Changes

===Water quality===

===Water quality===

Many green roofs receive only rainwater, which is relatively clean when it lands. As such green roofs can contribute contamination, most notably in [[Water quality#Nutrient|nutrient]] leaching during early establishment. Reported values of total [[phosphorous]] in green roof runoff vary from less than 0.1 ppm to over 10 ppm.   

Many green roofs receive only rainwater, which is relatively clean when it lands. As such green roofs can contribute contamination, most notably in [[Water quality#Nutrient|nutrient]] leaching during early establishment. Reported values of total [[phosphorus]] in green roof runoff vary from less than 0.1 ppm to over 10 ppm.   

But, in dense urban centres, green roofs are increasingly being used to receive irrigation from harvested rainwater. Current Ontario Building Code permits the use of rooftop runoff to be reused in this manner, so long as it is 'free of solids'.

But, in dense urban centres, green roofs are increasingly being used to receive irrigation from harvested rainwater. Current Ontario Building Code permits the use of rooftop runoff to be reused in this manner, so long as it is 'free of solids'.

A 'closed loop' system can be created by coupling a rainwater harvesting system to a green roof.  by catching and reusing runoff, the only water leaving the system is through evapotranspiration. This prevents any runoff from leaving the site and so prevents any nutrient loading to the environment.

A 'closed loop' system can be created by coupling a rainwater harvesting system to a green roof.  by catching and reusing runoff, the only water leaving the system is through evapotranspiration. This prevents any runoff from leaving the site and so prevents any nutrient loading to the environment.