How Deep Should Fence Posts Be for a 6ft Fence?

As a rule of thumb, a fence post should go about a quarter to a third of its length into the ground. For a standard 6ft (1.8m) fence that means around 600mm below ground, and 750mm or more on exposed, windy boundaries or soft ground. Gate posts need to go deeper than fence posts because they carry the gate's weight every time it swings.

Fence post depth by fence height

These are sensible minimum depths for timber or concrete posts set in concrete, on ordinary firm ground:

• 0.9m to 1.2m fence: about 450mm to 500mm in the ground.

• 1.5m fence: about 500mm to 600mm.

• 1.8m (6ft) fence: about 600mm, more if exposed.

• 2.0m fence: about 600mm to 750mm.

• Gate posts: 600mm to 750mm or more, with a larger post section.

Wind is what pulls fences over, and a tall solid fence acts like a sail. The more exposed the boundary and the more solid the fence, the deeper the post needs to go. Open styles such as a slatted fence let some wind through and put less load on the posts. On exposed plots, we often go deeper than the minimum as a matter of course when we put up fencing and gates in Kettering.

How long should the post be?

Work out the height above ground, then add the depth. Remember to include the gravel board.

For a 1.8m fence with a 150mm gravel board, the fence stands about 1.95m tall. Add 600mm in the ground and you need a post of about 2.55m, so in practice a 2.7m post. Without a gravel board, a 2.4m post gives 1.8m above ground and 600mm below.

Buying posts too short is one of the most common reasons DIY fences lean within a couple of years.

How wide should the post hole be?

A good guide is a hole about three times the width of the post. For a 100mm square post, that's roughly 300mm across. It gives enough concrete around the post to resist leverage without wasting material.

Put 100mm or so of gravel or hardcore in the bottom of the hole before the post goes in. It lets water drain away from the end grain of a timber post, which is where rot starts.

Concrete or postcrete?

Both work if the post is deep enough.

Postcrete is a rapid-setting dry mix you pour into the hole around the post and wet with water. It sets in minutes, which makes it quick for single posts and repairs. It's usually more expensive per post.

Mixed concrete is cheaper in bulk, and on a long run of posts it's what most fencers use. It takes longer to set, so posts need propping plumb while it cures, and you shouldn't hang heavy panels or a gate until it has hardened.

Either way, finish the top of the concrete just above ground level and slope it away from the post, so rain runs off instead of pooling against the timber.

Concrete fence posts vs timber posts

Concrete posts don't rot, so the depth you set them at is the main thing that decides how long they stay upright. Slotted concrete posts take panels and gravel boards straight in, and mortised concrete posts take the rails of a close board fence.

Timber posts are easier to fix to and look better with a timber fence, but the section at ground level is where they eventually fail. Pressure-treated posts rated for ground contact, a gravel base for drainage and a sloped concrete top all extend their life.

Metal post spikes that are driven into the ground are quick, but they aren't a substitute for a concreted post on a tall fence in exposed ground.

What changes the depth

• Soil type. Soft, sandy or recently disturbed ground needs deeper posts. Firm clay holds well once concreted.

• Exposure. Open fields, hilltops and gaps between buildings funnel wind onto a fence.

• Fence style. Solid fences take more load than slatted or open ones.

• Obstacles. Roots, old foundations and drains can stop you reaching full depth. Move the post position rather than settle for a shallow post, and always check for buried services before digging.

Fence post questions, answered

Can fence posts be too deep?

Not really, but beyond about a third of the post length you're spending more on post and concrete for little extra strength.

How long does concrete need before fixing panels?

Postcrete is usually firm within minutes to an hour, depending on the product. Mixed concrete needs at least a day, and longer before hanging a gate.

Why does my fence lean after a storm?

Usually the posts were too shallow, the concrete too small, or a timber post has rotted at ground level.

Setting posts properly is most of the work in a fence that lasts. It's how we build every fence and gate, and the same approach goes into the footings for decking. Get in touch if your fence needs replacing.