Fence Post Foam vs Post Fixing Mix (Postcrete)

When installing a fence, one of the most important decisions is how to secure the posts. Traditionally, post-fixing mix (often referred to as Postcrete) has been the go-to option, but expanding fence post foam has emerged as a modern alternative. Each method has its own advantages and drawbacks, so which one should you choose? 

What is Fence Post Foam? 

Fence post foam is a two-part expanding polyurethane product that, when mixed, creates a lightweight but strong foundation for fence posts. It is designed to replace traditional concrete-based mixes, offering a quicker and easier installation process. 

Why is Fence Post Foam Gaining Popularity in the UK? 

While post foam has been widely used in countries like the US for some time, it is now becoming more popular in the UK. Many DIYers and professionals are drawn to its speed and convenience, especially for small-scale fencing projects where handling heavy bags of Postcrete can be a hassle. With more brands introducing foam products tailored for UK soil conditions, it is increasingly seen as a viable alternative to traditional concrete. However, its growing popularity does not mean it is suitable for every project. While foam provides a strong initial hold, questions remain about its long-term durability in the UK's wetter climate, especially in high-wind areas. 

Pros of Fence Post Foam 

• Fast setting - It expands and sets in minutes, allowing you to install posts quickly. 
• Lightweight and easy to handle - No heavy bags of mix to carry. 
• No water required - Ideal for remote locations or areas without access to water. 
• Works in various weather conditions - It can be used in cold or wet conditions where concrete might struggle. 
• Expands to fill gaps - It creates a secure hold by expanding into the hole. 

Cons of Fence Post Foam 

• More expensive than Postcrete - It generally costs more per post compared to traditional mixes. 
• Not as strong as concrete over time - While it provides a solid hold, foam can degrade faster than concrete, particularly in high-wind areas or where heavy loads are involved. 
• Difficult to adjust posts - Once expanded, the foam sets quickly, leaving little room for repositioning. 
• Can be less durable in wet conditions - Some foams are more susceptible to water absorption and degradation over time. 
• Can rise out of the hole - If too much foam is used or the post is not held firmly in place, the foam can expand above ground level, making the base of the post look unsightly. This excess foam often needs to be trimmed away once fully set. 
• Can force the post to move - Because foam expands as it sets, it can push the post out of alignment if not properly secured. This means installation methods need to be adjusted slightly compared to Postcrete. Posts should be held in place with braces or supports until the foam has fully expanded and set to ensure they remain straight. 
• Better suited for non-structural posts - Foam is fine for applications like verge protection posts, signage, or marker posts, as long as they are not subjected to force. However, fences put far more strain on the post due to wind loads and movement, meaning concrete is usually the better option. If a verge post secured with foam is hit, it may break away more easily than one set in concrete. 

What is Post Fixing Mix (Postcrete)? 

Postcrete is a pre-mixed, fast-setting concrete designed specifically for fixing fence posts. It is a dry blend of cement, sand, and aggregate that hardens when water is added. 

Pros of Postcrete 

• Stronger and more durable - It provides long-term stability, especially for fences in high-wind areas. 
• Cost-effective - It is generally cheaper than post foam, making it ideal for larger fencing projects. 
• Widely available - It is easily found in most DIY stores and builders’ merchants. ✔ Resistant to water and ground movement - Concrete is less likely to degrade over time compared to foam. 

Cons of Postcrete 

• Takes longer to set - It typically requires at least 10 to 15 minutes to harden, with full strength achieved in hours or days. 
• Heavy and difficult to transport - Bags of Postcrete can be bulky and require significant effort to mix and pour. 
• Requires water - Unlike foam, Postcrete must be mixed with water, which can be inconvenient in remote locations. 
• Can crack over time - In areas with heavy frost or movement in the ground, Postcrete can crack, reducing its effectiveness. 

Which Should You Choose, Foam or Post Mix? 

The best choice depends on your specific needs. 

For quick installations: Fence post foam is ideal if you need a rapid installation process and do not want to deal with heavy materials. 

For long-term durability: Postcrete is the better option if strength and longevity are priorities, particularly in windy or wet conditions. 

For non-structural posts: Foam is a good choice for verge protection posts or other markers that do not experience lateral force, though it may not hold up if struck. 

For fences and high-stress applications: Postcrete remains the stronger and more reliable option, especially where posts must withstand wind and movement over time. Read our blog on a fence exposed to strong winds or Windproof Fencing.

For ease of installation: Foam is lightweight and does not require water, but posts must be braced properly to prevent movement as it expands. 

For cost-effectiveness: Postcrete is the more budget-friendly choice, especially for larger projects. 

Key Installation Considerations

 If using foam, it is important to modify installation techniques compared to concrete: 

Brace the post firmly - Since foam expands rapidly, it can push the post out of position if not held in place. Use supports to maintain proper alignment. 

Use the correct amount - Overfilling the hole can cause excess foam to expand above ground level, requiring trimming. 

Work quickly - Once mixed, foam expands in seconds and begins to harden within minutes, leaving little time for adjustments. 

Check product suitability - Not all foams are created equal. Some are rated for higher load-bearing applications, while others are only suitable for light-duty posts. 

For traditional Postcrete installations, ensure that: 

The hole is deep enough - Typically, a third of the post's height should be below ground for stability. 

Water is added first - This allows the Postcrete to mix properly and set correctly. 

The mix is compacted - Avoid air pockets by ensuring the Postcrete fully surrounds the post base. 

Overall Fence Post Stability

Fence post foam is gaining popularity in the UK as an alternative to traditional Postcrete, particularly for those looking for a fast and convenient installation method. It is a great option for lightweight, non-structural posts, such as verge markers or signposts. However, for fencing, which places more strain on the post due to wind and movement, Postcrete remains the better choice for long-term strength and durability. 

One key consideration with foam is that it expands as it sets, which can push the post out of position if not properly braced. Additionally, excess foam can rise out of the hole, creating an untidy finish. While foam is useful in certain applications, those installing fences should weigh the benefits of speed against the need for long-term strength and adjust their installation methods accordingly.

The strength and durability of a fence rely heavily on the quality and proper installation of the fence posts. While the choice of post-fixing material, whether fence post foam or a traditional concrete mix, plays a role in stability, it is the post itself that forms the foundation of the entire structure.

A strong and long-lasting fence requires sturdy, well-chosen posts that suit the conditions they will be exposed to. The material, whether timber, metal or concrete must be capable of withstanding environmental pressures such as wind, moisture, and ground movement (read our blog on Concrete vs wooden fence posts). If the post is weak, prone to rot, or lacks sufficient load-bearing strength, no fixing material will compensate for its deficiencies.

Correct installation is just as important. The depth at which the post is set, its positioning, and how well it interacts with the surrounding ground all influence the fence’s overall stability. A post that is too shallow or poorly placed will cause instability, regardless of how secure the fixing material appears to be.

Other factors, such as soil type and drainage conditions, also impact the long-term integrity of the post. Even the best fixing methods cannot fully prevent issues like rotting in wooden posts or corrosion in metal ones if they are not properly treated and installed.

This is why choosing fence posts with long guarantees is so important. A manufacturer’s guarantee provides reassurance that the post is designed to last and has been tested for durability. High-quality timber posts should be pressure-treated to resist rot and decay, with some suppliers offering guarantees on wooden fence posts of 25 years or more. Metal posts should be galvanised or coated to prevent rust, with guarantees that reflect their expected lifespan.

A longer guarantee is not just a marketing promise. It is a reflection of the confidence a manufacturer has in the durability of their product. It also protects homeowners and businesses from unexpected repair or replacement costs. Investing in posts with a long guarantee ensures the fence remains strong for years, reducing the risk of failure and the need for costly maintenance. 

While post-fixing materials help anchor a post in place, they do not determine the overall strength of a fence. The durability and stability of the fence depend on the quality of the post itself, the way it is installed, and the assurance that it has been built to stand the test of time.