Composite Decking, Not Fire Resistant but Reduced Flame Spread


Nature and fire are two things many of us love about being outdoors. Something that can both be enjoyed from a backyard deck. A place to gather around a fire or sit back and soak up nature’s view.

Unfortunately, both pleasures come with risks. Risk of burning your deck. But can these be minimized by installing composite decking?

Composite decking is not fire-resistant. It will burn and become deformed if exposed to high levels of heat. Composite decking is designed to slow flame spread, with most having a class C flame rating, similar to wood decking, but some are rated as high as Class A, equivalent to concrete products.

The class rating of composite decking impacts you in a number of ways. The top two are, enjoying a fire on composite decking and building a deck in a Wildland Urban Interface areaOpens in a new tab..

Fire-resistance of the composite decking will determine what types of fire you can enjoy on your deck. From grills to fire pits, don’t forget fire tables. Decking fire resistance will limit the fire you enjoy. Included in this is the fire’s fuel source. Propane fires burn differently than wood fires, determining where each one is appropriate. The same can be said about charcoal grills.

Wildland Urban Interface is more of a legal and permitting question designed for your safety. If you ever watch a brush fire and the speed it spreads when the wind is blowing, you know what I mean. The fire may not be on your deck, but the flames can quickly jump from the brush to your deck, consuming your house in minutes if the wrong material is used.

Composite Decking and Fire

Before jumping into the fire, what is composite decking?

Composite decking is a wood alternative that has become extremely popular in today’s deck construction. There are different brands of compositeOpens in a new tab. materials, but most deckings are a combination of plastic and wood with a polymer cap. The core is roughly a 50/50 mix of recycled wood fibres and plastic.

Originally introduced to reduce maintenance of a deck but now also are being designed to minimize deck fires.

The difference in fire resistance between brands and decking lines has more to do with the addition of chemicals to the formula than the material composed of.

For instance, Azek has a Class A rating while Trex DeckingOpens in a new tab. has a Class C rating. Comparing brands is essential to select the right composite for your deck. Not all composite decking is rated the same. Many only match the fire rating of wood decking, but other is far superior.

Is Composite Decking Fire Resistant Enough to Have a Fire on Your Deck?

Are you asking for trouble for enjoying a fire on your composite deck?

This is a very good question.

“Residential fires start outside 4% of the time.1 Your grill, smoker, fire pit, and even dry vegetation can be sources of outdoor fires.”

SafeWiseOpens in a new tab.

You don’t want your home to become the 4% that get burned.

And despite higher ratings of some composite decking for flame spread does not mean that enjoying a fire pit on your deck is safe.

We need to differentiate between composite decking flame spread and fire resistance.

Flame spread is primarily a surface burning characteristic of materials, and a flame-spread rating is a way to compare how rapid flame spreads on the surface of one material compared to another.”

CWC

Fire Resistant: so resistant to fire that for a specified time and under conditions of a standard heat intensity, it will not fail structurally or allow transit of heat and will not permit the side away from the fire to become hotter than a specified temperature”

Merriam-Webster

Matt Risinger from the Build Show does an excellent job of running a few tests on composite decking revealing the level of fire resistance. He is comparing TimberTech composite decking, Azek PVC, cedar and Ipe.

So, the results may not be universal. Just like flame spread, manufacturers modify their composite to be more fire-resistant than TimberTech has done or not modify it, making it burn easier. But it does give us an idea of the fire resistance of composite decking.

Summary if you don’t want to watch the video. Matt does the following tests on the decking.

1: The Open Flame Test

  • Cedar quickly burned
  • Ipe was charred by the fire but remained structurally strong
  • TimberTech resisted till the cap melted, and then the core started on fire
  • Azek was severally damaged by the flames but didn’t start on fire

2: The Heat Retention Test

  • TimberTech (composite) became the hottest after being in the fire. Retaining the heat much longer than the other decking

3: The Consistent Flame Test

  • TimberTech and cedar quickly transferred the heat. Showing almost no stopping of heat transfer
  • Azek took about 30 seconds for the heat to transfer but than became hot on the opposite side of the flame.
  • Ipe, with its high density, did not transfer the heat from the flame to the other side of the decking

4: The Kindling Test

  • After 10 minutes in the fire, all four burned up. None of the current deckings is fireproof.

These results are not surprising. Trex also is very upfront about the effects of heat and fire on their composite decking.

“Trex decking will soften as low as 176°F (80°C)”

TrexOpens in a new tab.

This is great for bending decking boards for curved decks. Looks amazing but also reveals the fact that composite decking is not safe to walk on if heated.

The decking profile should also be considered when evaluating fire resistance.

In 2003 University of California Forest Products LaboratoryOpens in a new tab. did multiple tests on various decking, wood, composite and full plastic. Their findings showed that solid deck boards perform better than scalloped (channelled) or hollow decking. They in particular, tested Trex’s solid board, I assume Transcend, but it could be Select which performed better against fire than scalloped boards. This was especially true for under-deck fires.

TimberTech’s scalloped board collapsed in 7 minutes from under deck fire, where Trex’s solid board did not.

Interesting, with fire from above the deck, Trex collapsed in 25 minutes, but TimberTech Scalloped, most likely Terrain did not from the test.  

These are fairly old tests, and there may have been improvements in composite decking fire resistance. Like how Azek decreased the flame rate of Azek decking.

Or how Trex’s Transcend and Select lines now have a Class B rating. Better than wood.

All this to say, composite decking is not fire-resistant.

Composite decking will melt and become deformed if overheated.

This requires proper care to be taken when enjoying a fire on a composite deck.

All fires should be raised off the decking to allow airflow to cool the decking.

You should also have decking protection like DeckProtectTMOpens in a new tab. or a heat reflection fire mat.

Trex recommends DeckProtectTM and for a wood fire is essential. Few other products match the level of heat and fire protection that DeckProtectTM offers.

To learn more about protecting composite decking from fire and heat, read Essential Elements You Need Under a Fire Pit on a DeckOpens in a new tab., where I go much more in-depth.

Another solution is using a fire table on your composite deck. Quality fire tables have a built-in heat shield protecting the decking under the table. And as the name says, being a table, the fire is raised, providing more room between the decking and fire, minimizing the chances of damaging the decking.

Summary of Composite Decking Fire Resistance

Because all composite decking can burn, I would discourage enjoying a wood fire on a composite deck. Yes, the flame spread has been slowed with Azek and Fiberon Paramount, but it is still flame spread. Even a slow-moving fire will leave a mark, ruining the look of your beautiful deck.

Always take care that the decking is never heated beyond 150°F (66°C). It can melt, which is worse than a dark spot on the decking. I would recommend protecting your decking even more than that. That it never gets close to melting.

Yes, enjoy your outdoor kitchen and grilling on the deck. But design and place your grillOpens in a new tab. with sufficient space or surrounded by material that is fire resistant. Allowing you to enjoy cooking and dining on your deck without damaging it.

Composite Flame Spread and Wildland Urban Interface.

The concern here is not if your composite decking will burn but how quickly the flames will spread. The situation is giving you sufficient time to escape from the fire.

If a spark from the wildland around your home, lands on your deck, will the decking slow the flames spread long enough for you to escape with your loved ones.

The minimum rating for decking in a Wildland Urban Interface is class C. But like Mike Holmes often says, “code is only the minimum” or some version of it. No decking should be used in a Wildland Urban Interface that is less than class C but would it not be better to install decking that is more flame-resistant. Reducing the chance of your house being engulfed by a wildfire.

What Do the Flame Ratings Mean?

Fire resistance ratings aren’t set by the composite companies but are done by third-party testing. In the United States, the standard testing is performed according to the American Society for Testing and Materials (ASTM) process. Which is probably the same testing that is performed in Canada by Canadian Construction Materials Centre.

These ratings also come into play for insurance purposes, so Underwriters Laboratories is also often involved in evaluating fire resistance. Providing the necessary information and testing for insurance companies.

  1. Class A flame spread rating 0-25
  2. Class B flame spread rating 26-75
  3. Class C flame spread rating 76-200
  4. Class D flame spread rating 201-500
  5. Class E flame spread rating over 500

0 being the best rating and 500+ the fire spreading the quickest.

It gets a little nerdy explaining exactly how the fire tunnel test is performed and the math used to determine a rating. But for a quick reference.

Oak flooring in the tunnel takes 5 minutes and 30 seconds to burn 19’6″ (5.9m), giving it a rating of 100.

When it comes to composite and PVC decking, the brand makes all the difference.

DeckingFlame Spread
Azek PVCOpens in a new tab.25 Class A
Fiberon Paramount PVC DeckingOpens in a new tab.30 Class B
TimberTech EdgeOpens in a new tab.Class B
Trex TranscendOpens in a new tab.Class B
DeckoratorOpens in a new tab.sOpens in a new tab. VistaOpens in a new tab.65 Class B
Southern Yellow Pressure-Treated DeckingOpens in a new tab.70 Class B
MoistureShieldOpens in a new tab.Class C
TimberTech ProOpens in a new tab.Class C
Fortress InfinityOpens in a new tab.110 Class C
Fiberon Good LifeOpens in a new tab.120 Class C
TrexOpens in a new tab.120 Class C
Deckorators Vault, FrontierOpens in a new tab.155 Class C

Source: Flammable Composite Decking Made Safer by Additives and PVCOpens in a new tab.

All these composite deckings on this list have sufficient fire resistance for building a deck in a Wildland Urban Interface area, but the top half would be better.

Azek is the highest rating for minimizing flame spread. Giving you the most time to get out of your house if your deck starts to burn.

Small side note, the importance of reading the fine print.

Trex’s decking lines vary in fire resistance. Select and Transcend, their Better and Best composite decking have a class B fire rating. Where their budget line, Enhance, only has a Class C rating. 

The difference in country ratings further complicates this. In the United States, Transcend has a Class B flame rating, but the Canadian Construction Materials Centre only gives it a class C rating. This may be an update in the additives Trex is adding to their premium boards which have not been accounted for in Canada yet but check the most current testing in your area before selecting your composite deckingOpens in a new tab..

Ryan Nickel

A Red Seal carpenter, passionate about building decks to be enjoyed.

Recent Posts