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UC4A vs UC4B pressure-treated posts: what to ask for in Florida

If a post is going straight into the ground, “pressure treated” is not enough information. The treatment rating decides how long it should last, and yards will quote different products unless you say which one you mean.

By Callum Weiss · Updated Nov 5, 2026

What UC4A and UC4B mean

Treated lumber is sorted into “use categories” (UC) by the American Wood Protection Association. Two matter for posts set in soil:

• UC4A: ground contact, general use. The common “ground contact” rating. Typical for fence posts and other parts that touch soil where replacement is not hard.
• UC4B: ground contact, heavy duty. A higher preservative loading for parts that are harder to replace or matter more structurally, or where the conditions are tougher.

Both are meant for soil contact. UC4B carries more preservative, so it normally costs more, and it is not stocked in every size at every yard.

Which one for Florida soil?

South Florida is hard on buried wood: warm, wet, often sandy or high-water-table ground, and active termites. That is a reason to take the rating seriously, not a rule that says one rating is always required. A few ways to decide:

1. Follow the spec. If a drawing, engineer, sign installer or permit names a rating, order that rating.
2. If the post is hard to replace or holds load (sign structures, pergola or deck posts, anything an inspector cares about), ask about UC4B.
3. If it is a light-duty, easily replaced post, UC4A may be what the job needs. Ask the supplier what they stock and what the treater recommends for your use.

What to put in your quote request

Write the line the way a yard can price it without a follow-up call:

That one line covers size, quantity, species, treatment rating, how the posts will be used and where they go. Missing any of those is the usual reason two quotes cannot be compared.

Check the end tag when it arrives

Treated lumber normally carries an end tag or stamp. It should show the preservative, the use category or retention, and the inspection agency. If the tag says UC4A and you ordered UC4B, sort it out before the posts are in the ground, not after.

Lifespan and warranties

How long a buried post lasts depends on the product, the soil, the water table and how it was installed, so nobody can promise a number for your site. Some treated products come with a limited warranty from the treater. Ask for the actual warranty document and read what it covers before you count on it.

Questions to ask the yard

• Is this UC4A or UC4B, and what is the preservative?
• Is the post Southern Yellow Pine or another species?
• Is it in stock or a special order, and how long does special order take?
• Can I see the end tag or the treater’s data sheet?
• Is there a warranty document from the treater?