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Deck Designs:  Foundation

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Deck Designs: Before You Start Digging Deck Footings

All too often deck designs begin with digging holes and filling them with concrete to make the deck footings. Unfortunately, deck designs that use this approach often result in deck footings that either sink under load or end up heaving in winter. In this section, there are deck footing tables and advice developed from numerous professional publications that will help you get your gazebo and deck designs right. Following this information on deck designs will ensure your deck footings sit flat and level for years to come.

More specifically, this section on deck designs will tell you how to determine the diameter and spacing between the deck footings- based upon your particular soil type. Although not difficult, most deck designs fail to take into account how the spacing between deck footings influences the number and size of floor joists and beams. Given this interdependency between soil type, spacing between deck footings, and lumber sizes, you can begin to see why good deck designs start with determining the soils bearing capacity BEFORE even thinking about sizing joists and beams. Good deck designs literally start from the ground up - deck footings first.

Deck Designs: Deck Footings - More Details

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 cannot be used in deck designs to size individual deck footings poured at shallower depths. Using bearing capacity tables designed for full basement homes for deck designs will result in deck footings that are undersized and tend to sink over time.

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, increases with depth. So the deeper you dig your 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 that are sizing 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.)

Deck Designs: Deck Footings That Resist Frost Heave

Good deck designs also take into account the effects of frost heave on deck footings. Luckily for most people with inadequate deck designs, the connection between the deck and the house usually has enough flex to tolerate deck footings that heave in winter. Consequently, even though many poor deck designs result in deck footings that undergo significant uplift in winter and produce an undulating deck surface over time, the flexible connection between the deck and the house prevents any serious damage.

Good deck designs resist frost heave. There are ways to easily determine and minimize the potential for heaving deck footings and thereby ensure a flat surface for the life of the deck. Read this section on deck designs and learn why using stubby sections of 4x4 posts resting in metal saddles is a bad deck designs practice and what alternatives you have to install your deck footings right.

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