Without any strenuous research, we can definitively state that exterior wood decks, as a residential dwelling amenity, have increased over the past 25 years. Accordingly, the 2012 edition of the International Code Council (ICC) International Residential Code (IRC) included Section R507 pertaining to the installation of those structures.
If you look up Subsection R507.3, it pertains to footings of exterior decks. Figure R507.3, included in Subsection R507.3, presents a variety of accepted footings for the support of associated posts.
Which footing will last the longest?
2018 IRC Figure 507.3 – Deck Post to Deck Footing Connection (Annotated)
If properly sized and installed on suitable soil, any of the above footing configurations would be quite adequate from a structural and approval standpoint. But, which is the best from a longevity standpoint?
Most people, including some contractors, are unaware that there are two primary categories of pressure-treated (PT) wood for residential construction. There are ‘above-ground’ and ‘ground-contact’, which are mainly determined by the amount and type of preservative treatment utilized in production.
All the above configurations should incorporate ground-contact PT lumber, though there may be some debate regarding configurations ‘C’ and ‘D’ as they are technically above ground. Following other ICC codes, above-ground PT lumber would be fully appropriate if the bottom of the post were set at least 8 inches above finished grade. Use of a column base bracket with a 1” offset would further improve the configurations.
Configurations A and B are problematic, and E and F very problematic for longevity, even when using ground-contact PT lumber. Regardless of whether the post base is the factory cut end, and therefore fully treated, those configurations will expose the lumber, particularly the core, to elevated moisture conditions ranging from damp to saturated with not much opportunity for drying.
Irrespective of treatment, the lower sections of these posts will deteriorate sooner than configurations C and D. The longevity of configurations E and F is further compromised above A and B with additional vectors for moisture entering the core of the post via the through-bolt perforations.
With the provision of vertical stability, lateral stability and maximized longevity, the use of configurations C and/or D is recommended for exterior deck construction. The incorporation of an offset base and field treatment of cut ends will only enhance the configurations and extend the life of the structure.
So if there’s a deck project in your future, plan for footings that will last.
