Will timber posts rot in concrete?

In this article, we will be looking at whether timber posts will rot in concrete. We will also be looking at ways you can protect posts, to avoid rot, and ensure that they last for a long time.

Whether you’re fitting a fence, installing decking, or any other job that requires posts, the most effective way of securing them is in concrete.

This is a relatively simple process; you just need to dig a hole and install the post. Your hole should be roughly two foot deep, and you should also allow at least two inch around the perimeter of the post for filling with concrete.

Generally, you will add some gravel into the bottom of your hole, in order to provide a base with drainage for the post. Following this, you can add your post and fill with concrete. A popular option for the concrete mix is postmix. This is a premixed, fast setting concrete, designed specifically for this job.

Obviously, you should also ensure your post is level and in the correct position before adding concrete or postmix. But providing you get this right; the process is pretty simple. Once the post is in position and the concrete is added, it can simply be left to go off.

If you’re using postmix, it will usually be hard within around twenty minutes. With that said, it should be left for at least 4 hours before you start applying any weight to it.

Once the concrete goes off completely, it will have a very low moisture content. The curing process in concrete requires water. Therefore, the level of moisture reduces over time, as the chemical reaction continues.

Any water that soaks into the post, will usually wick up and be evaporated above ground level.

Concrete will not cause your fence post to rot. However, rot is quite common where the post enters the ground. This is because water can pool around the post, causing it to become saturated. If this happens, rot can occur.

Ways to stop rot in timber posts

If you are installing timber posts outdoors, you should be using treated timber. The good news is a decent pressure treated timber can last for 10-20 years outdoors, without any further protection.

However, in areas where there is a lot of moisture, this could deteriorate faster. A good example would be water pooling around the base of your posts. Once your post mix goes off, it will generally have a flat surface, where your post enters the ground. This means water can easily well at the base. The excess moisture can cause the wood to rot prematurely.

One simple way to avoid this, is to create a slope that runs water away from the post. This can be done by applying cement and tapering it away from the timber. Now when water runs down, it will hit the sloping cement, and be carried away from the wood.

This simple trick alone, could add years to the life of your timber posts.

Applying additional wood preservatives

Another good solution is adding additional treatment to the wood. This would be things like stains and preservatives.

If your posts are exposed (for example fence posts), then this is quite common, and will not only improve the appearance, but it will also add an additional layer of protection.

Generally, a good wood preserver will give 3-5 years of protection for your timber.

These types of preservative offer surface protection and do need to be re-applied every few years to get the best results.

If you are already using a pressure treated timber (which you should be), then adding additional preservatives means your timber will have very good protection.

Pressure treated timber already has a type of preservative running right through the wood. This is forced into the timber with immense pressure during the treatment process. As a result, it adds a treatment all the way through the timber. This is why pressure treated timber can last so much longer than an untreated alternative

However, even this pressure treatment will wear over time. By adding additional preservatives and stains at a surface level, you will protect the timber further, and potentially extend the life of the pressure treatment.


Concrete will not cause your timber posts to rot. In fact, the concrete will usually have a much lower moisture content than the wood itself.

Your main area of concern is directly above ground level, where your post enters the ground. This is an area that is most at risk when you install external posts, for things like fencing and decking.

As we mentioned, the best thing to do to avoid rot at the base of your posts is a simple two-step process:

  1. Direct water away from the bottom of the post. Simply tapering cement away from the post will achieve this quite easily.
  2.  Add additional preservative treatments. Not only will this add extra protection, but when it is visible, it will also be more aesthetically pleasing.