Sun Shining Through a Water Puddle on Plastic Causes Fire

When I was a kid, we used to try to do start a fire using sunlight and a magnifying glass and I was never really successful. This weekend, I almost caught our deck and ultimately our house on fire mistakenly using the same science.

I aded heavy clear plastic to a metal pergola we have in our back yard so that everything underneath it would remain dry. Later, I planned to add a decorative canvas shade over it to prevent the sun from beating down on us.

After I added the plastic, which is a very durable plastic much like the windows in a pop-up camper, it actually did rain quite heavily and successfully, everything under the pergola remained dry. On Sunday, the day started out cloudy and as I was sitting on the deck under the structure the sun finally came out. No more than five minutes later I began to smell smoke; what seemed to be like wood burning. I figured the neighbor must be burning wood so I paid little attention. Within a minute or two I realized I could actually see the smoke in the air and it was coming from the deck I was sitting on.

Sure enough, the sun was shining on the puddle of water on the plastic that I had just installed and it was beaming down onto the treated lumber which made up the deck. I was shocked as the pin pointed beam was actually igniting the wood to the point that it was smoking heavily and there was actually a flame. It was shocking to see that within five minutes, the heat from the sun magnified through the water and plastic so much that it was igniting the wood 14 feet below.

Looking back on this, I was sitting on the deck because I was waiting for my wife to be ready to leave for a graduation party. Had we left without discovering this phenomena it would have almost certainly caught the deck on fire, the wooden chairs, the wooden fence, and most likely would have spread to our home.

What a freak accident this would have been and we probably never would have discovered what actually started the fire. I certainly wouldn't have thought it was the sun, shining through the water and the plastic creating and laser beam intense enough to ignite a wooden deck. We dodged a major bullet.