Guardrails for decks must meet specific requirements as are laid out in the International Residential Code.  The most basic regulation is that all guardrails must be at least 36” in height. 

A variety of styles are allowed as long as the interior sections of the rail don’t possess any openings large enough to pass a 4” diameter sphere through.  In the case of guardrails for stairs there is an exception that allows up to a 6” diameter sphere through the triangle opening formed by the stair riser, stair tread, and bottom rail.  The guardrails must be strong enough to withstand a concentrated 200 lb force anywhere along the top of the rail.  To achieve this you should space rail posts no greater than 6' apart.

Handrails are required for stairs and must meet standards as specified by R311.5.6.3 in the IRC code.  The top edge of the handrail must be placed between 34” and 38” above the nosing of the stair treads.  Handrail ends must be returned and terminated at rail posts.  The handgrips must allow a minimum of 1-1/2” space between the handrail and the guardrail or wall.  A variety of gripping surfaces may be acceptable but must meet requirements for gripping surface.  Flat 2x4 and 2x6 handrails are not acceptable.  A circular cross section of a handrail must have an outside diameter of between 1-1/4” and 2”.