DEAR TIM: I am thinking of building a deck and am leaning towards composite decking material for the decking, railings, and other accessories. I am wondering about positive and negative feedback you may have heard on these products. Are there any other things I should be concerned about regarding the construction and long-term maintenance of the materials? Joe R., Jefferson County, CO

DEAR JOE: Outdoor deck components including decking, railings, spindles, post wraps and caps, etc. that are made using a mixture of plastic and wood or other cellulose material are a rapidly growing building products category.

You were possibly wooed by the extensive amount of advertising that this industry is spewing out to consumers and contractors alike. Some of this advertising states that the composite decking materials will not rot. These particular claims made by some of the manufacturers, though, may come back to haunt them.

These composite decking materials first started to appear in the early 1990's, and one of the first manufacturers intended for its products to be very environmentally friendly. They made their composite decking material by combining recycled plastic milk cartons and discarded shipping pallets. Many of the other competing products are now made using virgin plastic and cellulose fibers or flour and/or a blend of virgin products with recycled materials. Some are still made with 100 percent recycled materials.

Depending upon whom you talk to, you will get both positive and negative comments about composite decking. Some homeowners on the East Coast were so unhappy with one of the brands of composite decking, they filed a law suit in a state court. Somehow this case was certified as a class action in that one state. Rather than fight expensive future legal battles in a number of different states, the decking manufacturer decided to agree to a national settlement. By doing this, any evidence or lack thereof concerning the plaintiff's claims was never disclosed in a public courtroom.

Several renowned scientists have also discussed composite decking materials in three separate professional white papers published between September 2001 and December 2002 in the Forest Products Journal. The findings in these three papers indicated that the wood fibers and other cellulose products used in the composite decking products they tested can and does rot if they're not treated with a preservative.

At first blush, this may not seem like a big issue; the average person might think that the wood or cellulose content of composite materials is low. But it appears many of the products have a cellulose and/or wood fiber content of nearly 50% of the volume of the product and in some it can climb to nearly 70%. Laboratory tests have shown that some of the composite decking materials can lose between 10 - 20% of their overall weight over time, which translates to a possible 40% or more loss of wood content due to rot.

In an effort to get to the bottom of the issue, I decided to interview 15 of the most prominent composite decking manufacturers. I sent them a series of written questions asking about their products, whether their raw ingredients are virgin or recycled, whether they were aware of the Forest Products Journal white papers, whether they use a preservative in their products and a few other generic questions. Only
one company of the 15 responded, and they provided concise written answers to my questions. I found the silence of the other 14 manufacturers to be very disturbing.

Further investigation revealed there is at least one preservative that can and is incorporated into at least one of the composite decking materials. This simple chemical - zinc borate - can be blended with the wood or cellulose component as the decking is manufactured. The borate in the preservative acts as a poison to many fungi that typically would consume the wood fiber and produce wood rot. The zinc borate preservative is also long-lasting. It can remain active in the composite decking materials for 20 or more years.

Most untreated lumber exposed to the elements rots over time. Don't think for a moment that the wood or cellulose fibers in the composite decking materials are totally surround and protected by the plastic component. That is not the case. The wood and/or cellulose fibers not only can be readily seen at the surface of the products and at all cut edges, they are randomly interconnected throughout the entire length, width and depth of each board. Water can and does soak into many of the composite decking materials and this water fuels the wood rot process in those materials that do not contain a preservative.

If you are going to use composite decking material, you should consider buying one that contains preservatives. Make the manufacturer prove it to you in writing. Don't count on the warranty as being proof that a preservative is used. More importantly, be sure to follow the written installation instructions to the letter. Creating gaps between decking boards, spacing of other components and support joist placement are critical. If you fail to install the materials correctly, you or your builder may void the warranty.

Important News:

August 19, 2005 -

The U.S. Product Safety Commission has issued a Recall for certain Geo-Deck™ Decking and Railing Materials.

As of this date, they have received 370 reports of accelerated degradation. The composite material can fail and create a fall hazard to consumers.

This is a very serious matter.

Click here to read the full text of the recall notice.