Flame-retardant Decking

The recent Colorado wildfires have brought the fire-retardant nature of all building materials into question, especially decking. Decks are not only large structures attached to homes acting as a net for falling embers, but decks often extend into less manicured sections of properties making them more vulnerable to burning foliage. And since most decks are attached to the rim of the house, a burning deck provides access for flames to enter the home.

Although there are a number of tests to evaluate the fire performance of combustible materials, with regard to wildfire, two tests are used – flame spread index and heat release rate. The flame spread rating of a material is determined by the Steiner Tunnel Test, in which a material is subjected to flames inside a controlled environment (tunnel). The materials performance is assigned numeric value which gives it a rating of Class A (1-25), Class B (26-75), or Class C (76-200). A material rated as Class A would have a lower flame spread, and therefore a better performance rating, than a Class C material. Concrete, for example, carries a Class A rating.

Most building codes stipulate that decking materials have a heat release rate of 25 kW/sq-ft or less and conform to Class B flame spread standards. Even in fire hazard severity zones, including some jurisdictions in California, decking materials that meet these two criteria can be installed.

Which decking materials meet these two criteria? Many of today’s “low-maintenance” synthetic decking materials meet these criteria, as does con-common redwood. Which decking products perform the best? PVC decking products, like Timbertech XLM and Azek score very close to the Class A flame spread. Exotic hardwoods, such as Ipe, also score better than most decking materials on the flame spread index.

Here are some flame spread scores for a few of the different decking materials.

Ipe: 30 flame spread rating (results vary depending on test and type of exotic hardwood)
Timbertech XLM: 30 flame spread rating (Class B)
Azek: 40 flame spread rating
Trex Transcends: 60 flame spread rating
Timbertech Twin Finish: 70 flame spread rating
Redwood: 70 flame spread rating
Western Red Cedar: 70 flame spread rating
Timbertech Earthwood Evolutions: 85 flame spread rating
Evergrain composite: 90 flame spread rating
Fiberon Tropics: 115 flame spread rating

In the end, many of today’s more common decking products perform well in terms of flame spread index and heat release rate, minimizing the risk that a fire will start on a deck and extend into the home. Are they absolutely burn resistant? Of course not, but when combined with a defensible space around the home, they provide a reasonable defense against fires.

We can confirm that this combination works from experience. Several years ago we built a custom deck in the Mountain Shadows area and used Timbertech XLM decking with a wrought iron railing. The home also had stucco siding and a tiled roof. It was the only home to survive the Waldo Canyon Fire on this particular street. The surface of the deck had burn marks resembling dropped cigarettes buttes, yet the deck never caught fire.