I frequently come across very nice looking decks, usually well-built except for one significant defect: they have unsafe, insecure (wobbly) guardrails. This is because most deck builders attach the railings improperly, often using a notched post connection method where the posts supporting the guardrail are notched where they fasten to the deck structure. This method is not preferred because it greatly magnifies the forces against the railing at the joint to the deck band joist, essentially creating a lever where the fulcrum is the point at the top of the notch in the post. This method is no longer allowed because modern codes require guard rails to withstand a 200 pound impact loads perpendicular to the railing. Since force = mass x acceleration a 200 pound man moving toward the railing can generate forces far greater than the 200 pound load requirement.
In addition, the attachment using the notched method turns the post into a lever so that the forces acting at the top of the railing is magnified to a much greater force below the fulcrum of the lever - the point where the notched post meets the top of the deck. As a result, a modest impact at the top of the railing can easily cause the post to break or cause the lag screws attaching the post to the deck to pull out of the band joist.
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Deck inspected as part of a recent home inspection with notched railing posts.
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