Grip Handrails & Tube Railings
Grip rails are essentially a secondary smaller sized tubular rail that is often exclusive for use on stairs only & secured to the inside of the actual stair railing frame.
The most common size of grip rails is 1.5 x 1.5 inches as this is considered by all codes to be the most comfortable and graspable dimension.
The use of grip rails for any particular job can be one of request by the owner for additional safety or in some cases they may be mandatory to satisfy code requirements.
Tubular style railings are also most often made from the same 1.5 x 1.5 inch pipe as grip rails & are almost always exclusive for use on stairs.
There are only certain circumstances where a railing of this type is allowed. Often they are only applicable when there is no danger of a fall hazard next to the railing. So they are commonly used next to retaining wall or landscape wall details.
The most common design is posts with a single top pipe as a top rail. Another common design employs a secondary or intermediate rail for additional safety and strength.