Synthetic deck collapse (C) Daniel Friedman Composite or Synthetic Deck Board Durability

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Composite & synthetic decking durability research results. Studies of the durability of composite lumber used for decks, ramps, and similar structures show the effects of moisture, freeze/thaw cycles, sunlight, temperature variation, and other factors that impact the appearance, durability, life & safety of plastic & synthetic decking.

Our page top photo shows the collapse of plastic-synthetic deck boards at a New York home while below we also include images of more contemporary plastic type deck boards.

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Durability of Synthetic or Composite Decks & Floors: Composite-Fiber Deck Collapse Photos

As our photos show, below, even though synthetic decking material may itself be durable, if the deck is not properly constructed there is risk of rot, and even collapse. At below left is a picture of more recent composite decking at a New York lumber supplier. This deck has been exposed outdoors continuously for several years. At page top and at below right we illustrate an older synthetic or composite deck board product that has failed.

Synthetic decking installation (C) Daniel Friedman 2013 Synthetic deck collapse (C) Daniel Friedman

While we knew that this entry deck at an upstate New York home was in bad shape, just how horrible it was was not completely apparent until demolition was begun during a home renovation.

The composite decking shown at left was installed directly over a pre-existing and rotted deck structure - not a smart idea.

This composite deck board photo shows that the deck actually collapsed during work on the structure, revealing rotted wood structure over which the deck boards had been added as a "repair". Luckily no one was injured.

Don't do this.

Research on the Durability of Composite & Synthetic Lumber Reports on Freeze/Thaw, Moisture & Temperature Effects

Reader Question

4/11/2014 Bill C said:

We are about to replace a front porch and we are considering using composite boards. do you know of any manufacturer that we should avoid because of poor results and durability?


Veranda composite decking warranty terms (C) InspectApediaBill, it's a topic worth some research, and indeed there have been a number of studies of the durability of composite lumber or "synthetic" lumber. Just above you can see illustrations of some early composite lumber that didn't fare so well.

Current suppliers of composite and plastic decking materials are listed along with their contact information at SYNTHETIC & COMPOSITE DECK SOURCES .

Experts from a variety of fields including the U.S. Forest Products Laboratory, the wood composite manufacturers, and the plastics industry have studied important synthetic-deck board and other composite lumber properties & durability including the structural strength, fire resistance, weathering, color fading, and other properties as well as studying the usefulness of protective coatings on these products.

A scan of recent research suggests that the effects of freeze-thaw cycles on composite lumber as well as effects of moisture, high temperature, and possibly sunlight seem to vary more by the mixture of composite ingredients than to point to a specific brand.

Pilarski and Matuana conducted some important research into the durability of wood-flour composite lumber in 2005 and 2006, listed in the citations below. Note this conclusion:

The loss in stiffness of the composites was statistically significant after only two freeze–thaw cycles, regardless of both the wood species and content. Conversely, the strength of the composites was not significantly affected by five freeze–thaw cycles at lower wood flour contents (50 and 75 phr). The deleterious effects of the freeze–thaw actions on the strength of the composites became apparent at higher wood flour content (100 phr) after only two freeze–thaw cycles for maple flour and five freeze–thaw cycles for pine flour. The property loss was attributed primarily to the water portion of the cycling, which appears to have led to the decreased interfacial adhesion between the wood flour and the rigid PVC matrix. - Pilarski (2005)

That we need to read studies of composite lumber performance with care and to make note of just what ingredients are in use is evident in a very different finding by Ronald Wolfe and Agron Gjinolli in a February 1999 USFPL research report.

A pilot study was conducted to assess the potential for using southern pine particles derived from construction waste to create a cement-wood composite suitable for exposed structural applications. Panels fabricated from copper chromium arsenate (CCA)-treated as well as untreated particles were cut into individual samples and tested for freeze-thaw durability, strength, and toughness. Results support the premise that these composites can be designed to meet the requirements for highway sound barriers. The results also show that these composites have energy-dissipating properties that could have special applications in structures where impact and dynamic load are a design consideration. - Wolfe (1999)

As a point of comparison with composite lumber typically used for deck or ramp boards, we report here as well on Green (2003) who examined the comparative effects of humidity and temperature on solid sawn lumber and (wood) composite lumber (LVL and LSL) used as structural members.

The effect of temperature on MOR of solid-sawn lumber is independent of grade. Composite lumber is more sensitive than solid-sawn to change in strength due to thermal degradation. The difference in MOR between species and product types may be less at low humidity levels than at high. The total effect of temperature on MOR can be estimated by adding the reversible plus the permanent effects. Available literature suggests that the wood used in attics of residential construction is not likely to experience significant accumulation of exposure at temperatures ≥66°C over the life of the structure. - Green (2003)

Finally you might want to take a look at Douglas Gardner (et al)'s research on the use of protective coatings for composite wood products, Gardner et als. (2001 & 2003) and at Stark & Gardner (2008) USFPL from whom we quote just below.

Wood - plastic composite (WPC) lumber is promoted as a low - maintenance, high - durability product (Clemons, 2002). However, after a decade of exterior use in the construction industry, questions have arisen regarding durability. These questions are based on documented evidence of failures in the field of WPC decking products due to such impacts as polymer degradation (Klyosov, 2005), wood decay (Moms and Cooper, 1998), and susceptibility to mold which negatively impact the aesthetic qualities of the product.

The industry has responded to problems associated with first - generation products by improving WPC formulations. Manufacturers have also made great strides in making more reasonable claims and in educating consumers on the proper care and main tenance of WPC products to maintain the aesthetic quality of the surface finish. Research groups throughout the world are working toward a fundamental understanding of WPC durability that will help improve and/or identify new strategies for protecting WPCs. WPC durability will continue to be an important subject regarding the use of these products in building construction and other related applications in the field. ...

The work summarized here suggests that controlling moisture is the key to not only decreasing losses in performance due to moisture absorption but also increasing weathering performance and resistance to biological attack. . - Stark (2008) 

Later work by Morrell, Stark et als (2011) continues this investigation. They conclude:

It is clear that the wood in WPCs must be protected from both biotic and abiotic damage; however, it is equally apparent that technologies are available to achieve this goal. As these products continue to evolve, we should expect to see increasingly durable materials that over - come the biological, moisture, and UV factors to produce materials that retain their appearance. - Morell (2011)

Webb (2011) writing in Pro Sales Magazine and later re-published online warns:

Wood and plastic typically make up roughly 85% to 95% of the ingredients’ total weight. Most of the wood consists of cast-offs from flooring factories. And except for virgin PVC flakes, all the plastic had a previous life as well. WPC and PVC manufacturers work constantly to get reliable streams of raw product, but if the wood flour has a different mix of species or the recycled plastic shipment was heavier on low-density polyethylene film, the manufacturer may need to adjust its formula.

... “It’s not a highly repeatable process,” says Tom Gramlich, chief operating officer of TimberTech, which makes composite and PVC decks, porches, railing, and trim. “Every night, our operator is fighting a different set of circumstances than the night before.”

... In fact, the testing protocols that companies use to justify their 25-year warranty claims are getting questioned.

... Well, since 2004, seven wood-plastic composite manufacturers and suppliers have faced lawsuits related to a host of problems, including fading and color changes, slippery surfaces, shrinkage, swelling, and mold. In at least two of those cases, a supplier and reseller were accused along with the manufacturer.

References & research on the durability of composite lumber, decking, trim

Deck & Porch Industry Associations


Continue reading at DECK FLOOR LAYOUT or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



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