I’ve come across a repeating set of misconceptions and issues among green roof beginners, and have prepared this short article to – i hope – bust some of these top green roof myths. Feel free to chime-in in the comments or on twitter – maybe we can get this up to a top ten list!
Myth 1: “Green roofs are maintenance free.”
Extensive green roofs need periodic maintenance during the first year as the plants are getting established. You need to closely monitor the moisture of the growing medium so the roots get established. There will also be some weeding although that will be minimal because the plants are elevated above the normal seed dispersal zone. In subsequent years you will have to occasionally check for adequate moisture when rainfall is sporadic and there will be cleanup that has to be done such as clipping dead growth and removing blown in leaves. Plants do not live by water alone and eventually whatever nutrients were originally in the planting medium will be used up, or become unbalanced. Plan on doing some fertilization periodically. There is always the likelihood that plants will die and have to be replaced.
Intensive green roofs have heavier maintenance needs that are similar to traditional gardens. Besides water and fertilizer the plants may need pruning or clipping depending on variety, size and habits. Just like in gardens that grow on the ground, pests and weeds will find homes in green roofs as well.
Green roof membranes and other components can get damaged or degrade over time and may have to be replaced.
Myth 2: “If you have a green roof you can’t use the roof for solar electric generation panels.”
Actually, green roofs and solar panels go well together. Green roofs will reduce the changes in temperature surrounding solar panels so they operate in a more favorable environment. The photovoltaics will work more efficiently, especially during periods of high heat. According to the International Green Roof Association the vegetation has to be of the type used in extensive green roofs and the photovoltaic panels have to be mounted above the plants.
Myth 3: “Green roofs leak.”
Any roof will leak if it isn’t properly constructed and maintained but there is little evidence that green roofs are more prone to leaks. There is evidence that green roofs protect the waterproof membrane below them from the damaging effects of the environment and ultraviolet light from the sun.
Myth 4: “Green roofs don’t need to be watered.”
While many sources are quick to site little need for irrigating a green roof, roof designers and architects typically plan for some type of irrigation especially during the roof’s establishment stage and most especially for intensive green roofs. Drought is more commonplace and even though the properly selected plants are drought hardy it is always best to consider how you will water the roof if necessary.
Myth 5: “Green roofs are too heavy for most buildings.”
For new construction the roof is designed for the load so its weight is already accounted for. Many existing roofs are designed for live loads (snow, water, wind) exceeding 40 pounds per square foot. Reinforcing well-built structures to handle the additional 20 to 30 pounds of load needed for an extensive green roof is often possible and affordable. Each roof has to be evaluated individually
Myth 6: “Green roofs don’t require insulation below them inside the building.”
The insulation value of green roofs is still under study and preliminary findings show the climate control benefits for buildings using green roofs come from factors other than insulation. The transpiration of plants coupled with their heat reflectance characteristics helps with keeping the building cooler in the heat. The frozen planting medium creates a constant 32 degrees F at the roof’s surface and reduces the building’s heating needs in the cold.