Timber posts almost never fail in the middle. They fail in a band a few centimetres either side of ground level, where the wood is alternately wet and dry and where oxygen, moisture and fungi all meet. A post support exists to move that band out of the equation by holding the timber clear of the soil on a steel foot. It is a small component that decides how long the whole structure lasts.
Why ground contact is the problem
Buried end grain behaves like a wick. It draws water up into the post, and the zone just above the soil stays damp long enough for decay to establish while still getting enough air to keep it going. Deeper down, waterlogged soil is often too oxygen-poor for rot to progress quickly, which is why a failed post so often snaps at the surface while the buried end still looks sound.
Preservative treatment slows this down but does not remove it, particularly where a post has been cut to length on site and the cut face is no longer treated. Lifting the timber out of contact is the more durable answer.
Bolt down against drive in
A bolt down support fixes to an existing concrete pad, slab or masonry with anchors. Use it where a base already exists or where the position has to be exact, such as a deck frame or a carport tied to a building. It is the more precise option and the easier one to replace later.
A drive in support has a long spike driven straight into firm ground. It avoids digging and concreting entirely, which makes it the fast choice for fencing runs in reasonable soil. It needs ground that will actually hold: soft, made-up or waterlogged ground will not develop the resistance the spike relies on, and stony ground can deflect it out of plumb.
Adjustable supports add a screw adjustment so the post height can be set after the base is fixed. On an uneven site or a deck being levelled across a slope, that adjustment saves a great deal of packing and re-cutting.
Matching the support to the job
Fencing: drive in supports where the ground allows, bolt down where it does not or where a wall is in the way. Consistency along a run matters more than the individual choice.
Decking and pergolas: bolt down onto pads, with adjustable supports where levels need trimming. These structures carry people and wind load, so the connection back to the base is doing real work.
Gates and heavily loaded posts: these see repeated dynamic load rather than steady load, so the fixing into the base matters as much as the support itself.
Size the support to the post section, not by eye. A post that rattles in an oversized socket transfers load through the fixings instead of through bearing.
Finish and fixings
Anything in ground contact or exposed to weather should be hot dip galvanized to DIN EN 1461. The coating has to survive being driven into soil or bolted down and then left outdoors for years, and a thin decorative finish will not.
Use fixings of a matching specification. Mixing metals in a wet outdoor joint sets up galvanic corrosion, and the fixings are usually the smallest part of the cost and the first thing to fail.
Common questions
No. That is the point of the drive in pattern: the spike develops its resistance from the surrounding ground. It does need ground firm enough to hold, so soft, made up or waterlogged ground calls for a bolt down support on a pad instead.
Because it keeps the timber out of contact with soil and standing water. Posts decay in the band around ground level where wood is alternately wet and dry; removing that contact removes the condition decay needs.
On uneven ground, or wherever levels have to be trimmed after the bases are fixed, such as a deck built across a slope. It replaces packing and re-cutting with a screw adjustment.