Porch roof framing isn't located within the 'building envelope' per se, and often prompts questions about the appropriateness of using Microllam® LVL and other untreated Trus Joist® products to support that framing. This seems particularly true when there is no finished ceiling over the porch, as with a rustic appearance where the roof joists are exposed to view. The thought being the support beams are exposed to atmospheric moisture, which isn't the case for interior beams within the building envelope. See Fig. 1 below for an example.
Fig. 1: Exposed roof framing
Trus Joist products, excluding Parallam® Plus PSL, are limited to covered end-use installations with dry conditions where the in-service equilibrium moisture content is less than 16 percent. Those conditions are specified in paragraph 5.3 of Weyerhaeuser's code report ESR-1387.
In general, porch roof applications as described would be considered dry use with framing ambient moisture content of 16% or less. Provided the materials are covered and thereby protected from rainfall, the rafters/ceiling joists/beams would generally not be required to be preservative treated materials. This environment would be the same as for a gazebo roof, or the roof over a carport, where treated framing is typically not necessary.
Microllam LVL and other untreated wood products (dimension lumber or engineered wood) are not appropriate in applications where the beam is directly exposed to rainfall (wet service). Fig. 1 above shows the rafters provide a substantial overhang which provides a dry environment for the support beam. Preservative treated products such as Parallam Plus PSL should be used in wet service applications where that protection is minimal or does not exist. More information regarding Parallam Plus PSL and wet use environments that would require treated material can be found in the Parallam Plus PSL Specifier's Guide.
Would add that the support beam's ends would benefit from metal (Galv. sheet metal or copper) flashing over the top/end/side grains with a drip profile. Best to 1/8" air gap flashing off wood surface to allow air flow and prevent capillary action if water migrates under flashing. And best to flash down fascia boards to help for a long wood life.
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