How to Install Posts for Automatic Gates
Posts for single swing gates need to be 4x stronger than those for a double swing gate because of the extra force that can be applied.
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 galvanised steel, 100 x 75mm season hardwood, 300mm core filled brick of 200mm square core filled concrete block posts for most gates up to 6m wide x 1.8m high. For larger gates to 12m long x 2m high a 100mm square galvanised steel, 150mm square seasoned hardwood, 400mm 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 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 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 wet, sandy or soft the post must go much deeper preferably into drier and harder ground below. If the ground is still soft 1m down then make the hole larger at the bottom and add lots of concrete. The top must have a wider area of concrete 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
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 to 15mm of material to the post, which can make a difference to measurements. For Sliding Gates it is a lot less critical.