Calculating Areas for COA’s 2014 Fire Flow Requirements for Residential Buildings

With the explosion of prices for land in the Austin area we have seen folks looking to build out as much as they can with either a larger single family home maxing out the FAR or by building a two family home or duplex on a larger property.  With this increase in density came an increase in concern regarding safety especially where fires are concerned.  To respond to this concern the city implemented a requirement for fire-flow pressures (gallons per minute that water comes out of a nearby fire hydrant).  This ensures that in the event of a fire, the fire department will have the resources to put the fire out efficiently. This new requirement can be a bit tricky to meet however, especially when building a two family home.  A licensed Architect will navigate these sections of the code when designing your home but we find that some folks are interested in what goes into this special new requirement and what it might mean to their new home design so we have listed some of the basics here.


‘Beginning in January 2014, the City of Austin began to formally review for the fire-flow requirement for houses larger than 3600 SF. Fire-flow for residential construction varies based upon size and construction type (see Table B105.1, 2012 International Fire Code) If a property does not have the required fire-flow, the code allows for alternate methods of compliance including the installation of sprinklers or an increase in the fire rating of the construction.’

– Excerpt from AIA Austin Advocacy

To see if your new home(s) will trigger this requirement the area must be determined – the “Fire Flow Area”. The fire flow area calculation comes from the 2012 International Fire Code Section B104 , copy pasted below and is a different area metric from both FAR, building coverage or GSF that’s used in other City of Austin calculations:

B104.1 General.The fire-flow calculation area shall be the total floor area of all floor levels within the exterior walls, and under the horizontal projections of the roof of a building, except as modified in Section B104.3.

B104.2 Area separation. Portions of buildings which are separated by fire walls without openings, constructed in accordance with the International Building Code, are allowed to be considered as separate fire-flow calculation areas.

Here’s the section that drives the 3600 s.f. trigger (For years, Austin Water Utility sized SF residential area water mains to deliver minimum 1000 gpm):

B105.1 One- and two-family dwellings. The minimum fire-flow and flow duration requirements for one- and two-family dwellings having a fire-flow calculation area that does not exceed 3,600 square feet (344.5 m2) shall be 1,000 gallons per minute (3785.4 L/min) for 1 hour. Fire-flow and flow duration for dwellings having a fire-flow calculation area in excess of 3,600 square feet (344.5m2) shall not be less than that specified in Table B105.1.

Exception: A reduction in required fire-flow of 50 percent, as approved, is allowed when the building is equipped with an approved automatic sprinkler system.


If it is found that you meet the size requirements and the Fire Flow rates are not sufficient in your neighborhood the City of Austin allows the following alternative compliance measures.

If you don’t meet the fire flow rates for a larger single family you are required to install a sprinkler system.  If you don’t meet them for two family residences the City requires that you increase the rating of the area separation wall to two hours as an alternative method of compliance.



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