Is Composite Decking Waterproof?

If you’re considering installing composite decking in a wet climate such as the UK, or around a wet area such as a pool, you’ll probably be wondering how well it deals with exposure to water.

So, is composite wood waterproof? The simple answer is no.

There is no type of decking that is fully waterproof unless it is made from purely synthetic materials. However, composite decking is one of the most water-resistant materials available and it is also very slip-resistant.

In this guide, we’ll take a look at just how well composite decking deals with water exposure and the advantages it has over other materials when it comes to water resistance.


What Is Composite Decking?

Composite wood decking is manufactured using a mixture of recycled plastic polymers, wood fibres, and bonding agents. A process called co-extrusion (Capped – Forma and Grande range) is used to combine and then melt these materials.

The materials are then passed through a ‘die’ which is a metal channel that gives shape to the material and forms it into composite decking boards.

As a combination of both plastic and wood is used to make composite it has many benefits when compared to other materials such as natural wood decking.

Composite deck boards are also very durable and are resistant to moisture, dampness, insects, mould, and rot. They also require very little maintenance and don’t need to be painted, stained or sealed.

The upfront cost of composite decking is higher than many other materials but due to its durability and longevity, it should save you money in the long term. It also has virtually no upkeep costs and only requires occasional cleaning to keep it looking its best.


Uncapped or Capped Composite Decking?

The two types of composite decking are uncapped and capped. Although both types are water-resistant capped composite decking has an extra layer of plastic polymer that gives it added protection. This makes it more durable and longer-lasting, and also increases its water resistance.

The main reason that capped decking boards are more water-resistant than uncapped decking boards is that the polymer shell prevents water from being absorbed into the core of the board.

Although uncapped composite decking is still water-resistant it does absorb water, which over a long period will eventually lead to some deterioration in the boards.

Capped composite decking is more expensive than uncapped but it may well be worth paying the higher price to increase the durability and lifespan of your composite deck boards.

Our Forma range of decking boards is made using an advanced co-extrusion process which gives them an incredibly durable outer layer and makes them even more resistant to water.

Capped boards also require even less maintenance than uncapped ones and dirt can easily be wiped away meaning they don’t need to be cleaned using a power washer.


The Benefits Of Water-Resistant Decking

You may be wondering why it’s so important for decking boards to be water-resistant. There are many reasons why this is the case. If the decking is exposed to water it will expand and contract until it begins to warp and lose its shape.

Over time, traditional wooden deck boards begin to break down due to moisture absorption and the resulting expansions and contractions loosen brackets, screws and nails.

Composite decking boards resist moisture exposure far more than natural wood boards meaning they don’t shrink or swell anywhere near as much. This is why it’s a good idea to choose composite decking boards for wet climates or use near water.



As well as being water-resistant, composite decking is also very slip resistant and is an ideal choice for decks around swimming pools or hot tubs. As composite decking has a textured finish it provides a better grip underfoot making it less slippery than other materials when wet.

Due to the mix of real wood fibres and synthetic materials used to make composite boards water evaporates from the surface very quickly and they usually dry within 30 minutes of exposure to water.

This means that composite decking is often the safest choice for your outdoor living space as deck boards that are slippery when wet can be very hazardous. This is especially important for countries with a high amount of rainfall such as the UK.



One of the greatest benefits of water-resistant composite decking is that it’s very resistant to rot. The plastic polymers that are used in its construction prevent moisture from penetrating the core of the deck boards and stop rot from taking hold.

Although uncapped composite deck boards are resistant to rot, capped composite deck boards are even more so. The extra layer of plastic polymer on capped boards prevents almost all moisture from coming into contact with the core.

This is one of the main reasons that composite decking has such good longevity and won’t need to be replaced for many years.

If you’re planning to install composite decking in an area where it will be exposed to water frequently, such as by a swimming pool, then it is definitely worth considering using capped decking instead of uncapped.


Mould and algae

Unlike natural wood boards, composite decking is resistant to mould and algae. As we mentioned earlier, composite decking tends to dry just 30 minutes after being exposed to water. This means that water doesn’t sit on the surface creating the conditions for mould and algae to form.

This not only improves the lifespan of the boards but also makes them very low-maintenance as you don’t need to regularly wash mould and algae away.

As with rot resistance, capped decking boards are also more resistant to mould and algae than uncapped boards. This is due to the fact that the polymer cap prevents the wood fibres in the boards from being exposed to moisture which can weaken resistance to mould, algae and mildew.


How Does Composite Decking Compare To Other Materials For Water Resistance?

As previously mentioned, there is no such thing as fully waterproof decking that’s not fully synthetic but some materials are far more water-resistant than others.

However, although there are decking materials that are as resistant to rot and mould as composite decking they tend to have other downsides when exposed to water.

So, let’s take a look at how the other most popular types of decking compare to composite when it comes to water resistance.


Wood decking

Traditional wood decking may be aesthetically pleasing but is probably the worst choice of decking for water resistance. It tends to absorb water easily especially in wet climates or in areas where water exposure occurs frequently.

When temperatures drop after a period of wet weather, moisture that has entered timber decking freezes and causes the boards to expand which can lead to cracking. The fact that water penetrates the cores of timber boards also makes them very prone to rotting, moulding and forming algae.

In order to try and mitigate the effects of water exposure wooden boards require a lot of maintenance and need to be treated, stained, painted and sealed regularly. This is not only time-consuming but also increases the cost of wooden boards throughout their lifespan.

Wooden decking is also notoriously slippery when it gets wet which can be hazardous if it’s used as a surround for swimming pools or hot tubs. There are several treatments that can be used to try and make it less slippery but again, these are costly and time-consuming.


UPVC decking

UPVC decking is made of a material called un-plasticised polyvinyl chloride. It is also often referred to as PVC decking or simply vinyl decking. It is a fully man-made material and is 100% plastic.

As it doesn’t contain any organic materials such as wood, it is fully water-resistant and doesn’t rot.

However, UPVC decking does get very slippery when it’s wet and is not a good choice for use in wet climates or around pools. The main reason for this is a lack of adequate drainage.

As it doesn’t absorb any water at all it means that water pools on the surface which makes UPVC decking very slippery. This can be reduced with good drainage and correct spacing between boards, but in heavy rainfall, this will still be an issue.

When compared to wood, UPVC decking is fairly resistant to mould, algae and mildew. But, if it isn’t cleaned properly and debris such as leaves, dirt and pollen are allowed to sit on UPVC boards then mould and mildew will start to develop.

This will lead to the boards becoming very slippery and will also increase the risk of the boards becoming stained or damaged. So, there is a price to pay for fully waterproof decking.


Aluminium decking

As aluminium is also made without the use of any organic materials it is very water-resistant. Like UPVC, aluminium decking doesn’t absorb any water and isn’t susceptible to rotting, which gives it a good amount of longevity.

Some aluminium decking designs can also be used to create a watertight surface so that you can use the space underneath for storage. The main issue with this is that by eliminating the gaps between the boards it is more difficult for water to be drained from the surface.

Aluminium decking can also become very slippery when exposed to water. In colder climates, such as the UK, this can become quite dangerous as ice can form on aluminium surfaces fairly easily when it’s wet.

This can create a very serious slip hazard which makes it one of the worst choices for outdoor areas that will be exposed to moisture regularly.



How much maintenance does composite decking need?

One of the main benefits of composite decking is that it needs very little maintenance. Unlike other decking materials such as wood, it doesn’t need to be painted or stained and doesn’t need to be cleaned often. We suggest lightly jet washing your composite deck once or twice each year, for lighter colours in the first generation composite range we would recommend 2-3 timers per annum. This will save you time and money in the long term.


How long does composite decking last?

Composite decking usually lasts between 25 and 30 years. Capped decking will last longer than uncapped decking due to the extra layer of plastic polymer that gives it added protection, durability and longevity.


Should I seal composite decking?

Composite decking doesn’t need to be sealed as it’s made from wood fibres and plastic polymers which make it resistant to water, rot and mould. However, sealing it can have some benefits.

If you seal composite boards it means you will have to clean them even less than usual. Although composite decking is already very low-maintenance this would reduce its maintenance requirements even further.


How often should I clean composite decking?

How often you should clean your decking varies depending on how much debris accumulates on your deck. However, you should clean it after it’s first installed and then around once every six months to keep it looking its best.

You can either clean it with hot, soapy water and a soft bristle brush or use a power washer.


Take A Look At Our Amazing Range Of Composite Decking Boards Today

Both our Clarity and Forma ranges of composite decking are highly resistant to water which gives them incredible durability and longevity. Our Clarity range comes with a 20-year warranty and our Forma range comes with a 25-year warranty.

We do this because we have absolute confidence in the quality of our boards and we know that they’ll be with you for years to come. So, if you’re ready to install high-quality composite decking boards in your outdoor space then order a free quote or sample today.

Gary is the founder and CEO of Ecoscape UK. Gary graduated from Liverpool JM University in 2007 with a 2:1 in Economics. A passion for innovation, design and sustainability, Gary has put to market numerous wpc products/systems, some of which have UK and European registered designs. Outside of work Gary enjoys spending time with his family, the great outdoors and watching Manchester United.


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