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Frequently Asked Questions
What does a post base actually do?
Two unrelated things - it anchors the post and it keeps it out of water.
Anchoring resists uplift and sideways movement.
Standoff keeps the end grain clear of standing water.
The second job is what decides service life.
Both have to be satisfied by the base chosen.
A base that anchors well but sits flush will still let the post rot.
Why does timber on concrete rot?
Concrete holds and releases moisture continuously.
End grain is the most absorbent surface on a post.
It wicks that moisture upward into the timber.
A post set flat on a slab rots from the bottom regardless of treatment.
The failure appears years later at the one point nobody can inspect.
By then the post usually has to be replaced rather than repaired.
How much standoff is needed?
More in exposed and wet locations, less where it is internal and dry.
An elevated base lets water drain away from the end grain.
It also lets air reach the end of the post.
Flush bases exist and are the wrong choice outdoors.
Splash height from the surrounding surface is worth considering.
Where the base will sit in a puddle, more standoff is always the safer specification.
Cast-in or post-installed?
Cast-in gives the better connection; post-installed gives flexibility.
Cast-in bases are set while the concrete is wet.
That demands accurate setting-out before the pour.
Post-installed anchors go into cured concrete.
They depend on embedment depth, edge distance and concrete strength.
A base near a slab edge is frequently limited by edge distance rather than the anchor.
What are adjustable bases for?
Correcting setting-out that was not perfect.
Concrete work rarely lands exactly where the drawing said.
An adjustable base lets the post move a little afterwards.
That matters on any run of posts that has to line up.
The adjustment is usually a slotted plate or a moving saddle.
It is worth having even where the setting-out is expected to be accurate.
What corrosion specification is needed?
Higher than almost anywhere else in this family.
The base lives at ground level in the wet.
It is frequently in treated timber and in contact with concrete.
Modern preservatives are aggressive to standard galvanising.
Fasteners must match the base's corrosion class.
Stainless is specified for coastal and permanently wet locations.
Does the base carry the post's weight?
It transfers it, and the bearing under it has to be adequate.
Downward load passes through the base into the concrete.
The concrete has to be able to receive it.
Uplift and lateral capacity are published separately.
Moment resistance is limited on most standard bases.
A post relying on its base to resist bending usually needs a different detail entirely.