Tonight is the second night of the November 2008 winter where the outside temperature is expected to drop below freezing. Last night there were low in the mid to low 20's all across North Florida and the same is expected tonight.
Expected seasonal weather like this creates a huge challenge for the vegetated roof designer in Florida. Not just cold weather alone - because many northern green roofs survive much colder temperatures.
No - it is the combination of two weeks ago we were experiencing heat and humidity at high levels and now the frost - little if any hardening off of the roof plants.
I pose a question here. What happens when many of the native plants here that are deciduous or produce leaf litter - what happens when leaf matter dies from frost or drought? One of the answers is that we see a marked increase in leaf litter buildup.
Leaf litter turns into fuel once ignited. Dead plant matter, dried by desiccating cold winds can easily and rapidly burn when exposed to a spark or heat source.
Fire occurs in nature to cleanse dead plant matter from the environment and return nutrients to the soil. The chance of a roof fire increase once flammable, dried leaf litter accumulates on a green roof.
ASTM has recognized this potential problem and is developing standards for green roof plants.
The good designer will also strive to incorporate plants that prevent or greatly reduce the chances of leaf litter build up.
As we enter the season where many plants are dropping their leaves and becoming dormant, we should ask ourselves as we design green roofs, about potential fire issues with the plants we are selecting.
Better safe than sorry.