Three Causes of Moisture Below the Roof Membrane

  February 14, 2012

Today's tip of the day is about three ways moisture can get underneath a single-ply roofing membrane. Because knowing is half the battle, understanding these three common causes can be the first step toward prevention.

First is lack of redundancy. If the seams on single-ply membranes aren't formed correctly, water has a good chance of getting underneath the membrane and leaking into the facility. Single-ply membranes - because they are really only one layer - really rely much more heavily on proper installation than do multi-ply membranes. Ensure that good specifications are written for the installation, and that the installer has plenty of experience installing your particular type of roof.

The second cause of water intrusion is failed flashing. Especially with EPDM single-ply membranes, shrinkage can cause the membrane to pull apart from the vertical flashing and the roof. This unseals the flashing and provides and easy avenue for water. With PVC and TPO membranes, it can be difficult to form flashings around unusual penetrations, like square posts or wide-flange beams. So, again, flashing installation is incredibly important. Make sure your contractor has experience and can show you exactly how he plans to build the flashing and why that construction will be successful at keeping water at bay.

And finally, the third cause of moisture getting underneath single-ply membranes is water vapor diffusion. What exactly does that mean, you may be saying? Basically, water vapor can flow through solid surfaces when the difference in moisture levels, say, in the air outside a building is much greater than inside a building. This happens mostly in temperate climates. That water vapor can then condense underneath the membrane and leak into the building. A solution for this problem is including an appropriate vapor barrier as part of the single-ply roof assembly.


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