April 30, 2011 | Ken from Orangeburg, SC asks:
“I am wondering is there a minimum height requirement for a
deck to allow for adequate ventilation under the deck. Is this
measured to the top or bottom of the decking timber?”
Glenn Mathewson, NADRA’s Technical Advisor, responds:
Hello Mr. Panitt.
Thank you for turning to NADRA with your questions about the Decking and Railing Industry. We are certainly happy to provide you guidance. The International Residential Code (IRC) and International Building Code (IBC) do not specifically regulate the ventilation of areas underneath exterior decks. IRC Section R408.1 describes the ventilation requirements for under floor spaces, including the minimum net free area, but it is in reference to areas under buildings that are enclosed by foundation walls. While it would certainly be good practice to maintain moisture control under decks that are very low to the ground, its not part of the minimum standard set forth by the IRC. However, in the case of manufactured decking or other products, the installation requirements of the manufacturer are essentially part of the code. This may be from a direct reference to, such as section R317.4.1 in the 09 IRC for wood/plastic composites or through approval as an “alternative”.
I know at least two prominent tongue-and-groove composite decking products that require a minimum height above grade to make up for the lack of air flow between the boards. In both cases the distance was 12 inches of vertical space beneath the bottom of the framing, though the percentage of perimeter where it was required varied. You must always adhere to manufacturer’s installation instructions, not only for good practice, but also for code compliance.
For spaced deck boards, the gaps would likely suffice for ventilation. They may be narrow, but they’re evenly dispersed. One thing to consider, however, is the size of the gaps and the material used. If swelling of wood, expansion of thermoplastics, proximity to heavy autumn leaf fall, long snow coverage or poor maintenance/cleaning is likely, the gaps may close. In that case, additional ventilation openings or greater clearance to grade would be a good plan.
I hope this information is of assistance to you and the visitors of the NADRA blog.
Glenn Mathewson, MCP