Tuesday, April 15, 2014

Plant Size Considerations for Green Roofs - Size and Weight Are Important!

Sometimes we may be inclined to add small succulents to a green roof planting scheme because we know they may be tolerant to just about every environmental factor encountered on a roof.
Agave are drought, salt, wind, heat and hurricane tolerant, but they will add weight to a green roof!
Succulents can be deceiving with respect to size and weight, especially to those who have not worked long term with these plants and seen their size at maturity.  What may be a tiny, perfectly sized immature succulent may turn into a very large and heavy specimen with age.
Potential Florida Green Roof plant or potential roof problem?
Root architecture too is another important consideration for the green roof designer to take into account.

Some succulent plants develop swollen root tubers for water and nutrient storage.  Additional water means heavier weight.  Moreover, some of these roots grow in a very aggressive fashion and can damage an underlying single ply membrane or asphalt shingle roof.
Potential green roof plants have also have aggressive root systems that can damage single ply or shingles
While some small succulents, like sedum may never grow six feet tall, others can.  Though the succulents and cacti you plant may do wonderfully for the first several years,  over time they may outgrow the roof's planting bed.

Problems arise when these overgrown plants present weight issues with respect to the loading capabilities of a supporting roof structure.  An extensive green roof with smaller plants may weight twenty to thirty pounds per square foot or 100 - 150 kg/sm.  The same extensive green roof with 5' to 6' agaves may weigh three or four times the original weight.

Wind resistant can be another important factor with large plants on a rooftop, especially in hurricane impacted areas.

When you increase a roof's live load fourfold you may cause structural failure issues.

Treat some succulents as potential large shrubs or small trees.  Remember, just because a small succulent looks just right for your green roof doesn't mean it will stay that way.

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