property line skylights, how to meet the code without a parapet

sometimes my blog posts might prove useful to other homeowners / architects / contractors. this post most likely will.

if you run into a situation where you want or need a skylight at your property line and you cannot install the code minimum height fire rated parapet there, read on.

the scenario:
we added on to the back of an R3 (single family) residence in san francisco. the addition was up against a neighboring property line. at the request of the neighbor, we agreed to minimize the height of the wall there by not having a parapet (you can do this by rating the roof for five feet instead where there is no parapet). we wanted skylights in that area to bring light into the addition, and the addition also had a deck at the roof, pushing the skylights to the perimeter. an initial read from a senior building plan checker was that they would allow a fire rated skylight there to accomodate the rated roof requirement.

the problem:
at a certain pitch off vertical, you can no longer claim that glazing holds their listed fire rating. it is easier for materials to restrain fire horizontally than vertically, for obvious reasons. there is no skylight system that has met any fire rating tests in a horizontal position (indeed there actually are no ASTM or UL tests out there for them to meet). to meet horizontal ratings, it must meet floor rating test methods, which then places it inside a type of assembly. there are glass floor systems now that meet rating tests for two hours, but i assure you these floor systems will exceed most budgets for this application, and i am unsure as to whether they can be used in an exterior application.

our solution:
we filed for a local equivalency, under san francisco's administrative bulletin, using AB-005. i would assume for those of you outside san francisco that you have a similar framework to work within. instead of trying to attempt the rated roof exception, there is another alternate in the code for R-2 and R-3 occupancies only. (section 705.11, exception 5) if the entire building has a minimum class C roof (again, not an issue in SF, which requires class A), you can forego the parapet with either non-combustible or fire treated structure for the roof for a minimum of 4 feet, or protect the deck and framing for that distance using gyp board. so we essentially said that a skylight made of noncombustible materials with fire rated glazing (tested vertically!) meets the intent of a noncombustible deck. we were made to use 90 minute rated glazing, but have since been told by the building official that they now would accept 60 minute glazing. the key here was to continually refer to the fact that the rating was tested vertically, and not refer to the skylights as having a listed fire rating. the skylights only needed to be considered non combustible, not rated.

side note for san francisco:
all of the building officials in SF are aware now that there is no such thing as a fire rated skylight and they will reject it outright. if the plan checker, for whatever reason, misses the skylight in the drawings, i assure you all field inspectors also know there is no such thing as a fire rated skylight and will reject it. the field inspector may say something along the lines of "your approval is for a fire rated skylight. what you are installing does not meet the requirements for a horizontal installation, therefore what you are installing does not comply with your approved plans."

road blocks:
to our knowledge, this is the first time san francisco has approved this equivalency. as such, we have had issues with all involved. the skylight manufacturer, even, has stalled in getting us an order going, throwing out caveats, voiding warranties (I now have finally overcome all of that with them). the field inspector, knowing this is a hot issue was flat out not going to approve it (that has now been ironed out). the plan checkers at the city have inserts in their own code books, as i am sure you are aware. the inserts are often sketches for various scenarios on many aspects of the code and what historically the department agrees to approve. they have sketches for skylights in roofs, given varying conditions, telling them what they can approve.

this now sets precedent (in san francisco at least) under the new code to allow skylights at the property line without a parapet for R2 and R3 occupancies.