Fence posts below ground level

15 Feb 2009
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United Kingdom
I need to have a 6' tall wooden fence extended. I will use 100x100 pressure treated wooden posts inserted into the ground and fixed with Postcrete or similar. The presence of tree roots makes the use of spikes impossible.

Many fencing suppliers and timber yards sell pressure treated posts that they say are suitable for use below ground. But they are not Use Class 4 certificated, which i understand is recommended for such use.

Is there a significant difference? Will UC4 posts last much longer? Are they worth the extra cost?

Thanks for any advice.
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I replaced my fencing about 10 years ago and I was advised if you can afford the extra cost use concrete posts. Wood will rot - treated or not. They have been in the ground now for 10 years and never moved.....the fence is as solid now as it was on day one. It does cost extra but will not need to be replaced whereas wood posts will probably last 10 years max then break att eh point they enter the ground.

I would suggest pricing up the wood posts and the concrete ones...
Thanks, Mark. I agree that concrete will be more durable. However, because this is an extension to a fence already in place, I want to use wooden posts to match.
I would argue the lifespan mentioned, they could be as low as ten years but they could easily last twenty or more, the standard of wood used, installation, local site weather conditions and existing ground conditions will all play their part. Haunching the concrete up above the ground level a little can help too.
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I agree with freddie, in particular the last part. Timber posts installed poorly with the concrete below ground level creates a lovely little reservoir right at the base of the post. This wet dry wet dry causes premature rotting. I have almost never seen a timber fence post break any other place than where it meets the ground.

The secret is to bring the concrete a fraction out of the ground and haunch it around the post to shed water.
Thanks for the tip about how to use concrete. But does anyone have advice or information about the 'added value' of posts that are UC4 certificated?
They should last longer in theory, maybe use one UC4 and one non UC4 and report back in twenty years with the results?
Neo has a point about keeping it dry at ground level as to rot it needs air and water. Something I did on the replacement timber columns on our house, is added a drip at the bottom of each one to divert any water away from the base.
Previously the old ones had been cemented in, but once the wood contracted the water ran in the gap and rotted the base completely.
However you have to weigh up the effort of doing that against the cost of replacing them. It was very expensive to replace our columns so I don't plan to do them again, but fence posts are less critical.

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