Surface Area to Mass Ratio, or simply the surface area of an object divided by its mass, is an important geometric factor when determining the propensity of a material to ignite and for a fire to spread on that material after ignition has taken place. As we previously discussed, ignition in solid materials such as wood is a surface event. In other words only the surface of the material has to reach the ignition temperature, not the entire mass. Increasing the surface area and reducing the mass of a material increases its ability to ignite and the ability for the fire to spread along that material more quickly.
Think about a block of wood and a piece of paper. They are essentially the same material in composition; however, they have significantly different surface area to mass ratios and different thermal inertias. Thermal inertia was discussed in a previous article so we will focus only on surface area to mass ratio in this article. A piece of paper has a much greater surface area but substantially less mass compared to a block of wood. So if at the same time, you put the wood and paper on a stove element that has a constant heat output, you will see the paper ignite first and the fire spread very quickly along its surface. The reason is that the paper has a lower thermal inertia and a higher surface to mass ratio than the wood.
Another example is to take the lumber you would use to build your backyard deck. Place the lumber in a tight stack. You have a large mass, but by tightly stacking the lumber you have reduced the exposed surface area available for a fire to spread along. Thus, the total mass of the wood will therefore burn relatively slowly. Now take that same stack of lumber and build your deck. You now have exposed all the surfaces (top, bottom and sides) of each individual piece of lumber so your surface area to mass ratio is greatly increased relative to the stacked lumber. This increased surface area to mass ratio will rapidly increase the rate of fire spread. In addition, the gaps between each individual piece of decking lumber will enhance air-flow or ventilation, further intensifying the fire. We will discuss ventilation effects in future articles.