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    4 3/8" requirement

    4 3/8" requirement

    New postby chris mc on Mon Mar 29, 2010 8:37 pm

    To settle a friendly dispute between two builders can you please explain exactly when and where you are allowed to have a 4 3/8" opening in a stair rail per the IRC.
    chris mc
    Posts: 22
    Joined: Mon Mar 22, 2010 10:36 pm

    Re: 4 3/8" requirement

    New postby Jerry Peck - Codeman on Mon Mar 29, 2010 10:04 pm

    Hi Chris,

    chris mc wrote:To settle a friendly dispute between two builders can you please explain exactly when and where you are allowed to have a 4 3/8" opening in a stair rail per the IRC.

    You included the key word in your question above ... "stair" rail.

    The guard rail along side a landing is 4 inches, the guard rail along side a floor area more than 30 inches above adjacent grade on the other side of the guard rail is 4 inches, the open riser opening is 4 inches, so why in heck did someone decide to use 4-3/8 inches ANYWHERE?

    Okay, to answer your question, though, the 4-3/8 inches is only applicable "on the sides of stair treads".

    (underlining and bold are mine)
    - R312.2 Guard opening limitations. Required guards on open sides of stairways, raised floor areas, balconies and porches shall have intermediate rails or ornamental closures which do not allow passage of a sphere 4 inches (102 mm) or more in diameter.
    - - Exceptions:
    - - - 1. The triangular openings formed by the riser, tread and bottom rail of a guard at the open side of a stairway are permitted to be of such a size that a sphere 6 inches (152 mm) cannot pass through.
    - - - 2. Openings for required guards on the sides of stair treads shall not allow a sphere 4 3/8 inches (107 mm) to pass through.

    Now let us go back to the previous ponderable of "so why in heck did someone decide to use 4-3/8 inches ANYWHERE?"

    First, let us start here:

    - R311.5.3.3 Profile. The radius of curvature at the leading edge of the tread shall be no greater than 9/16 inch (14 mm). A nosing not less than 3/4 inch (19 mm) but not more than 11/4 inch (32 mm) shall be provided on stairways with solid risers. The greatest nosing projection shall not exceed the smallest nosing projection by more than 3/8 inch (9.5 mm) between two stories, including the nosing at the level of floors and landings. Beveling of nosing shall not exceed 1/2 inch (12.7 mm). Risers shall be vertical or sloped from the underside of the leading edge of the tread above at an angle not more than 30 degrees (0.51 rad) from the vertical. Open risers are permitted, provided that the opening between treads does not permit the passage of a 4-inch diameter (102 mm) sphere.

    Now let's go back to the 4-3/8 inches allowed for the guard on the side of the tread ... if a small child was on the tread and could become entrapped in an opening larger than 4 inches between the tread and the tread above (the 4 inch maximum allowed opening for open risers), would not that same small child be just as likely to become entrapped in that 4-3/8 inch opening on the side of that very same tread it was sitting/laying on? Does having two different opening sizes right next to each other seem logical?

    Okay, now lets add another oddity into the mix: go back up to R312.2, Exception 1. above where it states "such a size that a sphere 6 inches" cannot pass through.

    Okay, now we have a small child on a tread, with the open riser limited to an opening of 4 inches, the adjacent triangular opening limited to 6 inches, and the guard opening above that bottom rail allowed to have an opening size of 4-3/8 inches - now for my question: What is going to make that small child turn toward the smallest opening of 4 inches at the open riser instead of the larger openings right next to it?
    Jerry Peck - CodeMan

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