Blocking or bridging can be used for a variety of purposes in deck building. Probably, the most commonly used technique is to install small pieces of material in a zig-zag pattern between the perimeter joists to create a rigid rim joist that prevents bounce and increases the strength of the rail attachment. Some deck builders install blocking throughout the frame to increase the strength of long joist spans and promote a more solid structure.

Often times, scrap materials from your framing lumber may be used for blocking. Always make sure that this material is either pressure-treated or naturally resistant to decay.

Another blocking technique involves laying the blocking level with the tops of the joists to create a solid surface around the perimeter to support a picture frame or racetrack perimeter board. Blocking can also be used in this manner down the center of a deck to provide support for a division board, a zipper pattern or herring bone pattern. Most creative decking styles like parquet etc. will require some blocking. 

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Reduce joist bounce with blocking.

Install blocking or bridging between joists at mid-span to reduce bounce. Install blocking along the perimeter rim joists to strengthen guardrails.

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Blocking the joists at mid span.

Snap a chalk line across the center of your joists to lay out your bridging. You can use scrap framing material for blocking and bridging.

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Toe nailing blocking with a nail gun.

Stagger the blocking material to provide space for nailing. Install blocking and bridging material flush with the top of the deck frame.