While you might expect the notch to be the weak point in the connection, in fact none of the test posts failed at the notch. Even so, notching should be avoided, because it does substantially reduce the strength of the post.

What is code for deck railing posts?

U.S. building code requires support posts for the rail structure to be no further apart than 6 feet on a deck with 4×4' deck rail posts and no more than 8 feet apart for decks with 6×6' deck rail posts. Posts should be no further apart than 5.5 feet on stairs.

How much can you notch deck post?

A joist can be notched at bearing locations at either the top or bottom without reducing its structural capability, as long as the notch doesn't exceed one-fourth the depth of the joist.