Flammable Composite Decking Made Safer by Additives and PVC


Decks are great to get outside, enjoy the sun and grill supper on. But can a composite deck start on fire from the heat of the grill? Or what about if your deck backs onto a forest reserve.  Is the composite decking flammable, increasing the chance of your house burning down?

Composite decking is flammable with a high flame spread, manufactured from plastic, which has a high heat release level. To reduce flammability, manufacturers add fire retardant chemicals such as ammonium polyphosphate to the core of composite deckingOpens in a new tab., reducing flammability to Class C or better.

Class C or Level III fire spread is comparative to wood decking. All composite deckingOpens in a new tab. sold in the United States, Canada or U.K. must have a minimum fire spread rating of C or better.  

Yes, Ipe is an exception for wood decking with an incredible Class A flame spread rating. That is hard to beat, and it’s hard to burn. Most other wood decking have a Class C or better flammability rating.

Before getting into the fun science facts of flame spread on composite decking.

If you build a deck near a forest or wildland, you should check if it’s approved for WILDLAND URBAN INTERFACE (W.U.I). Meaning the decking is Class C or better for flame spread. Not amazing, but low enough to be safe in areas with potential of brush fires.

Now for the fun science regarding composite decking flammability and decking that is better rated against fire spread.

Before getting to far into talking about decking flammability, a little clarification.

Composite (HDPE) decking being made from plastic can melt. When we are talking about flammability, we are not talking about melting or other ways that plastic decking can become deformed from heat. We are simply talking about how fast your deck will become engulfed in flames.

Melting and burning are two different things. Take vinyl siding, which you always need to ensure that your barbeque on your deckOpens in a new tab. is not too close to. Take KaycanOpens in a new tab. Vinyl Siding as an example; it has a Flame Spread rate of 20, lower than any composite decking I know off. But set up your barbeque to close, and it will melt right off your house. 

Level of heat resistance is another conversation. 

Explanation of Levels of Flammability of Composite Decking

Composite decking is rated level 3 or class C in flame spread or better. To help you compare composite decking flammability, let me explain the rating system. The best-known test is one developed by the American Society for Testing and Materials (ASTM) Test Method E-84, commonly known as the tunnel test.

  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+ being the worst.

The number value is created by comparison with two materials as reference, cement board which does not burn is rate 0 and red oak which is rate 100. Not sure why they picked red oak, but that’s their other reference point.

Class D and E are liquids and flammable gases. Leaving only the first three as relevant for discussing decking flame spread.

To test decking, they place a piece of decking into a combustion chamber that is 25′ (7.5m) long. Started a fire on one side, which is approximately 5 ½’ (1.5m) and test to see how quickly the flame spread to the end. Red Oak flooring takes 5.5 minutes to spread to 19 ½’ (5.85m) Which earns it a score of 100.

In theory, all things being equal if you had a 20′ wide red oak deck, with a grill on one side. If the grill was to burst into flames. You have a little over 5 minutes to get off the deck before you get burned.  

This doesn’t necessarily mean that if you install composite decking with a 75-fire spread rating, you now have 7 minutes to get off the deck. Not sure why you are just standing there anyways. The math is a little more complicated than that.

For the actual math equation, you can go to NRC-PublicationsOpens in a new tab..

But for here, we will just say. If the number is less than 100, it will take more than five minutes to burn. Higher than a 100, you better run your decking is going to burn fast. The lower the number, the slower the decking will burn.

Comparing Flammability of Composite Decking

Of course, not all composite decking has the fire rating. Here are a few to get an idea of which are safer and which are more flammable.

Composite 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
Deckorator 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
Composite Decking Flammabilty Comparision

As the chart shows, there is a wide variation in flammability and flame spread within composite decking.

You have Azek PVC decking on the top of the chart, with a rating of 25. The only decking listed as Class A. The best rating any decking can get for a fire rating. The same flame rating as Ipe decking.

Then on the bottom, with the fastest flame spread, is Decorator’s Vault and Frontier decking lines. With a flame spread rating of 155, Class C.

Roughly in the middle is your traditional Southern Yellow Pine, the most common tree used for treated decking. Coming in with a rating of 70, class B. Actually, fairing better than Red Oak, the critical reference point for flame spread.

Interesting side note, HDPE, the common ingredient of composite decking, has a Flame Spread rating of 98Opens in a new tab., not sure what their doing to make the decking more flammable than both pine and HDPE, the main ingredients.

On the other hand, what are they doing with composite decking to reduce its flammability rate?

Reducing Composite Decking Flammability. 

There are two ways manufacturers commonly reduce composite deckingOpens in a new tab. flammability.

  • Increase Wood Content
  • Add Fire Retardant Chemicals

The first is the easiest. As noted, pine has a lower flame spread rating than HDPE. By adding more wood fibres to the blend, composite decking flammability is reduced.

“Reducing the polymer content by increasing the wood fiber content has been shown to improved fire performance in the cone calorimeter test.”

U.S. FOREST SERVICE, USDAOpens in a new tab.

Wood fibres will vary how hot and quick they burn. Which limits how much you can reduce the flame spread. On top of flame spread, it’s the protection of wood fibres with HDPE that make it low maintenance decking. Limiting the amount of wood fibres you can have in composite decking.

But why does Azek PVC decking with no wood fibres have the best flame spread rating?

Two things are working to Azek’s advantage. First, PVC has a lower flame spread rating than HDPE.  

“PVC is less flammable than most polymeric materials, natural or synthetic and it will not normally continue to burn unless a source of a sizeable fire exposure remains present.”

Vinyl Info

AzekOpens in a new tab. has gone even further to reduce the flammability of their decking. They did what many composite manufacturers do, adding fire retardant to the composite decking.

“Adding fire retardant chemicals, particularly ammonium polyphosphate, was also shown to be effective in improving the fire performance.”

U.S. FOREST SERVICE, USDAOpens in a new tab.

By increasing the fire-retardant’s volumes or strength in the decking, reducing the speed the flame spreads across your deck. Reducing the potential for fire damage or injury with additional time to react to fire spread.

Testing Fire Spreading of Decking

Rather see it than read about it?

Matt Risinger does some fun fire test with Ipe, Azek, TimberTech and Cedar decking. With very similar results to what we just discussed.

Ryan Nickel

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

Recent Posts