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Do composite deck boards get hotter than hardwood deck boards? They sure do!

In an average year Adelaide gets fewer than 90 overcast days. This is one of the reasons we are global leaders in the uptake of rooftop solar panels. What it also means is that our decks get punished by a lot of direct sunlight every year. Couple direct sunlight with a 40 degree summer day and you will have one hot deck under your feet whatever it is made of. But what decking boards are the coolest in direct sunlight? And is there much of a difference between different decking boards how hot they get in direct sunlight?


To answer this question we laid out a range of different timber and composite decking boards in direct sunlight over the course of a week and took regular temperature measurements with an infrared thermometer.


Now in the interests of full disclosure the thermometer we used was designed to tell you how hot your BBQ is. It is in fact manufactured by Matador and sold at bunnings for $35, but to counter the possible ‘noise’ in our data we did take a total of 306 measurements over 5 days. So it may be that in some cases our readings are a little out, but there was great uniformity in the relative differences between the decking boards we measured and as such we are confident in the averaged results below.

The great decking board heat experiment!


To make our 306 measurements easy to understand we have given each board a rating. This rating is the average temperature measured on each day expressed as a percentage of the ambient temperature on the day (as measured in the shade). So, as you would expect, given the boards we were measuring were in direct sunlight every measurement is over 100. Here is a summary of our rating equation expressed (somewhat) mathematically.


Average temperature measured per board on a particular day X 100 / the ambient temperature in the shade. So for instance if the measured temperature was 30 degrees Celsius and the average board measurement for that day was 60 degrees Celsius our rating equation would be as follows,


60 X 100 / 30 = 200


As we anticipated the composite boards we tested got considerably hotter than the hardwood boards. Also as anticipated the darker hardwood boards got hotter than the lighter hardwood boards. This relationship between colour and heat however did not always carry over to the composite boards. On some occasions the lightest composite boards recorded higher temperatures than the darker composite boards. This result caused plenty of re-measurements but each time we got the same result. In the averages however the lightest composite board (Alpine Ash) was definitely the coolest and was even cooler than our hottest hardwood board.


Ranked from coolest (congratulations to Spotted Gum for winning our ‘Fonzie award’) to hottest below along with their relative heat ratings are our results.


To give you an idea of what these differences mean in actual degrees C, on the 21st of February at 1:10pm with the ambient temperature at 35 degrees C here were the temperatures we measured. Our collection of boards had been out in direct sunlight all day by this stage.


Note that on this occasion all the Hardwoods are cooler than all the composite boards. At the top end of the scale Estate Brown and Leatherwood have also traded places (by 0.2 of a degree).


What is really noteworthy here is the difference between the hardwood Spotted gum and the composite Leatherwood – 16.6 degrees Celsius. Here is where we can get a little unashamedly scientific! ASTM C1055 (Standard Guide for Heated System Surface Conditions that Produce Contact Burn Injuries) recommends that pipe surface temperatures remain at or below 60°C. The reason for this is that the average person can touch a 60°C surface for up to five seconds without sustaining irreversible burn damage. Pipes that are hotter than this must be insulated to avoid human contact.


So according to ASTM C1055 (Google it, we didn’t make it up!!) all our decks need to be insulated when the temperature gets above 33°C. What we can tell you from a rather unscientific viewpoint is that in our experience, decks are comfortable to walk on until they get above the 70°C mark. We are unscientifically accounting for this extra 10°C through the fact that most people have tougher and thicker skin on their feet than they do on their hands. Above 70°C you really start to notice the heat.


So what does all this mean for your proposed deck? If it is in the shade, not much. Whilst we did still see differences on cloudy days the range was closer to a 5°C split coolest to hottest, with the measured temperatures in the 40’s where the ambient temperature was in the high 20’s. If however your deck is in full sun there are some real considerations for you here. Our advice if you have your heart set on a darker colour composite board would be to look at adding a pergola or some shade sail to your construction. All of which we can help you with here at Paul’s decking and pergolas.

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