Tuesday, May 10, 2011

Green Roof Plants and Root Barriers - Root Tips and Seams

Today's technical insight focuses around the situation of plants having agressive root growth habit near waterproofing or root barrier membranes.

Sometimes the membrane installation will have a seam just in the place you wish it did not have a seam.

Seams and seam welds are normally designed to hold up against stormwater and normal use but an agressive root tip can quickly and easily penetrate the welded area between two layers of barrier membrane and create a leak.
Green Roof Membrane Seam and Plant Placement

Yet seams are usually very strong.  Think of water supply pipe and the glue used to attach both pieces - the resulting bond is permanent enough to prevent pressurized water from leaking out of the pipe and the same holds true for a properly welded waterproofing or root barrier membrane.

But some roots have a way of exploiting even the smallest cavity along a seam, finding their way through the bond and creating an eventual issue.

An easy way to circumvent a potential problem is to try and plant or place the green roof plants on the side of the top membrane layer.

Root architecture studies on most green roof plants show a growth pattern radiating away from the center of the central stem.

By placing the plant on the upper membrane layer the roots will generally have a tendency to grow over and down and away from the exposed seam edge.

However by placing the plant on the side of the membrane seam having the lower layer, the roots naturally grow directly towards the exposed membrane seam edge.  This placement creates a distinct disadvantage and potentially allows for outward expanding root tips to find voids or cavities in the membrane seams.

Importantly, not every green roof plants will have well behaved roots that grow outwardly.  Some species, like bamboo and other plants have roots that grow in all directions and will seek out seemingly all potential cracks and seams.

So the planting tip included here is not a solution to green roof plant problems, it is a precautionary measure, one that should be implemented whenever possible.

If your membrane is seamless you don't have to worry as much.  But many roofs have seams as well as many membranes, both root barrier and waterproofing.

Know your seams.  Find them visually the first time ou walk on a roof of a potential green roof project. Keep their locations in mind and on your design sketches and drawings.  Then properly place plants to avoid seam root tip intrusion.

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