Why Is My Composite Deck Warping? 4 Common Problems and Solutions

One of the benefits of composite decking is that its wood fiber and plastic composition are less prone to warping than wood. Whereas wood expands and contracts when exposed to moisture and temperature fluctuations, composite decking is more stable. However, because composite materials still experience some level of expansion in warm temperatures and after prolonged exposure to moisture, they can still warp. Are you wondering, “Why is my composite deck warping?” Here are a few common problems and how to fix them.

1. Thermal Expansion

While composite decking is generally stable, it may still be susceptible to thermal expansion, particularly when exposed to prolonged periods of direct sunlight and extreme heat. Thermal expansion is more likely to cause issues if the decking is in contact with a fixed object, such as a wall or nearby boards, because the board lacks sufficient space to expand.

How to Fix It

The right solution depends on how badly it is warped. If only a few boards are warped, you can replace these yourself if you feel comfortable with basic carpentry projects. To replace a board:

1. Measure the area that needs to be replaced.

2. Remove the affected board.

3. Replace the areas you removed with a new board of the same material and color, taking care to provide adequate spacing from nearby boards and walls.

4. Secure the boards in place with fasteners, following the manufacturer's installation instructions.

For severe warping throughout the deck, call a professional, as you may require more substantial repairs. In some cases, the whole structure may need to be replaced.

2. Poor Material Quality

Low-grade composite decking is far more susceptible to warping than higher-quality materials. It is more prone to moisture absorption, thermal expansion, and buckling from inadequate support. If you purchased bottom-of-the-line composite decking or your deck installer used low-quality materials without informing you, this could be why your decking is warping.

How to Fix It

If only a few boards are warped, you may be able to replace them using the method above. However, in most cases, those with low-quality decking should contact a professional to see if the damage should be repaired or if the whole deck needs to be replaced with a new structure made from higher-quality materials.

3. Improper Installation

Many installation errors can cause composite decking to warp. Examples include:

• Boards spaced less than 8 mm apart so they cannot naturally expand in high temperatures or allow for proper drainage, causing water pooling

• Boards placed within 15 mm of walls or other immovable objects, limiting their expansion

• Incorrectly spaced joists or support beams that do not evenly distribute or support weight on the structure

• Inadequate ventilation under the deck that traps moisture, causing swelling  and mold or mildew buildup that can damage the deck frame

• Overly tightened screws that prevent the natural expansion of the composite material

How to Fix It

In most cases, these structural issues require the expertise of a professional. If the deck is still new, it may still be protected by a craftsmanship or manufacturer warranty. Ask your installer for details.

4. Moisture Damage

Composite decking is more moisture-resistant when it comes to composite decking versus wood, but it is not entirely impervious to moisture and may incur moisture damage. If the boards absorb enough moisture, they can warp over time. This problem is more likely in decks made from low-quality materials or those with poor drainage, leaving them prone to standing water.

How to Fix It

If you only have a handful of warped boards, start by replacing any damaged boards using the previously mentioned technique. Take care to ensure any new boards you install have sufficient space to allow for ventilation and drainage.

If the damage is more extensive, you may need to hire a professional to assess the damage and recommend the best remediation option.

When to Call a Pro

You should call a local deck builder whenever your composite deck has warping on more than a few individual boards or on support beams or joists. The deck builder can inspect the damage and advise you on whether the cost to build a deck would be lower than your deck repair costs or if your deck needs to be replaced due to safety concerns. If you don’t feel comfortable doing any repairs on your own, you may want to hire an expert to replace even a single warped board.

How to Prevent Warping in Composite Decking

The best way to prevent composite decking from warping is to use high-quality materials and an experienced deck builder at the start of your project. Proper installation and high-quality materials are far less likely to warp, reduce the amount of necessary maintenance, and significantly increase the longevity of your deck. A good installer can even recommend the best material for your location, as some composite decking options offer advanced UV or moisture protection.

If you install your deck in an area with high temperatures and a lot of direct, prolonged sunlight, consider installing a sunshade or awning to reduce the amount of UV exposure it receives.

Finally, keep a regular maintenance schedule to prevent warping. Regular cleaning can help prevent moisture buildup by removing dirt and debris that can trap humidity and rain.