Rot, termites, and keeping wood alive

Wood outdoors in north central Florida is under pressure from two directions. Decay fungi need moisture, and this climate supplies it generously between the summer storm season and year-round humidity. Subterranean termites need moisture and soil contact, and the University of Florida's extension service records native subterranean termites occurring throughout Florida, so the exposure exists in every county in the state.

Neither of those is a reason not to build in wood. They are the reason preservative-treated lumber is the standard framing material here rather than an optional upgrade, and the reason a handful of construction details matter more in this climate than they do in a dry one.

How decay works

Decay fungi need four things: food, oxygen, a suitable temperature, and moisture. Wood supplies the first, air the second, and a Florida summer the third. Moisture is the only one anybody can control, which is why every meaningful durability decision on a deck is really a moisture decision.

Wood that gets wet and dries out is fine. Wood that stays wet is not. That distinction explains most deck failures here: the boards on top dry in the sun and last, while the joist ends packed against a wall, the post bases sitting on concrete, and the wood behind an unflashed ledger never dry and go first.

What preservative treatment does

Pressure treatment forces preservative into the wood so that fungi and insects cannot use it as food. It works, and it is, and it is the main reason a treated frame outlasts an untreated one by decades in this climate.

Two things it does not do. It does not make wood waterproof, so treated lumber still swells, shrinks, and weathers, and still benefits from being kept out of standing water. And it does not treat wood the preservative never reached, which brings us to the two failure points below.

Use categories: ground contact and above ground

Preservative-treated lumber is sold in use categories that describe where it belongs. Material rated for above-ground use carries a lighter treatment suitable for wood that stays clear of the soil and can dry out. Material rated for ground contact carries a heavier treatment for wood in or near soil, or in a location where it will stay damp.

Posts, anything set in or close to the ground, and stair stringers that touch the ground all need the ground contact rating. Using above-ground material at grade to save a little on the material line is the single most common reason a Florida deck frame is soft at the base after eight or ten years while the boards on top still look perfectly fine. It is also invisible on a finished deck, which is why it belongs on the quote in writing.

Cut ends

Preservative penetrates from the outside in. Every time a board is cut, the saw exposes wood in the middle that the treatment never fully reached, and that fresh face is often the one placed against a beam, a post, or the ground.

Field-applied preservative on cut ends before the piece is installed takes seconds and closes off the easiest route in. It is one of the clearest tells of a careful frame, and it is something you can ask about directly.

Termites and soil contact

Subterranean termites live in the soil and reach wood through it, which is why soil contact is the risk factor that matters most. A treated post correctly rated for ground contact is doing its job. An untreated timber landscape edging butted against a deck post, a stack of offcuts left under the deck, or cardboard and debris in the crawl space beneath it are all providing a route and a food source that the deck's own material was chosen to avoid.

Keeping the ground under and around a deck clear of untreated wood and cellulose debris is free, and it does more than most things that are not.

The details that keep wood dry

• Post bases that hold the end grain of a post clear of the concrete, rather than standing it in whatever water pools there.
• Flashing above a ledger, tucked behind the wall covering, so wall runoff passes over rather than behind it.
• Gaps between deck boards kept clear, because packed debris holds moisture against the wood underneath all year.
• Grade that falls away from under the deck, so the area beneath it is not a puddle after every storm.
• Downspouts that discharge away from the deck rather than under it.
• Airflow underneath, which is why a very low deck over damp ground is a harder structure to keep dry than a raised one.
• Corrosion-rated fasteners and connectors, since a failed connection is a structural problem even where the wood is sound.

Where to look on an existing deck

Rot announces itself in predictable places. The base of every post at the concrete line. Joist ends where they meet the ledger at the house wall. Stair stringers where they meet the ground. The underside of boards in permanently shaded spots. And anywhere the deck meets a planter, a step, or landscaping that keeps wood damp.

A screwdriver is the whole toolkit. If it pushes in, the wood has lost structure, and how far the problem extends is a question for somebody who can open it up. The repair page covers what usually turns out to be behind it and where the repair-versus-replace line sits.

What composite changes, and what it does not

A capped composite or PVC surface removes decay risk from the boards, because there is nothing in a cap for fungi to feed on and PVC has no wood content at all. What it does not change is the frame. The joists, beams, posts, and ledger under a composite deck are the same treated lumber with the same requirements, and a twenty-five year surface on a frame detailed carelessly is still a deck with a problem underneath it.