 6 in zone

Any framing lumber within 6 in of the ground has to carry preservative treatment. That catches post bottoms, the underside of a low frame, stair stringer ends, and any blocking that ended up low. It is a simple rule with an ugly failure mode, because the parts it covers are the parts holding the deck up.

Cuts are the whole game

Factory treatment does not go all the way through. It is a treated shell around an untreated core, which is why incising exists at all: those dimples let the preservative deeper into species that resist it.

The moment you cut a treated post to length, you expose that untreated core, at the end grain, which is the thirstiest surface on the piece. And that is usually the end going closest to the ground. Field applied end cut preservative is a five dollar bottle and a brush, and it is the difference between a post that rots at the bottom in eight years and one that does not.

Cut the post from the top where you can, so the factory treated end stays at the bottom. When you cannot, treat the cut.

Wood never sits on ground contact concrete

Concrete is porous. A pier in the ground wicks moisture up, and a post standing on it drinks that moisture through its end grain continuously. The separation options the code names are all cheap:

• 0.05 mm (2 mil) polyethylene film
• Roll roofing
• A metal post base or saddle anchor, which also gets the post up off the surface where water sits
• A neoprene spacer

A proper post base is the best of these because it adds a lateral connection at the same time, and it keeps the post out of the puddle that forms on top of every pier.

Treated, cedar, and what each is good for

Cedar is a fine deck surface and a poor foundation material. The heartwood is naturally decay resistant. The pale sapwood in the same board is not.

Fasteners have to match the treatment

Modern preservatives are more corrosive to metal than the old ones were. Two consequences that come up constantly:

• Use hot dipped galvanized or stainless fasteners, connectors and hangers, or whatever the treatment manufacturer names. Plain zinc plated screws will rust out of a treated joist.
• Do not use aluminum flashing against treated wood or galvanized fasteners. It corrodes, and it is exactly where you cannot see it happening.

Joist hangers matter twice over here, because a hanger also has to be installed with the specific fasteners its manufacturer names. Not the screws you have. The ones in the instructions.

Resisting the weather, not just the ground

The requirement is broader than the 6 in zone: the construction has to resist deterioration from exposure to the elements, through factory applied treatment or suitable site applied coatings or stains. In practice that means the framing is treated and the surface is finished.

A surface finish is not a code obligation you can point to on a cedar deck, and it is the difference between fifteen years and thirty. Cedar likes an oil once a year. Treated pine takes a stain every two or three. Composite takes a hose.

Handling treated lumber

Wear gloves and a dust mask when cutting it, cut it outdoors, wash your hands before eating, and never burn the offcuts. The manufacturer publishes handling guidance with the product and it is worth the two minutes.