Riku Ylipelkonen, Owner, Standard Building Advisors and FRSA Technical Advisor
How wet is too wet? In the Florida roofing industry, this question prevails as one that never seems to have a straight-forward answer. From dimensional lumber to engineered wood products, roofs use a variety of products that must account for moisture and how to optimize their service life. Just like most other building materials, dimensional lumber has changed. The technology and evolution of growing trees is just one of the reasons we don’t have access to the density, species and pricing of the previous generations. One big reason is demand. The construction industry is an important component of the economy. The population of Florida is estimated at over 21 million people and growing and Florida is one of the fastest growing states in the country, thanks to a variety of reasons. With this growth, the demand for housing materials follows.
While most new residents of Florida love the warm weather and easy access to water, these qualities also help make the environment one of the toughest in the world for building materials to last. Building materials are assaulted with higher temperatures, higher moisture and, sometimes, higher winds. Name the building material used and there is an easily correlated deterioration caused by moisture. Steel experiences rust. Concrete encounters degeneration from rebar decay causing spalling and water causes structural cracking. Wood undergoes rot. All these conditions are accelerated with higher ambient temperatures and higher levels of available moisture.
Water absorption is a concern for building materials and their ability to maintain physical properties and structural integrity. Wood products naturally absorb moisture for the most part. Whether it’s dimensional lumber, plywood or oriented strand fiber board (OSB), the content of moisture is somewhere around eight to twelve percent when it is ready for use. Wood products can absorb more moisture depending on ambient conditions. The secondary consideration is for the permeability of moisture in the same product. If a sheet of plywood gets wet from rain, the permeability of the sheet helps to move moisture through the entire product and then the same process in reverse dries the plywood back out to be ready for use.
How the wood product is processed or prepared can also play a role in the ability to control moisture absorption and permeation. How a product is able to absorb, permeate and release moisture must be considered for each particlar type of use. There are several treatments used for controlling absorption in building materials. Painting or coating is a liquid-
applied membrane that blocks moisture absorption and release. Pressure treated lumber and epoxy coated rebar are good examples of products that will maintain physical properties and structural integrity for longer periods of time than their non-coated counterparts. As the size of materials increases, the cost to treat also increases. Also, the intended use of the products can help to maximize the efficiency and cost to control the moisture content. As an example, steep slope roof decks that use OSB or plywood usually rely on the roofing system design to control moisture. When a self-adhered underlayment is applied to the roof deck, the individual sheathing must absorb, permeate and release moisture with the remaining five sides of the product – those being the four seam sides and the backside of the sheet. Since all six sides of the products are not sealed, the remaining open surfaces must be allowed to perform moisture permeation and release to keep the sheathing serviceable. The attic should have ample air flow and exposure so that the ambient air can carry out the extra moisture and heat.
The concept is the same for low slope decks, . The use of a vapor barrier as a base sheet is also sealing the surface of the applicable building materials. The challenge grows further when a deck vapor barrier is used, insulation and cover board are installed on top and then everything is capped with a single-ply membrane or built-up roof that does not allow moisture to permeate and be released into the ambient air. The vapor barrier and top waterproofing system basically form a bag, so construction conditions and moisture control are crucial to a long service life.
The process of construction is also a consideration for how moisture and heat can attack building materials. Steel nails and screws can lose the integrity of their protective coatings as they are driven into their final installed position. Old nails and screws in the roof deck are often corroded because their protective coating was compromised and it lived in a wet sponge known as the plywood roof deck. Fascia boards rot because they were not able to dry out, possibly from water ponding due to poor design or moisture saturation of adjacent materials.
Roofing gets better every day. New materials and designs are a part of that. Controlling heat and moisture, however, is just as much a requirement with the new as the old. The final thought to take away is this: circulating air is an efficient and effective way to move heat and moisture away from building materials. The more this is accomplished, the longer the materials will last.
Riku Ylipelkonen, Owner, Standard Building Advisors has been in the roofing industry for 15 years. He is an engineer and is working as a consultant with FRSA.
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