Posts for sliding gates need only be light duty because they do not bear the weight of the gate just stop it from falling over. They should be at least 65mm square (1.5") galvanised steel, 100mm (4") x 75mm (3") season hardwood, 300mm (12") core filled brick or 400mm (16") square core filled concrete block posts for most gates up to 4m (13') wide x 1.8m (6') high.  For larger gates to 6m (20") wide x 1.8m (6') high a 150mm (6") square galvanised steel, 300mm (12") seasoned hardwood, 600mm (24") core filled brick or concrete bock posts are recommended.

Installing a timber or metal gate post

If the ground is hard you need to dig a hole 100mm (4") wider than the post, then put a small amount of concrete mixed quite dry (less water than normal) into the hole followed by the post and more dry mixed concrete rammed around the post checking the plumb from both sides as you go with a spirit level. You can get bags of pre-mixed concrete from you hardware store that you just add water too if you don't already have any concrete on the site.

You won't need any temporary bracing for the post as the rammed in concrete mixed quite dry will hold it in place and won't shrink much as it sets. You just need to stop anyone from leaning on the post for a few hours until the concrete is hard enough. Allow a week or two for the concrete to harden up property before hanging a gate.

If the ground is very firm posts need only go 500mm (20") in the ground. If the ground is very wet, sandy or soft the post must go much deeper normally at 1.2m (4') or where ever the ground becomes drier and harder. If the ground is still soft 1.2m (4') down then make the hole larger at the bottom and add lots of concrete. The top must have a wider area of concrete too or be hard against a concrete driveway. Do not use drainage gravel around a gate post to drain it, as this can move around and cause problems.

Installing a Brick or Concrete Block Post 

Brick or Concrete block posts should be core filled with concrete and have reinforcing steel up the centre that ties in with reinforcing steel in a deep footing. It should NOT be made from solid bricks set on a shallow footing as the weight of the gate is more than likely to pull it over, crack it in the middle somewhere or it will move around mysteriously.

Finishing Gate Posts

Galvanized Steel Gate posts can be powder coated to match the finished gate before being installed or the post can be painting once installed.  For new posts that haven't been in the weather for a while use an etch primer first so the paint will stick.

Concrete block posts should be hard plastered before a swinging gate is measured up because this adds 10mm (3/16") to 13mm (1/2") of material to the post, which can make a difference to measurements.  For Sliding Gates it is a lot less critical.