Concrete-in post holders are commonly made for square timber posts of 70×70 mm, 90×90 mm and 100×100 mm, with U, L, H and T cradle profiles for different post positions. The cradle holds a set post width, so measure the actual post and match it to the holder's internal dimension. For gate and corner posts that carry side loads, choose a larger section and a deeper spike.

Set a concrete-in post holder deep enough that the spike is fully embedded in a solid footing, typically a hole around 300–600 mm deep depending on post height and ground conditions. Dig below the frost line, part-fill with concrete, push the holder in and adjust it level and plumb, then let the concrete fully cure before loading the post. Taller fence, pergola and gate posts need a deeper, wider footing to resist wind and side loads.

Concrete-in post holders are typically hot-dip galvanised steel, which resists corrosion where the spike is embedded in concrete and moisture is present. Hot-dip galvanising gives a thicker zinc layer than electro-galvanising and is preferred for permanent below-ground and in-concrete use. The exposed cradle that carries the timber also benefits from the galvanised coating, keeping fence, deck and pergola supports protected for many years.

Choose the cradle profile by the post's position: a U profile suits a single post held on three sides, an L profile suits a corner or edge post, an H profile clamps the post on two opposite faces, and a T profile suits a mid-run post where a rail or panel meets it. Match the profile to how the post is loaded so it is supported against the main direction of push. All profiles come in the standard 70×70, 90×90 and 100×100 mm post widths.