How to Waterproof Under Composite Decking Boards

M. John

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, under the decking may not be suitable for you 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 allows 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. But 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 store things beneath the decking as well as do 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 decking material. The water is channelled away, thereby keeping the under of the decking dry.

The drawback of the under decking ceiling system 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. In addition, the under decking drainage system is designed to divert water away from the decking joist and beam. These prevent the wooden joist 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. The steepers will raised 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.


You can waterproof under composite decking board if you use the under-decking drainage system. Additionally, you can use decking flanges, decking sleepers and interlocking systems to waterproof under composite decking.

Tags: waterproof decking

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