Destruction of Metal Flashing by Water Intrusion

The building of structures creates joints between floors and walls, along supporting-beam joints and between conjoining sections of floor decking. These joints spaces are openings, and even if they are very tight, water will still seep into these narrow cracks. Construction experience shows that more than just caulking material must be used to secure these joints. The metal strip or flashing along the edge of your decking, along a wall, around your deck post and around supporting-beams, performs this very important function. It is placed anywhere there is a joint that water could seep into.

Most residential-construction flashing today is made from light-gauge aluminum. Aluminum is affordable, easily bent to fit the conjoining surfaces and can be painted. However, it is prone to rust and deteriorate if the paint-cover is removed, or if it is exposed to saltwater. Aluminum also tends to corrode if it is placed over concrete, masonry, or pressure-treated wood because of the alkaline nature of these materials. Therefore, the metal flashing needs to be compatible with the building materials that it is placed over. Some of the other flashing metals that are available are copper, galvanized aluminum, and galvanized steel, to name a few.

The joint between the flashing and support structure is sealed with a caulking compound to ensure that it is water-tight. If there is a breach anywhere in the system, the first to be exposed to water is the flashing that will begin to rust. This oxidation process, once started, will continue along the surface of the metal, slowly separating the caulking material and the paint protecting the flashing. As the moisture works its way into the underlying structure it causes a separation of the metal flashing, further breaking the seal.

As this destructive spreads, water will seep into the wood structures below causing rot. The rotting wood will separate from the metal flashing allowing even more water to enter, further spreading the damage deeper and along the under surface of a decking.  As the water further penetrates, deep wood support structures, such as wood beams and joist will decay and fail. The rotting wood under the decking or in the support beams will not be noticed because it is most often out of view. Failure of the rotted wood-supports can be catastrophic.

A tragic example of the importance of metal flashing is demonstrated in the collapse of a balcony in Berkeley, CA. Apparently, a group of people standing on the balcony exceeded the ability of the balcony to support them, collapsed and 6 died as they fell 50 feet to the ground. Inspection of the structure showed that the support beams were rotted at the junction of the balcony and the wall.

Commenting on the accident Oakland civil and structural engineer, Gene St. Onge, said that water damage probably caused the balcony to break off the building as he noted the rotted wooden-support beams. He is quoted as saying “All water has to do is get in there and start seeping into the joint and into the wall. In a short time it can rot the wood, which can give away.” He continued and stated that improper installation of the waterproofing may have been at fault.

This is a clear example of how important the proper installation of flashing is, and how it protects the underlying support structures if properly installed. Inspection of the other balconies on the same building showed no deterioration of their waterproofing or support structures. This brings up several important facts about metal flashing:

  • Proper installation is a must.
  • The flashing joints need to be caulked.
  • Routine inspection of the metal flashing and caulking should be performed yearly.
  • Re-caulking should be done and timely repair or any deteriorated flashing on a routine basis.
  • Inspection of exposed support beams and joists should be done yearly.


Pli-Dek, Inc manufactures a wide variety of specialty coatings for the building industry; waterproof deck coatings, fluid applied waterproofing membranes, epoxy flooring and concrete coatings. As experts within the specialty coating niche for over 35-years, Architects, Contractors, and Owners depend on Pli-Dek’s commitment to unparalleled performance and service.