Hacking the Building Code: Part Five - Occupant Loads and Construction Type
In the wake of the tragic Ghostship fire in Oakland, I got more involved with our local planning and building departments, as well as some local artist spaces and maker associations who were looking for advice. I also have taken many complex and unusual projects from concept to finished construction in many different cities. From this experience, I’ve realized that a guide to interacting with building officials and codes for people looking to do creative things with space was sorely needed. This series of blog posts hopes to be that guide. I’ll be posting these over the next few months with the intent to make a comprehensive guide.
The prior posts in the series are available here:
Hacking the Building Code: Part one - What are building codes and where do I find them?
Hacking the Building Code: Part Two - Building Officials and how to approach them.
Hacking the Building Code: Part Three - Use is the Key Factor
Disclaimer
This series is purely for informational and educational purposes and does not constitute professional or legal advice. While this information is provided by Because We Can, and while we endeavor to keep this information up to date and correct, we make no representations or warranties of any kind, express or implied, as to the accuracy or completeness of this information. We do not provide any guarantees or warrant and do not accept any liability on your actions from this information. Any reliance you place on such information is therefore strictly at your own risk, and we highly recommend you hire a building professional (like us) to review your specific case.
Occupant Loads & Construction Type
In our last posts, we covered Use and Occupancy. In this (rather long) post we’ll get into how specific Occupancies determine how many people can use a space or building, what a building has to be in order to support that use and number of occupants, and some of the other rules that can kick in depending on how many people are going to be within a space.
Occupant Load Factor
Every Use of a space drives something called the Occupant Load Factor. You take the area of the space, you divide it by this factor, and that gives you the number of occupants, or Occupant Count, you have to design the space to be able to accommodate. The Occupant Count is a theoretical assumption of how many people would be in a building or space in its normal everyday use. This number is very important, for that’s how many people you’ll need safe exits for, how big those exits and hallways / stairs to get to those exits need to be, what sort of hardware has to be on the exit doors, how many restrooms you’ll need to have, and more.
For example, if you’ve got an office space, the Occupant Load Factor is 100 in the office areas. So if your open office area is 6,000 square feet, that means the space has to be legal to have up to 60 people using it (6000 / 100 = 60). But let’s say your building also has a 1000 square foot storage area, and three big conference rooms that total 600 square feet as well. Storage space only has an Occupant Load Factor of 300, so it would at most have four people in it (rounded up), but an Assembly space such as your conference rooms might have an Occupant Load Factor of 15, which would give you 40 more Occupants there as well. If these additional forty-four people are going to be exiting through the same open office area, the hallways / stairs and exits from that area now have to work for 104 people instead of just 60, etc.
An exit, and the ‘path of travel’ you use to get to that exit, are required to be a certain size and have certain features depending on the number of people that will potentially need to use it when the building is fully occupied, and in some cases certain occupant counts or distances to an exit will require multiple exits regardless of their size. This is why theaters and sport areas have lots of large exits located all over the place that ideally go straight outside, and a small three-story office building still has to have multiple staircases within it.
You can get the Occupant Load Factor from your local Building Codes or officials, there’s usually a table that calls them all out. The 2019 International Building Code has it as Table 1004.5, but your local rules might be different, so it’s always best to check in with a building official or professional.
Again, where this gets really critical is when you’re changing the Use of a space from what it was before, for by changing the Use, you chance the Occupancy, and by changing the Occupancy, you change the Occupant Load Factor, and by changing the Occupant Load Factor you change the theoretical number of people you have to have the building safely work for. Hence why changing an old warehouse into an event space can be tricky, for a warehouse only needs exits and bathrooms for a small handful of people, whereas an event space could easily have ten times or more people within it.
In some cases you are able to negotiate to either have less people in a space, and thus less strict rules that have to be followed. For example, if you have a space that would typically be a A-3 assembly occupancy, but you limit the number of people allowed in the space to under 50, there’s a special exemption that lets you classify that space as a less-strict B occupancy instead. Hence why you see in many small cafes and bars the sign ‘Max Occupancy 49 people’. By voluntarily limiting the number of people, you can negotiate with building officials and find exemptions that make it easier to use a space. So even if the Occupant Count from the Occupant Load Factor is over 50, by limiting to under 50 in this case, you don’t have to have a second exit, extra bathrooms, more fire-resistant construction, etc.
Or in some cases you can negotiate to have more people in a space that what is set by the Occupant Load Factors by going the extra mile. Adding extra exits, fire sprinklers when you don’t already need them, higher fire-rated construction, more signage, and other extra efforts when negotiated ahead of time with the building officials can sometimes get them to allow you to go over what the Occupant Load Factor says a space should be limited to.
One-time events can usually go over the allowed number if proper steps are taken as well. Party permits, approvals from the Fire Marshall, trained staff at key locations to help people to an exit in case of an emergency, additional signage, and more can all make it fully legal to have many people within a space for parties, events, and conventions that what’s normally allowed.
Finally most building permit applications require an ‘exiting plan’ that shows the Occupancy, Occupant Load Factor, and the calculated number of Occupants for every space within a building; as well as the path of travel from those spaces to the exits, what the ‘Fire Rating’ is of those parts of the building, how wide those exits are, and how many people would be using each exit. This sort of plan summarizes everything that the Building Department cares about when it comes to Occupancy and Exiting, and can be very useful to already have when trying to figure out how to affordably change the Use of various spaces within a Building. It’s something you can hire us or any other competent commercial Architect to put together as part of a project assessment, so that you can really understand just how much work your project might be!
Construction Types & Fire Ratings
The Occupancy, and thus the Use, of a building or space also drives what it’s made out of for fire safety. For example, with the tragic Ghostship fire that inspired me to produce this series, it’s my understanding that a major factor in the travesty was that there was only one main exit stair from the upstairs event space, and that stairway was made from salvaged, unfinished wood (some from pallets even!) and was open to the downstairs space. So once a fire started downstairs, the only exit stair caught on fire too, and there was no safe way for people upstairs to exit. Had it been a legal event space, there would have been more than one exit stair, and those exit stairs would have been required to be much more fire-resistant and made from ‘non-combustible’ materials.
The way the Building Code handles this issue is that Buildings are classified into ‘Construction Types’ based upon how fire-resistant they are, which is dependent upon what they are generally made out of. There are five main types, denoted for some reason with roman numerals, going from the most fire resistant “Type I” to the least resistant “Type V”. How big of a building or space of a certain Occupancy is allowed is determined by these Types. And in some cases, certain Occupancies aren’t even allowed if the building isn’t a minimum Type. It’s easy to classify most buildings, as you’ll see below with each Types definition.
Additionally, specific parts of the building are ‘Fire Rated’. This is a measure of how long it would take for a fire on one side of something to reach the other side. So, for example, a One Hour Rated wall is made of materials such that a fire of a certain size on one side of the wall would take an hour to reach the other side of the wall. This ‘Fire Rating’ is used for all sorts of construction; walls, floors, ceilings, doors, windows, stairs, and even whole assemblies of these parts. So a ‘Two Hour Rated exit corridor’ is an entire hallway to the exit where all its walls, the floor, and the ceiling would hold back a fire for two hours to allow people to safely exit the burning building in that time span.
There are five levels of Fire Rating, from the most fire-resistant ‘Four Hour’ rating, to a slightly less fire-resistant ‘Three Hour’, ‘Two Hour’, ‘One Hour’, and finally the ‘Unrated’ for everything else. In order for part of your building to be Fire Rated, it has to match lab-tested and proven pre-defined constructions, and have to follow specific building codes exactly (down to nail placements!) in order to work. Chapter Seven in the International Building Code is all about this, and there are construction guides, such as the UL one here, and the USG one here, that show some of the pre-approved ‘assemblies’ that work.
The tests are actually really cool! They build the structure as it is defined, and then do a controlled burn on one side of it under strict conditions and measure the results. There’s some neat videos of the process, like this one:
But this is why it can be challenging even for building professionals to determine what Fire Rating parts of an older existing building’s construction are, or what needs to be done to get them to be properly Fire Rated. As there are lots of different pre-approved ways to accomplish it, but they are all complex and demanding, and to prove that something was built correctly to a specific specification you might have to take some of it apart for proper inspections. This is where a building professional with lots of experience in accessing older buildings for creative new uses (such as BWC!) can go a long way to affordably figuring out what’s really possible.
This is also why building with non-traditional materials, such as with hay bales, can be so challenging, for until there is a proper UL tested and approved assembly to reference, the Building Department won’t approve it without some major work on your part to either prove that it works, or do something they feel makes it work (like add in additional fire sprinklers that weren’t required before).
Finally most materials fall into a ‘Combustible’ vs. ‘Non-combustible’ classification, where ‘Non-combustible’ = a material that can’t burn at all under normal circumstances (i.e. concrete, steel, glass) and ‘combustible’ (everything else). Sometimes a combustible material isn’t allowed within a certain construction type, fire rating, assembly, or even exit pathway; which is why exit hallways can be a drab combo of raw concrete and painted drywall.
Here’s a quick summary of each Construction Type:
Type I
Type I buildings are the most fire resistant. Nothing flammable can be used for its exterior walls, structure, interior walls / floors / ceilings / etc. In some classifications flammable finishes can be used, but in the most restrictive classification even that isn’t allowed. Examples are skyscrapers, hazardous modern factories, and concrete bunkers. There are two subclasses of Type I buildings, based upon the Fire Ratings of their parts, with a Type IA being more fire-resistant than a Type IB.
Type II
Type II buildings are mostly educational buildings, newer commercial buildings, shopping malls, and smaller theaters. Again, the exterior walls, floor, and frame are non-combustible; but don’t have to be as highly fire-rated as a Type I building, and some combustible interior elements are allowed. There are two subclasses, with Type IIA following more strict testing standards than Type IIB.
Type III
Type III buildings are probably the most common type for an older existing commercial or a newer residential condo / apartment building in a city, where the exterior of the building is made of inflammable rated materials, but the ‘guts’ of the building are not. Think of your typical older warehouse, small office building, or larger condo building; concrete or brick exterior walls with wood structure and floors inside. It, too, has two subclasses, with a Type III-A having more fire-resistant ‘guts’ in it’s interior structure and floors / roof, and a Type III-B that only requires the exterior walls to be fire-rated.
Type IV
Type IV buildings are mostly historic, pre-1960, for they have a main structure made of heavy wood timbers 8” or larger. It’s extremely unlikely for a modern building to fall into this classification, but some older factories, churches, and warehouses do. They are very similar to a Type III-B building, in that only the exterior walls need to be fire rated, but they are generally more fire resistant than the following Type V building.
Type V
Type V buildings are the least fire-resistant, and are mostly made of wood. Houses, smaller apartment buildings, and some smaller commercial buildings fall into this category. Here, too, there are two subclasses, with a Type V-A requiring the exterior walls, interior structure, and floors / roofs to be at least 1 Hour Fire Rated (the lowest, easiest rating to produce) and a Type V-B that doesn’t have many limits on what you can use to built it with.
Construction Limits
As was stated at the beginning of this section, there are limits on how big of a building or space of a particular occupancy can be. The Construction Type will set what how tall a building of a particular Occupancy can be, how many stories it can have, how much square footage, and more. Chapter 5 of the International Building Code goes into lots of detail on this, with lots of tables where you can look up what type and size of Occupancy is allowed within a certain Construction Type.
So it’s best to do an initial assessment of any existing building to determine its Construction Type to better understand if the Use, and the number of people, you’re planning on is allowed in that specific kind of building (or part of it). It could be that a space that works just fine for a small Café won’t work if that Café is expanded into a large restaurant that takes over the rest of the all-wood building.
Additional Fire Rated Constructions
So while the Construction Types set the overall requirements for what has to be fire rated parts of the building, there are other rules that also trigger additional rated elements. These are too numerous to list, but we’ll cover the big ones you’re likely to encounter.
Separations between certain Occupancies are required. For example, no rated separations are required between an Business occupancy and a F-1 light manufacturing one. But an Assembly occupancy of a certain size has to be separated from a Business occupancy with two-hour rated construction. In some cases, certain occupancies aren’t even allowed to be connected at all, and have to be completely separate. Like a Residential use being combined with a Hazardous use. Table 508.4 in the International Building Code shows all of the variations and what has to be separated from what and how. Note that in many cases, having fire sprinklers makes the Rating requirements much lower (that’s what the ‘S’ vs ‘NS’ at the top of the chart is for).
Also it’s typical that the hallways going to an exit, exit corridors / stairs, and exits themselves have to be Fire Rated, even when the surrounding parts of the building might not need to be. For example, in a Type III-A larger building, the stairways and hallways to them will all be 1 Hour Rated or more, even when the rest of the interior construction isn’t.
Finally, specific Uses and even equipment may require additional Fire Rated construction around them, such as a commercial kitchen or an industrial oven within a larger non-rated space.
Use sets the Occupancy, Occupancy sets what kind of building is allowed, what Fire Ratings the parts of it have to have, and how many people can use it.
In our next section, we’ll get into Applications, Plans, Permits, Variances, and Reviews!