Tall Deck on a Sloped Lot
When deck posts exceed what the prescriptive code tables allow, it's time to consult a structural engineer for post sizing and possible bracing.
My client asked me about building a 16-ft. by 20-ft. deck over a walk-out basement on a sloping lot. Because of the elevation change, the posts look like they will need to be a little more than 13 ft. tall, although the slope makes it hard to measure exactly. Is there a prescriptive way to support a deck with pressure-treated posts this high above the ground? If I place the posts on concrete piers with post bases, is the section of pier above grade part of the equation? And is there a good way to brace the posts so they will resist flexing?
— Rob Wotzak; New Milford, Conn.
Editorial Advisor Mike Guertin Replies
Table R507.4 in the IRC lists the maximum deck-post height for 4x4s through 8x8s of different lumber species based on the typical live and snow loads and the tributary deck area that a post supports. Post height is measured from the top of the pier to the bottom of the beam. The table limits maximum post height to 14 ft., even for 8x8s.
Your first step is determining the exact height of the posts. If they’re 14 ft. or less, you can use the table to determine which post size you’ll need based on your load conditions and tributary area. You may be able to use the code table even if the distance between ground level and the beam is a little greater than 14 ft. by extending the top of the concrete pier above grade.
You can extend the piers above grade by 12 in. safely in most situations. Just ensure the piers are plumb and the post-base hardware is centered on the piers—good practice for any footing and pier installation. There is no prescriptive post-bracing requirement in the IRC other than anchoring to the pier and fastening to the beam.
If the posts are still taller than 14 ft., you’ll need to consult an engineer. The engineer may recommend taller piers based on soil conditions, pier diameter, and the depth of the footings, or design a bracing system that resists compressive deformation of post heights taller than the code table allows.
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Do you have more questions about building a tall deck on a sloped lot or something else? Get answers you can trust from the experienced pros at FHB. Email your question to Experts@FineHomebuilding.com.
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