In areas where full basements are the norm, typical codebook soil bearing tables are written for 7 foot deep and 16" wide full perimeter house footings. Although the values in these tables work well when designing for full basement homes, they can not be used in deck designs to size individual deck footings poured at shallower depths. Using bearing capacity tables designed for full basement homes in deck designs will result in deck footings that are undersized and tend to sink over time.

In case you're interested, the reason that good deck designs do not use full basement soil-bearing tables to size deck footings is that soil pressure, and consequently it's bearing capacity, increase with depth. That is to say, the deeper you dig deck footings, the greater the pressure from the soil above and surrounding the hole. Good deck designs take into account the fact that the deeper you dig the deck footings, the more weight the soil can bear. Understanding this, you can see why tables written for deep house footings can't be used in deck designs to size shallower deck footings. (Ex. A poor soil at 7 feet can bear 2,200 lbs/sq.ft., but at a depth of 4 feet, the same soil can only carry 1,600 lbs/sq.ft..)

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