A Deck for the Decades

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It shouldn't be too difficult to request variations from the code for specific reasons, but be forewarned that changes may have to be approved by a structural engineer. Those who want some structural references for planning should consult an engineering text or reference manual. From the Walls In by Charles Wing ($14.95, Atlantic-Little, Brown Co.) contains an excellent section for beginners on understanding the elements of structural analysis.

Pier-Group Pressure

For our purposes let's assume all decks have three parts: the foundation, made up of the piers and posts; the framing, including beams, joists and decking boards; and what I'll call the details--necessary extras such as railings, steps and benches.

Over the long term, the foundation is the most important because it supports the whole works. The foundation members can be designed for an attached deck (in which the platform is bolted to the house on one or more sides) or a freestanding structure, separated from the house by an inch or more.

The number of posts needed depends on the size of the deck, or more specifically on the length of clear (unsupported) span between each post. This in turn depends on the dimension of the beams being used to frame the platform. The size of the posts, in depth and width, should increase as they get longer, or as they bear the greater "tributary" weight of a larger beam-an engineer's way of saying that a stronger girder will channel more stress to fewer posts.

In specific terms, a 4 X 4 post placed every 8' is OK if the exposed portion isn't over 8' in height and if the rest of it extends 2' into the ground. But the same post supporting a 12' section would be near capacity; it should be upgraded to a 4 X 6.

However, at a 14' height and the same span, a 4 X 6 is too small. just as a tape measure is at greater risk of bending as it's extended, a post is more susceptible to buckling as it gets longer. Hence, decks over 20' off the ground generally need the blessing of a professional.

Piers or footings are the solid anchors that posts need to remain in place. A pillar imbedded in the ground should have 20% of its total length buried, or no less than 2'. If a concrete footing is poured, it's supposed to be at least 8" deep and four times as wide as the broadest side of the post. Occasionally, a block wall might be used as an anchoring point; in this case, the cores in the block should be filled with concrete and reinforcing bar.

THE BASICS: FRAMING AND FOUNDATION

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