How to Waterproof Under Composite Decking Boards

M. John
Learn Decking
How to Waterproof Under Composite Decking Boards

Taking the time and effort to add waterproofing to the substructure and frame of your composite decking area will extend its service life.

The spaces between your ultra decking allow for easy water drainage on the decking. It allows water to drip, thereby keeping the decking dry. But, the decking may not be suitable if you want to do anything under the composite boards. To keep underneath your decking dry, you have to prevent water from dripping through the spaces. How can you waterproof under composite decking boards?

Why You Should Waterproof Under Composite Decking Boards

Waterproofing under composite decking boards can prolong the lifespan of the decking joists. The gaps left between the composite decking allow the easy drainage of water from the decking.

Ground level composite decking

It can be very useful to take the extra effort to waterproof your decking

This water can drip on the wooden decking joists, and when this repeatedly happens over the years, it might weaken the joists. When you waterproof under the composite decking, the area beneath the deck will be dry. It will keep the joist from absorbing water as well as lengthen its lifespan.

Additionally, when the underside of the decking is dry, you can use the area beneath the decking for other things. You can use the underneath of your decking for storage or other activities.

Ways to Waterproof Under Composite Decking Boards

Composite Decking Flanges

First, you can waterproof under composite decking boards by using flanges. The gaps between the composite decking allow water to flow easily. To waterproof the underside of the composite board, use rubber flanges to fill the gaps. Applying flanges on the spaces between the decking will keep water from dripping under the composite decking boards. Some flanges are designed to serve as a mini waterway to flow toward the decking edges.

Under Deck Ceilings System 

A second way to waterproof under composite decking is by attaching a ceiling system beneath the decking. The ceiling keeps the underside of the decking dry by drawing water away from the decking. These ceiling systems often use vinyl or aluminium panels to dispose of the water that seeps through the decking gaps. The water is channelled away, thereby keeping the underside of the decking dry.

The drawback of under decking ceiling systems is that it can damage the wooden joist. It allows water to drip over the decking joist, which can weaken the joist. The joist will rot or decay over time since it’s exposed to water. Apart from that, the under-decking ceiling system prevents air from getting beneath the decking. It can trap moisture, thereby preventing air from drying the wet joist.

Interlocking decking systems

The interlocking decking system is another way to make your decking waterproof. Some of these systems have in-built water troughs that keep water away from the decking. When using the interlocking decking system, ensure that there is no gap between the system and the house. A gap between the system and the house can leave room for water damage.

Under-deck drainage systems

Furthermore, you can waterproof under composite decking with the under-decking drainage systems. The under-decking drainage consists of a method in which a waterproof gutter system is installed below the composite decking. It is usually done during the installation of the composite decking. The gutter system is installed above the decking joist and is designed to divert water away from the decking joists and beams. This prevents wooden joists from absorbing water or moisture while keeping the underside of the decking dry.

Roof Deck Sleepers

Lastly, you can waterproof under composite decking by making use of deck sleepers. Deck sleepers will raise the decking above the ground to keep water from beneath the decking. You can use a piece of wood to lift the decking from the ground. The wood should be waterproof so that it won’t decay over time. Additionally, this method allows air beneath the decking and thus keeps the decking away from water. You should also avoid using chemically-treated sleepers to raise the decking as it may not be suitable for use in residential areas.

Damp Proof Course

Many of the professional decking installers and landscape gardeners that regularly purchase Ultra Decking composite decking boards will place damp proof course (DPC) tape on to the top and bottom of the joists. Any water dripping through the decking board gaps simply bounces off the DPC tape preventing any water from penetrating the joists. In addition, some professional installers go further and source a colour match paint or sealant similar to the decking colour and add an extra layer of water protection over the top of the DPC tape.

Can You Submerge Composite Decking in Water?

Can You Submerge Composite Decking in Water? No. Ultra-Decking composite boards should not be fully immersed underwater. Ultra-Decking composite boards are water-resistant materials. Since composite decking can resist water, most people think that composite decking is also waterproof. There is a clear distinction between a water-resistant material and a waterproof material. You can build composite decking in regions with high levels of moisture. Some homeowners even install composite decking close to their swimming pools or hot tubs because of its water-resistant abilities.

Since composite decking is water-resistant, can you submerge composite decking in water? No. Composite decking is exceptionally durable and resistant to the elements. It is also resistant to water and moisture. Composite decking can survive in a moist environment. A capped composite decking will not swell or absorb water if you install it in a moist area. The plastic material in the composite decking gives it the ability to resist water. Water cannot easily penetrate a water-resistant material, but it can still absorb water when completely immersed in water.
You cannot submerge composite decking in water. Submerged means complete immersion of the composite board in the water. A composite board can only repel water but will absorb water if you place it inside water.

Why Composite Decking Boards Are Waterproof

When choosing decking materials, you have a few options that you can choose. One of the most well-known selling factors of Ultra Decking composite boards is their waterproofing. Unlike wood, composite decking has better water resistance, along with the issues related to moisture. But the question is, what makes composite decking boards waterproof? And why should you choose waterproof materials?

It is Part Plastic

One of the main reasons why composite decking materials are waterproof is because of their design and makeup. Composites, also known as WPC or wood-plastic composites, are materials made from both thermoplastics and wood fibres. People mistakenly group it with PVC, but vinyl decking is pure plastic without any wood parts. The process of producing composites involves extruding the components, melting them and forcing them to shape into decking boards.
Because of the melted and mixed wood and plastic, it no longer absorbs as much moisture, making it water and moisture-proof. WPC no longer experiences many issues associated with wood use, such as mould growth and rotting. Another problem with wood decking is warping due to moisture absorptions. While composites can still bend, it is not because of moisture and humidity. Instead, composites and other synthetic materials undergo thermal expansion.

2nd Generation Composite Decking is Capped/Shielded

Another feature that makes composite decking waterproof is because of its capping. Capping is exterior layers of thermoplastics, such as PVC, polyethene, or polyurethane, present in almost all new 2nd generation composite decking boards. While uncapped boards are waterproof by themselves, capped composites further protect the decking from problems aside from moisture.
If water and moisture are an issue, you will want to use capped composites over uncapped ones. However, there are various types of capped decking, according to how many sides have exterior capping. Most of the boards today either have three or four sides covered. Each one has its advantages and disadvantages. But the question is, which one is more waterproof?

Full-Capping – Full-sided capping has a polymer exterior on its four sides. Because of this, it has superior protection against moisture absorption. Composites with full capping are also reversible. Because of this, not only do they look beautiful on outdoor decks but above ground level floor decking as well. However, there are a few reports of swelling on the exposed ends on some low-quality boards. Since the board is fully coated, there are no other places for water to drain other than the ends. Full-capped composites are also generally more expensive.

Three-Sided Capping – Aside from fully-capped, there is also three-sided capping for composites. As far as durability goes, it is also resistant to water and moisture. But compared to full-sided capped composites, they can be slightly more prone to moisture absorption. However, since the underside of the boards is uncapped, it has better water drainage and is less prone to swelling due to moisture. They are also often cheaper than four-sided capped composites.

Why Choose Composite Decking?

When building decking, there are many reasons why you should go with composites. Many use it for its superior waterproofing. As mentioned, water can cause various problems with traditional decking materials. Wood is an excellent example of a material that can have issues regarding moisture. When exposed to moisture, it can warp, grow moulds, and rot. Composite decking is less likely to have these problems. Because of this, they are the superior choice if you want waterproof decking.
Another reason composites hold better than any other materials when it comes to moisture is that it is easier to maintain. Composite decking requires less effort and time to clean. Even if the water is stuck on the deck for a long time, it will just evaporate or drain. As for wood, you will have to dry it as soon as possible. Naturally, you still want to dry any spills on your decking as much as possible, whether it is wood or composite.
Another advantage of composite is that it does not need painting or sealing for it to be resistant to moisture. Because of this, you do not have to spend money on buying these products. The truth is, you can save more money when choosing composite decking due to its water resistance and low maintenance. A wooden deck may prove to be more expensive, especially with regular sealing, painting, and staining.


Can you submerge composite decking in water? The response to the question is No. Composite decking can repel rainwater that falls on it. When you submerge composite decking in water, it will absorb water. Although the capped composite board has high water resistance, it will still absorb water in a minimal amount if you submerged it in water.

You can waterproof under a composite decking board with the use of an under-decking drainage system, decking flanges, decking sleepers and interlocking systems.


Leave a Reply

Your email address will not be published.